Based on a Dead Rail Society Facebook post by Rich Steenwyk, I investigated the suitability of using the Pololu TB9051FTG ($11.95, 2.6A continuous, 1″ x 1″) and the Pololu TB67H420FTG ($19.95, 3.4A continuous, 1″ x 1.2″) as a DCC amplifier in conjunction with the ProMiniAir Receiver for Dead-Rail operation. This post investigates their feasibility and shows connection details.
Feasibility and Connections
The Pololu TB9051FTG is 1″ x 1″
The TB9051FTG truth table below shows the proper bipolar operation on highlighted rows. Note the pin values for EN and ENB. Based on this truth table, this device should be capable of delivering bipolar DCC to the decoder.
The TB9051FTG truth table
Connections for the Pololu TB9051FTG are shown below. When connected to a “large” decoder such as the LokSound 5 XL shown here, a current-limiting resistor is required to prevent the TB9051FTG from shutting down. The Negative Temperature Coefficient (NTC) resistor is initially at 1 Ohm, but with increased current demand, it heats up, reducing its resistance and voltage drop to a low value. This solution is superior to a high-wattage constant 1 Ohm resistor CVP recommends for its smaller Airwire CONVRTR receivers.
The ProMiniAir Receiver/TB9051FTG connections
A close-up of the connections for the Pololu TB9051FTG is shown below. Bipolar 3.3V DCC outputs and +5V from the ProMiniAir Receiver are required for proper operation. Note the jumpers that set EN to VCC (High), ENB to GND (Low), and OCC to VCC (High, retry after shutdown).
Close-up of ProMiniAir Receiver/TB9051FTG connections
Example DCC output for the decoder provided by the ProMiniAir Rx/TB9051FTG combination is shown below.
DCC output from the ProMiniAir Rx/TB9051FTG combination
A demo of the Pololu TB9051FTG in dead-rail operation, which will NOT operate with the LokSound 5 XL (5A max) without a current-limiting resistor, is shown below.
Demo of the ProMiniAir Rx/TB9051FTG combination
Now let’s turn to the larger, more expensive TB67H420FTG.
The Pololu TB67H420FTG is 1″ x 1.2″
The TB67H420FTG truth table below shows the proper bipolar operation on highlighted rows. Note the value of PWMx must be High. Based on this truth table, this device should be capable of delivering DCC to a decoder.
The TB67H420FTG truth table
Connections for the Pololu TB67H420FTG are shown below. When connected to a “large” decoder such as a LokSound 5 XL, a current-limiting resistor is NOT required to prevent the TB67H420FTG from shutting down. However, the TB67H420FTG is more expensive and larger than the TB9051FTG.
The ProMiniAir Receiver/TB9051FTG connections
A close-up of the connections for the Pololu TB9051FTG is shown below. As with the TB9051FTG, bipolar 3.3V DCC inputs from the ProMiniAir Receiver are required for proper operation. Note the single jumper connecting VCC (High) to HBMODE that turns on the single-output function. Jumpers between A+/A- and B+/B- deliver the maximum single DCC output of 3.7A.
Close-up of the ProMiniAir Receiver/TB9051FTG connections
Testing with the TB9051FTG when interfaced with a ProMiniAir Receiver was successful.
Conclusion
Both the Pololu TB9051FTG ($11.95, 2.6A, 1″ x 1″) and TB67H420FTG ($19.95, 3.4A, 1″ x 1.2″) can be configured to deliver full-power DCC to a decoder when used in conjunction with a Dead-Rail receiver such as the ProMiniAir Receiver. They require five connections to the ProMiniAir receiver, including 3.3V or 5 V bipolar DCC outputs. The lower power TB9051FTG does require a current-limiting resistor for some decoders that produce large “in-rush” currents during power on, and the TB9051FTG does not.
The Adafruit DRV8871 amplifier is perhaps a better choice because:
Cheaper: $7.50+s&h
Comparable output: 3.6A
Smaller: 1″ x 0.8″
Requires only four connections to the receiver: GND, VPOWER, DCC+, and DCC-
The disadvantage of the Adafruit DRV8871 and the Pololu TB9051FTG is they require a current-limiting resistor for some larger decoders. The Pololu TB67H420FTG does not need this current limiter, but it’s slightly larger and more expensive. You decide!
Sometimes folks want radio control of their locomotives but prefer to use track AC (including DCC) instead of a battery to provide DC power for the radio receiver and amplifier. This post shows how to repurpose the DCC converter PCB customarily provided with the ProMiniAir transmitter to convert track AC to DC power for the ProMiniAir receiver.
DCC Converter Modifications
The original purpose of the “DCC Converter” is to use a DCC throttle’s Track Right/Track Left output and convert it to 5V DCC and 5V power for the ProMiniAir transmitter. See the Figure below.
The original purpose of the “DCC Converter” is to provide 5V power and 5V DCC signals to the ProMiniAir transmitter.
One of the strengths of the modular approach used for the ProMiniAir transmitter and receiver is that you can “repurpose” components. The DCC Converter can be modified to use the AC track input to provide filtered, higher-voltage DC power.
Below is the repurposing idea: add a large capacitor (in series with a 100 Ohm resistor and a 1N4001 diode) across the “+” and “-” terminals of the rectifier and route out the rectifier’s DC output. Smaller onboard capacitors (10uf and 100nf) also filter out higher-frequency noise that large capacitors sometimes do not effectively filter.
One end of a 100 Ohm resistor and the + terminal of a 1N4001 diode are connected in series (and in parallel to each other) to the capacitor’s + terminal. The other end of the 100 Ohm resistor and the – terminal of the diode are connected to the rectifier’s + terminal. The capacitor’s – terminal is directly connected to the rectifier’s – terminal to form the DC ground. My thanks to ScaleSoundSystems.com for the idea of adding a resistor and diode in series with the capacitor.
When the throttle is turned on, the 100 Ohm resistor prevents an “in-rush” short circuit that might cause the throttle to cut off. When charging, the 1N4001 diode is reverse-biased with a large resistance. If AC power is interrupted, current flows out of the capacitor through the low resistance path of a forward-biased 1N4001 diode to maintain DC power output.
How keep-alive works. The resistor regulates charging, and the diode regulates discharging.A large capacitor (along with a 100 Ohm resistor and a 1N4001 diode) can be added to the DCC Converter to output heavily-filtered DC power.Connections between the AC-to-DC Converter with a large keep-alive capacitor and the ProMiniAir receiver/amp. The switch is NOT required if a large keep-alive capacitor is not used.
In fact, it is possible to forgo the large capacitor since these onboard capacitors do a pretty good job of “cleaning up” the DC output of the rectifier. This is a good option if space is at a premium.
The filtered DC output can now provide DC power to a ProMiniAir receiver/amp, just as a battery would. If the added capacitor is large enough, it will function as a “keep-alive” capacitor many DCC decoders use to prevent track power interruptions.
An Example
The photo below shows a real-world example of the conversion using a 10000uf “keep-alive” capacitor originally used with a Zimo decoder. The size of the capacitor dominates that of the DCC converter!
A modified DCC Converter using a very large 10000uf “keep-alive” capacitor. Close-up of the modified DCC Converter
The oscilloscope trace below demonstrates the ability of the modified DCC Converter to produce clean DC power for your ProMiniAir (or other) receiver.
Track AC input and filtered DC power output from a modified DCC Converter with an added large (10000uf) capacitor
Note how “clean” the DC power output is (13.8VDC). Square wave track inputs at 16.8V are a severe test because they produce frequencies at odd multiples of the square wave’s frequency, e.g., at 6KHz, 18KHz, 30KHz, etc. for the example above, but very little of these frequencies “bleed through” to the DC power output.
Simplifying further, we can use the DCC Converter without an added capacitor, relying on the onboard capacitors to filter the rectifier’s “+” and “-” output. This option might be useful if space is at a premium.
Modified DCC Converter with NO capacitor
The DC power output is still clean, but you lose the “keep-alive” that a large capacitor provides.
Track AC input and filtered DC power output from a modified DCC Converter with NO added capacitor
Conclusion
So there you have it – the DCC Converter can be slightly modified to provide filtered DC power and “keep-alive” capability. The modified DCC amplifier with a sizeable keep-alive capacitor costs $10 + shipping. Without the capacitor, the modified DCC Converter is $7 + shipping.
As some of you may know from my previous postings or other sources, if you try “raw” transmission of DCC from standard DCC throttles, such as with the Tam Valley Depot DRS1 transmitter, to Airwire receivers, you probably won’t get consistent control – I didn’t. This failure set me on the road to devise the ProMiniAir transmitter that would work with CVP Airwire receivers using DCC generated by standard DCC throttles, including the superb “open-source” WiFi-equipped EX-CommandStation created by the folks at DCC-ex.com. Of course, the ProMiniAir receiver is fully-compatible with Airwire throttles.
My web research and discussions with fellow dead-railers led me to believe you might solve the compatibility problem by providing frequent DCC “Idle” messages. Once the dust settled on the ProMiniAir firmware we made available on our GitHub site, the ProMiniAir transmitter worked pretty well with Airwire receivers! Besides CVP Airwire transmitters, the ProMiniAir transmitter is the only currently-manufactured transmitter that works with Airwire receivers.
After this success, we have worked hard to ensure that the ProMiniAir transmitter (and receiver) are compatible with multiple product lines, including CVP Airwire, Tam Valley Depot DRS1 transmitters and receivers, Gwire transmitters and receivers (available but no longer manufactured), Stanton Cab transmitters and receivers, and the no longer manufactured NCE D13DRJ.
OK, you may ask, what’s the point of this post? Well, I’d like to share some further research on the source of the CVP receiver’s incompatibility and its consequences on updates for the ProMiniAir transmitter/receiver firmware.
Further Investigations
OK, I based our initial success in making the ProMiniAir transmitter compatible with CVP Airwire receivers on observing how well the ProMiniAir worked with Airwire receivers. Yep, numerous inserted DCC IDLE messages from the ProMiniAir transmitter seemed to keep the Airwire receivers reasonably “happy,” responding to throttle speed/direction commands and function activation.
However, sometimes the Airwire receiver seemed a bit slow to respond to function activation… And some customers (who are hopefully still friends) sometimes noted this slow response. Could this be improved?
Still, I hadn’t analyzed what an Airwire throttle was sending in detail, so I purchased a simple logic analyzer from Amazon to look at the actual DCC transmitted by an Airwire throttle. I also needed the “pulseview” software from sigrok.org and a DCC decoder add-on. To properly analyze DCC, I modified the add-on and will make it available on our GitHub site.
The figure below is what I observed by firing up my original ProMiniAir transmitter integrated with a WiFi-equipped EX-CommandStation, and using the iOS WiThrottle app to send throttle commands to a ProMiniAir receiver. The figure below shows the “raw” digital output from the ProMiniAir receiver’s transceiver.
The raw DCC data received by a ProMiniAir receiver from a ProMiniAir transmitter integrated with a WiFi-equipped EX-CommandStation
The waveform is what you would expect. A “one” end packet bit from the previous DCC packet and then a series of 15 “one” preamble bits followed by a “zero” packet start bit that signals to the decoder that a DCC command is coming. I observed no significant or consistent DCC “errors” in the collected data.
The preamble to a packet consists of a sequence of “1” bits. A digital decoder must not accept as a valid, any preamble that has less then 10 complete one bits, or require for proper reception of a packet with more than 12 complete one bits. A command station must send a minimum of 14 full preamble bits.
The data below is what I observed by firing up an Airwire T5000 transmitter and looking at the “raw” digital output pin from the transceiver (radio) on the ProMiniAir receiver.
The raw DCC data received by a ProMiniAir receiver from an Airwire T5000 throttle
Well, well. Now we see an NMRA-permissible (see line 121 of NMRA Standard S 9.2) but non-DCC transition pair, called a “cutout,” with a 1/2 “one” and 1/2 “zero” pair after a valid “one” end packet bit and before a very long (30 “one” bits) preamble. If you try to send shorter preambles, say 15 “one” bits, the Airwire receiver will NOT work consistently despite the NMRA standard stating that a decoder must not require more than 12 complete “one” bits in the preamble. So, the Airwire receiver is placing a non-standard requirement for a “long” preamble of “one” bits before it will operate consistently.
Timing of a 1/2 “one” and 1/2 “zero” cutout
While reviewing the DCC sent from an Airwire throttle, there were NOT an unusual number of DCC “Idle” messages sent by the Airwire transmitter. But, by sending tons of short (3 bytes) DCC “Idle” messages, the ProMiniAir transmitter was sending just enough “one” bits to keep the Airwire receiver functional. I am not privy to the details of Airwire’s receiver firmware, so my success was based purely on empirical observation without underlying “insider” knowledge.
So what? With this knowledge, I felt it essential to make some ProMiniAir firmware changes.
Firmware Changes to the ProMiniAir Transmitter
Based on this new information, to improve compatibility with Airwire receivers, we have modified the ProMiniAir transmitter’s firmware to ensure a 1/2 “one” followed by a 1/2 “zero” cutout comes after the end packet “one” bit, and before at least 30 “one” bits are in the preamble. This change is NOT harmful to other wireless receivers, including the ProMiniAir receiver. The ProMiniAir transmitter and receiver still insert DCC “Idle” messages when possible to keep decoders “happy” while waiting for valid DCC messages from the throttle.
Along with these firmware changes, which will be made available on our GitHub site, you can set the number of “one” bits in the preamble by going into OPS mode at address 9900 (transmitter) or 9901 (receiver) by setting the value of CV242. If you set the value of CV242 to 0, the firmware sets the number of preamble bits to a “reasonable” value of 16 (receiver) or the number of preamble bits the throttle sent (transmitter). If you set CV242 to less than 12, it will be reset to 12 to ensure decoders are “satisfied” with the number of preamble “one” bits.
You can also change the duration of the cutout’s second 1/2 transition with CV240. By default, a CV240 value of 27 makes the second 1/2 transition a “zero” with a duration of 116us. If you do NOT want a cutout inserted, you can set the CV240 value to 141, which will make the duration equal to that of the cutout’s leading 1/2 “one” (58us), resulting in an output 1/2 “one” and 1/2 “one” pair, simply increasing the number of preamble “one” bits by one.
Example CV240 values to control cutout duration
So, how well do these modifications work for Airwire receivers? It isn’t easy to quantify, but the Airwire receiver’s red data LED remains “on” more consistently with less “flicker,” and the receiver’s DCC output to the decoder contains “cutouts” (with a duration of about five preamble “one” bits) just before the preamble. These characteristics are now very similar to those from an actual Airwire throttle. See the comparison figures below. It’s difficult for me to test the more practical aspects of these improvements, but the decoders continue to operate as I would expect, perhaps with somewhat less time delay. Other users may be able to test under more stressful conditions.
CONVRTR DCC output to the decoder from an Airwire transmitter. The cutout duration does NOT matter to the decoder.CONVRTR DCC output to the decoder from a PMA transmitter with updated firmware. The cutout duration does NOT matter to the decoder.
Conclusion
We now have a better idea why Airwire receivers do not work well with output from a typical, NMRA-conformant DCC throttle sent wirelessly and how to better cope with Airwire receivers’ unique DCC requirements by sending very long DCC preambles, preceded by a 1/2 “one” and 1/2 “zero” cutout.
Raw empiricism often leads you to workable, pragmatic solutions, but a little “looking under the hood” for the “how” and “why” almost always pays dividends. If you own a ProMiniAir transmitter and are not satisfied with its performance with Airwire receivers, don’t hesitate to contact me about how I can provide you with an update to see if your performance will improve.
Typical application. In some cases, such as the Airwire transmitters, the throttle and transmitter are combined. Also, the receiver and amplifier may be integrated, such as for Airwire and Tam Valley Depot receivers. The ProMiniAir transmitter and receiver require a “DCC Converter” or “DCC Amplifier” provided as part of the purchase.
I was inspired to fully develop a wireless DCC transmitter and receiver by two sources: Martin Sant, who runs the BlueRidge Engineering website, and an article by Mark and Vince Buccini titled “Build Your Own Wireless DCC System” that appeared in the April, June, and August 2014 editions of Garden Railways magazine. These back issues are still available.
The Buccinis showed that it was possible to home-build a wireless DCC system. And Martin became a great collaborator who concretely started me with the initial version of the “ProMiniAir” wireless DCC transmitter/receiver hardware and the wireless DCC software for the Pro Mini microcontroller board. I am deeply indebted to these people.
Note: Some photos may show older versions of the ProMiniAir. Also, previous versions of the ProMiniAir receiver and transmitter used 9000/9001 for their DCC address, respectively, which we changed to 9900/9901. Photos and examples may use the now-obsolete addresses.
Update for New Versions of the ProMiniAir Transmitter and Reciever
Please see this post on an important update on the ProMiniAir transmitter. It is now completely stand-alone; just plug in power and use your cell phone app to control your locomotive.
The new completely stand-alone ProMiniAir transmitter. Just plug in power, use your smartphone app to connect to the WiFI-equipped EX-CommandStation, and control your dead-rail locomotive.
The ProMiniAir transmitter and receiver have been significantly reduced in size: 1.1″ x 0.8″, making it possible to mount the ProMiniAir receiver and a tiny DCC amplifier in tighter spaces and some HO locomotives.
The new ProMiniAir receiver and small amplifier (3.6A)
Feature Comparisons
My goal for offering the ProMiniAir receiver/transmitter is to provide those interested in “dead-rail” (radio control, battery power of a model railroad locomotive) inexpensive wireless, DCC compatible transmitters and receivers for radio-control of model railroad locomotives in the US/Canadian 915MHz ISM band – the same band and protocol as used by Tam Valley Depot (TVD), CVP Airwire, NCE/QSI Gwire, and Stanton Cab. Also, you can operate the ProMiniAir transmitter and receiver in the European ISM band at 869.85MHz, and we have verified interoperability with Tam Valley Depot European DRS1 transmitters and receivers.
A note about channels: modern CVP Airwire transmitters and receivers can all operate in the Airwire channels designated 0-16 using current Anaren AIR transceiver chips. Older wireless transmitters and receivers from Tam Valley Depot and Stanton Cab used the Linx ES series transmitter or receiver chip that only operated at 916.48MHz with slightly different specialized radio settings from the Airwire channels. I call this channel 17. In most but not all cases, these Channel 17 devices are interoperable with Airwire Channel 16 @ 916.36MHz. Also, European versions of these older transmitters and receivers operated on 869.85MHz; I call this Channel 18. Here’s my unofficial Table of channels and frequencies.
The ProMiniAir has some features that may be more interesting than commercial offerings. See the Comparison Tables below.
Name
Airwire Receiver Compatible?
Channels
Power Level Adj
Any DCC Input
TVD DRS1 Transmitter
No
Ch 17 (or 18(E))
No
Yes
Airwire T5000
Yes
0-16
Yes
No
NCE Gwire Cab
Yes
0-7
Yes
No
S-Cab Throttle
No
17
No
No
ProMini Air Transmitter
Yes
0-17, 18(E)
Yes
Yes
Comparison of wireless DCC transmitters
In fairness, the manufacturers of the Airwire T5000, the NCE Gwire Cab, and the S-Cab Throttle hand-held throttles never intended to interface with standard DCC throttles. But, as Tam Valley Depot recognized, it is advantageous to use any device that supplies DCC to the rails and transmit this DCC wirelessly to DCC-compatible receivers.
A notable limitation of the Tam Valley Depot DRS1 transmitter is that it does not provide DCC “IDLE” packets that the Airwire receivers require unless the original DCC throttle does so (most, if not all, do NOT). Also, the Tam Valley Depot DRS1 transmitter can only broadcast on one Channel (near Airwire Channel 16, which I have designated Channel 17 @ 916.48MHz).
Shown in the Table below are the comparisons for wireless DCC receivers.
Name
Channels
DCC Filtering?
Channel Auto Search
TVD DRS1, MK IV
0-17, 18(E)
None
Yes
Airwire CONVRTR
0-16
Always On
Yes (Limited)
QSI Gwire
0-7
None
No
S-Cab LXR receiver
17
None
No
ProMini Air
0-17, 18(E)
None or On
Yes
Comparison of wireless DCC receivers
The most notable difference among the receivers is “DCC filtering,” i.e., how the receiver behaves when losing a valid RF DCC signal.
When the TVD DRS1 or QSI Gwire receivers lose a valid RF signal, they output random pulses to the decoder. I have discussed the pros and cons of this in another post.
On the other hand, the Airwire CONVRTR outputs constant-level DC when it loses a valid RF signal or doesn’t receive enough DCC “IDLE” packets. Again, as discussed in another post, the DCC decoder may halt the locomotive dead in its tracks when it receives this constant-level DC, which may or may not be what the user wants.
The Airwire CONVRTR performs “DCC filtering” by periodically evaluating whether it’s receiving DCC “IDLE” pulses. So, even if a stream of completely-valid DCC packets is received, but there are few or no “IDLE” packets, the Airwire CONVRTR will become inactive and output constant DC to the decoder.
These characteristics of the Airwire receivers are why Tam Valley DRS1 transmitter will usually NOT work with Airwire CONVRTR receivers because the DRS1 will not insert additional DCC “IDLE” packets! The Tam Valley Depot DRS1 transmitter is a passive participant: if the input DCC throttle doesn’t produce frequent DCC “IDLE” pulses, then the Tam Valley Depot DRS1 will not transmit frequent DCC “IDLE” pulses.
Stanton designed the S-Cab LXR-DCC receiver specifically for the S-Cab Throttle’s intermittent DCC transmissions. Like the Airwire CONVRTR receivers, the LXR outputs a constant DC voltage when a valid RF signal is lost.
Via OPS mode (by default at address 9901), you can reconfigure ProMiniAir’s output behavior when a valid RF signal is lost. The first option (CV246 -> 0) selects the output of DCC IDLE messages (which the decoder is “comfortable” with, rather than random pulses that might “confuse” the decoder). The second option (CV246 -> 1) selects the output of constant-level DCC.
This reconfigurability makes the ProMiniAir receiver a versatile wireless DCC receiver. The ProMiniAir receiver’s RF DCC detection technique is more sophisticated than Airwire’s. The ProMiniAir receiver detects how long it’s been since it received ANY valid DCC packet. And, after a preset time interval (which is reconfigurable via OPS mode, changing the CV252 value in 1/4 second multiples), the ProMiniAir receiver will output either the DCC “Idle” messages (DCC filtering “off”) or output constant-level DC (DCC filtering “on”). When DCC filtering is “on,” and there is no valid RF signal, the DC level output is reconfigurable via an “OPS” mode setting of CV248 (-> 1 for positive DC, -> 0 for 0V DC) at the ProMiniAir’s DCC address.
Once a valid RF signal is received again, the ProMiniAir receiver detects this condition. It outputs these valid DCC packets to the “DCC amplifier” that sends “track-level” DCC to the decoder.
Another important feature of wireless DCC receivers is Channel selection and searching.
IF YOU SET SOME JUMPERS, the TVD DRS1 receiver will “listen” on a fixed Airwire Channel. Otherwise, the DRS1 will automatically search the Airwire Channels for a valid RF signal if you do NOT insert the jumpers. This behavior may or may NOT be a good idea if multiple wireless DCC transmitters transmit simultaneously on different Channels. And changing the Channel selection behavior (fixed Channel or auto-scan) requires physical access to the receiver to connect or disconnect jumpers.
On startup, the Airwire CONVRTR “listens” for a valid RF signal on its “startup” channel (which is reconfigurable by accessing a CV using the wireless throttle’s “OPS” mode). If the CONVRTR finds no valid RF signal after a given time, the CONVRTR will switch to Channel 0. This behavior is usually a good idea.
Like the Airwire CONVRTR, on startup, the ProMiniAir receiver will “listen” for valid RF on its “startup” Channel (default, 0) stored in EEPROM memory. This startup channel is changeable using the transmitting throttle’s “OPS” mode by setting CV255 to 0 through 18 at the ProMiniAir transmitter’s DCC Address (default, 9901). Like the TVD DRS1 receiver, if the ProMiniAir does not find a valid RF signal on its startup channel, the ProMiniAir receiver will then auto-scan Channels 0(A), 18(E), 17(S), 1(A), 2(A), …, 16(A) (in that order) for valid RF signal (A=Airwire channels, E=European channel @869.85MHz, S=S-Cab Channel @ 916.48MHz). This scan sequence guarantees that a wireless DCC transmitter (if one is available) is selected, but only if the ProMiniAir does NOT find a valid RF DCC signal on its startup Channel from another wireless DC transmitter.
Once “locked on” to a Channel. The ProMiniAir Receiver will continue to “listen” on this Channel, even if the transmitter is turned off or the signal is lost. This allows the ProMiniAir Receiver to pick up signals on the “locked on” Channel once the transmitter is turned back on or the signal is re-established.
If the ProMiniAir receiver finds no valid RF DCC signal on any Channel on startup, it will select Channel 0 and wait for a valid RF DCC signal. Also, upon reset, the ProMiniAir’s Channel search process will be unchanged: it will try the “startup” channel stored in EEPROM memory, then try auto-searching Channels, and if all else fails, wait on Channel 0.
So, in summary, we are offering the ProMiniAir DCC transmitter and receiver to provide a low-cost alternative with features not entirely found in commercial offerings.
You are provided with a few additional components when buying a ProMiniAir receiver or transmitter. In the case of the ProMiniAir transmitter, we include a simple “DCC Converter” PCB that converts DCC output to the track into Ground, 5V power, and 5V logic DCC. These outputs supply the ProMiniAir transmitter with power and DCC packets to transmit, so no additional power supply is necessary.
For the ProMiniAir receiver, we include a low-cost “DCC amplifier” that converts the ProMiniAir receiver’s 5V logic DCC back to DCC. In its typical configuration, the onboard DCC decoder would pick up from the track (again, discussed in detail below). The ProMiniAir receiver can be powered directly from the battery or a small external 5V power supply.
This modularity keeps costs down, allows for easy replacement of components rather than the entire assembly, and enables the use of commodity components less susceptible to supply-chain disruptions.
ProMiniAir transmitter connectionsStandalone ProMiniAir transmitter connectionsProMiniAir receiver connections for a DRV8871 (3.6) amplifierProMiniAir Receiver connections for a Cytron MD13S (13A) amplifier
And you will need an antenna of your choosing! I love antennas, but your antenna requirements are too diverse to offer a “one size fits all” antenna solution. We provide an FCC/IC-approved Anaren “whip” antenna that connects to the U.FL connector on a 10-pin transceiver daughterboard. This antenna should work well for most transmitter applications and is FCC/IC-approved for “intentional radiators.”
For the ProMiniAir receiver, some can use the small whip antenna without modification; others will need to run an antenna connecting cable to a small, externally-mounted antenna. We discuss several excellent antenna options below.
Documentation
The definitive source of information for the ProMiniAir transmitter and receiver is available here.
Kit Assembly
We no longer offer the ProMiniAir as a kit.
Firmware Installation
The ProMiniAir Tx and Rx are provided with the firmware already loaded. These instructions are only for advanced users who want to update the firmware.
The source code is available from this GitHub site. Locate the source code in a directory where the Arduino IDE can find it. You should retain the subdirectory structure to access the “project” with the Arduino IDE.
How to download the GitHub zip file that will maintain the directory structure
If you want a transmitter or receiver, edit libraries/config/config.h to select the “define” for the transmitter or receiver.
For a receiver (Rx), config.h should look like this:
...
// #define EU_434MHz
/* For World-Wide 2.4GHz ISM band*/
// #define NAEU_2p4GHz
//////////////////////////////
// Set Transmitter or Receiver
//////////////////////////////
/* Uncomment ONLY ONE #define*/
/* For receiver*/
#define RECEIVER
/* For transmitter*/
// #define TRANSMITTER
/////////////////////////////////////////////////
// Set the default channel for NA/EU 900MHz only!
/////////////////////////////////////////////////
#if defined(NAEU_900MHz)
/* Uncomment ONLY ONE #define*/
/* To set the default to NA channel 0 for 869/915MHz ISM bands only!*/
#define NA_DEFAULT
/* To set the default to EU channel 18 for 869/915MHz ISM bands only!*/
// #define EU_DEFAULT
#endif
//////////////////////////////////////////
// Set the transceiver's crystal frequency
//////////////////////////////////////////
/* Uncomment ONLY ONE #define*/
/* For 27MHz transceivers (e.g., Anaren 869/915MHz (CC110L) and Anaren 869MHz (CC1101) radios)*/
// #define TWENTY_SEVEN_MHZ
/* For 26MHz transceiver (almost all other radios, including Anaren 433MHz (CC1101), 915MHz (CC1101), and 2.4GHz (CC2500) radios)*/
#define TWENTY_SIX_MHZ
...
If you want a transmitter (Tx), then config.h should be
...
// #define EU_434MHz
/* For World-Wide 2.4GHz ISM band*/
// #define NAEU_2p4GHz
//////////////////////////////
// Set Transmitter or Receiver
//////////////////////////////
/* Uncomment ONLY ONE #define*/
/* For receiver*/
// #define RECEIVER
/* For transmitter*/
#define TRANSMITTER
/////////////////////////////////////////////////
// Set the default channel for NA/EU 900MHz only!
/////////////////////////////////////////////////
#if defined(NAEU_900MHz)
/* Uncomment ONLY ONE #define*/
/* To set the default to NA channel 0 for 869/915MHz ISM bands only!*/
#define NA_DEFAULT
/* To set the default to EU channel 18 for 869/915MHz ISM bands only!*/
// #define EU_DEFAULT
#endif
//////////////////////////////////////////
// Set the transceiver's crystal frequency
//////////////////////////////////////////
/* Uncomment ONLY ONE #define*/
/* For 27MHz transceivers (e.g., Anaren 869/915MHz (CC110L) and Anaren 869MHz (CC1101) radios)*/
// #define TWENTY_SEVEN_MHZ
/* For 26MHz transceiver (almost all other radios, including Anaren 433MHz (CC1101), 915MHz (CC1101), and 2.4GHz (CC2500) radios)*/
#define TWENTY_SIX_MHZ
...
Two further options are available. The first option selects the crystal frequency of the FCC/EC-approved transceiver: 27MHz (Anaren) or 26MHz (Ebyte). The second option specifies North American or European default use.
After you complete downloading the firmware into the Pro Mini, please do not remove the USB connection from the computer until the “secondary” LED, which indicates attempted communication over the SPI (serial peripheral interface), flashes on (it will not be bright). This step ensures you properly initialize the EEPROM!
You load the firmware into the Pro Mini MCU using an “AVR ISP,” such as the Sparkfun Pocket AVR Programmer or a less-expensive clone. This “ISP” downloading mode will bypass and erase the bootloader to directly load the firmware into the Pro Mini MCU. On boot-up with the bootloader now erased, the Pro Mini MCU will almost instantly supply “5V logic DCC” to the DCC amplifier, which provides the DCC decoder with standard DCC waveforms. There is no “boot-up DC” or need to set CV29, bit2=0. (I set it anyway.) With this solution, all DCC decoders I’ve tried (ESU, Zimo, MTH) startup without the “boot-up jerk.”
This “ISP” form of loading firmware is not as extensively used by folks using the Arduino IDE, but ISP loading is easily accessible within the Arduino IDE. The overly-brief method of ISP programming steps is the following:
Remove the transceiver daughterboard and the jumper (if inserted).
Connect the USBtinyISP (or other) Programmer (with power switch ON to supply 5V DC to the ProMiniAir PCB while programming) to the 6-pin connector on the ProMiniAir.
From the Arduino IDE, Select Tools → Programmer → “USBtinyISP” (or whatever ISP programmer you use).
Select the AirMiniSketchTransmitter sketch.
Select Sketch → Upload using a Programmer.
The Arduino IDE will compile the sketch and download the resulting firmware to the Pro Mini via the USBtinyISP, bypassing (and erasing) the bootloader.
Once the ProMiniAir receiver or transmitter firmware is installed in the Pro Mini and inserted into the ProMiniAir PCB, the ProMiniAir is ready for integration!
Integration
You must establish several connections to complete the ProMiniAir receiver (Rx) integration or transmitter (Tx).
Overview of Connections
See the picture below for an overview of the connections to and from the ProMiniAir. Which connections you use depends on whether the ProMiniAir will act as a receiver (Rx) or a transmitter (Tx). THERE IS NO PROTECTION AGAINST INCORRECT BATTERY OR EXTERNAL POWER CONNECTIONS!!! You will destroy the ProMiniAir immediately if you reverse the GROUND and POSITIVE POWER SUPPLY connection!
Data and power connectionsfor PMA RxData and power connections for PMA Tx
The Anaren and Ebyte transceiver daughterboards have a versatile U.FL plug for antenna connections. You can plug in either the Anaren whip antenna we provide or a U.FL-to-SMA or U.FL-to-RP-SMA cable that screws into a remotely-mounted antenna. Also, a two-pin output provides Ground and the DCC input to (Tx) or output from (Rx) the RF transceiver board, serving as signals to an oscilloscope for waveform review. See the figure below for details on these connections.
ProMiniAir antenna connector (female RP SMA) and transceiver DCC input/output
The ProMiniAir has several connections that provide an AVR programmer, I2C display outputs, and 5V logic DCC inputs or outputs. See the photo below.
ProMiniAir connections for AVR programmer, I2C display output, and 5V logic DCC input or output
We will break down these connections for the ProMiniAir receiver and transmitter in the following two sections.
Receiver Connections
Several options exist for providing power, starting with the ProMiniAir configured as a receiver (Rx). The first option is to use external battery power and jumper the +5V and +5V (Battery) pins to use the onboard 5V regulator to provide board +5V supply.
ProMiniAir power connection options (for Rx only, the Tx receives power from the DCC Converter).
Since you may not like the heat generated by the onboard 5V regulator when you supply power with external battery power and install the jumper, as an alternative, you may use an external +5V power supply, as shown below, where the external power supply provides Ground and +5V. Of course, you do NOT install the jumper.
ProMiniAir receiver powered by an external +5V power supply (older PMA version, but the connections are the same for newer versions)Close-up of ProMiniAir receiver power connections to an external +5V power supply (older PMA version, but the connections are the same for newer versions)
The ProMiniAir receiver must connect to an external DCC amplifier that converts the 5V logic DCC from the ProMiniAir receiver to DCC A/B that a DCC decoder requires. This DCC amplifier uses battery power and the inputs from the ProMiniAir receiver to provide the power and DCC messages, coded as a bipolar DCC waveform, to the decoder for both power and DCC messages. These “DCC amplifiers” are usually medium to large amperage amplifiers that accept pulse width modulation (PWM) input to provide precision output control for electric motors. The maximum PWM frequency of these amplifiers is usually high enough (> 20kHz) to reproduce DCC packets accurately.
Depending on the particulars of your installation, the author will provide an appropriate DCC amplifier as part of your PMA Rx purchase.
Close-up of the inputs to the DCC amplifier from the ProMiniAir receiver
As shown below, some DCC amplifiers have specialized connector configurations for a GROVE-compliant amplifier.
Example of another DCC amplifier’s connections to the ProMiniAir receiver
Integration of the ProMiniAir Receiver into a Locomotive
Of course, the real purpose of the ProMiniAir receiver is to integrate it into a locomotive for wireless DCC control using an onboard battery as power. An excellent high-power (13A continuous) DCC amplifier may be purchased here, as shown below. Unless determined otherwise for size constraints, this Cytron MD13S amplifier is the one we provide with the ProMiniAir receiver. You can successfully use more expensive high-amperage amplifiers (about $30 US as of 2020) found at Pololu here or here. These amplifiers are smaller (0.8″ x 1.3″) than the Cytron.
ProMiniAir receiver integration with battery power, DCC amplifier, and antenna (older PMA version, but the connections are the same for newer versions)Example Installation
Transmitter Connections
Let’s turn the ProMiniAir used as a transmitter (Tx) of DCC messages from any DCC-compatible throttle.
The photo below shows the connections between an interface board that takes throttle DCC A/B inputs (“track” DCC) and rectifies these inputs to provide Ground and +5V power supply output. This “DCC Converter” PCB also “taps off” the DCC A input and converts it to a 5V logic DCC output suitable for the ProMiniAir transmitter. These outputs provide the ProMiniAir transmitter with Ground, +5V power, and 5V logic DCC input.
We provide the “DCC Converter” PCB as part of your PMA Tx purchase.
Photo of ProMiniAir receiver connections to a “DCC Converter” PCB that supplies the ProMiniAir transmitter with Ground, +5V power, and 5V logic DCC. The ProMiniAir transmitter does NOT connect to a battery or use the jumper connecting +5V to +5V (Battery)!Close-up of ProMiniAir transmitter connections to the “DCC Converter” PCB. The jumper connecting +5V to +5V (Battery) is NOT used! (older PMA version, but the connections are the same for newer versions)
The user can change the ProMiniAir transmitter’s Channel (Airwire channels 0-16, S-Cab channel 17, and EU channel 18) and Power Level (0-10) by setting the DCC throttle’s address to that of the ProMiniAir transmitter’s (9900 by default). Then, using the throttle’s OPS mode, change the value of a configuration variable (CV255 for Channel: 0-16, and CV254 for Power Level: 0-10), exit OPS mode, and change the throttle back to the locomotive’s DCC address.
Receiver/Transmitter Antenna Connections
For the ProMiniAir transmitter, we strongly urge you to use the FCC/IC-approved Anaren “whip” antenna supplied with the surface-mounted transceiver to a 10-pin interface daughterboard. This whip antenna/transceiver combination is FCC/IC-approved as an “intentional radiator.” You can purchase antennas for the ProMiniAir transmitter online from many sites for experimentation purposes. For fixed installations of the ProMiniAir transmitter, we suggest reputable products from Linx, such as their SMA one-half wave antennas with an internal counterpoise. You can find these antennas at Digi-Key, e.g., ANT-916-OC-LG-SMA ($10.55) and ANT-916-CW-HWR-SMA ($12.85). The former antenna has a slightly better gain (2.2dBi versus 1.2dBi) but is somewhat longer (6.76″ versus 4.75″).
Linx half-wave antennas. The ANT-916-OC-LG-SMA has a better gain than the ANT-916-CW-HWR-SMA at the expense of being 42% longer.
For the ProMiniAir receiver or the ProMiniAir transmitter, where a small, remotely-mounted antenna is needed, we again recommend Linx antennas such as the ANT-916-CW-RCS or ANT-916-CW-RAH.
The ANT-916-CW-RCS is an excellent choice for a small antenna with a 3.3 dBi gain. It is available from Digi-Key or Mouser and note the male RP SMA connector.The ANT-916-CW-RAH is another excellent choice for a small antenna (2.2 dBi) available from Digi-Key or Mouser. The connector shown here is a male RP SMA, but male SMA connectors are also available from Digi-Key and Mouser.
Diagnostic Outputs
The ProMiniAir receiver or transmitter provides diagnostic outputs that are not required for operation but are helpful for troubleshooting or just for fun:
You can monitor the transceiver’s output (in Rx mode) or input (in Tx mode) on the output DIP pins described above.
“I2C” outputs can drive an inexpensive two rows 16 columns I2C LCD.
The 2-pin connector provides Ground and the RF transceiver’s transmitted or received DCC signals. An oscilloscope can monitor these signals.ProMiniAir receiver/transmitter connections to an I2C LCD (older PMA version, but the connections are the same for newer versions)Close-up of ProMiniAir receiver/transmitter connections to an I2C LCD (older PMA version, but the connections are the same for newer versions)
The ProMiniAir software automatically searches for a valid LCD I2C address on boot-up. Please make sure you connect only ONE display to the ProMiniAir.
You can also change the ProMiniAir’s DCC address using the throttle’s “OPS” mode. For the transmitter, you use the DCC throttle that connects to the ProMiniAir transmitter (by default at DCC address 9900 (previously 9000)). For the ProMiniAir receiver, you use the wireless DCC throttle transmitting to the ProMiniAir receiver (by default at DCC address 9901 (formerly 9001)). The EEPROM permanently stores the changed address, but this new address is not operative until you power cycle the ProMiniAir.
Configuration and Testing
We default-configured the ProMiniAir receiver and transmitter to operate on Airwire Channel 0. This default can be changed by setting the DCC address to 9901(Rx)/9900(Tx) (the default, which can be changed as described in the Users Manual) to access the ProMiniAir transmitter and in OPS or Programming-on-the-Main (POM) mode setting CV255 to the desired Channel. Valid channels are 0-17 for North American operation or Channel 18 (869.85MHz) for European operation.
Should the ProMiniAir receiver fail to detect valid DCC packets on its default channel during startup, it will cycle through all Airwire Channels to find a Channel producing valid DCC packets. If this cycling fails to find a valid Channel, the ProMiniAir receiver will change to Channel 0 and wait for a valid RF DCC signal. This channel change is not permanent, and on a restart, ProMiniAir will revert to its default channel.
Several other configuration options are available through “OPS” mode programming, as described in the ProMiniAir Users Manual.
We strongly urge the user to test the ProMiniAir before the final deployment. At the least, an inexpensive I2C LCD can be purchased here or here (and numerous other locations) to gain insight into the ProMiniAir’s state. This display is particularly beneficial when using the ProMiniAir as a transmitter.
Examples of Testing (Advanced)
This section is only for the advanced or adventurous. In the examples below, the Yellow waveform is the signal from/to the RF transceiver for Rx/Tx, respectively. The blue waveform is one Channel of the resulting DCC (Rx) sent to the decoder, or DCC received from the throttle via wireless transmission (Tx).
Receiver Testing
The photo below shows the ProMiniAir operating as a receiver. Of course, an RF transmitter wirelessly sends DCC packets. This transmitter may be a dedicated wireless DCC throttle, such as the Airwire Tx5000. Or, it may be a transmitter that converts standard “track DCC” to wireless DCC, such as the Tam Valley Depot DRS1 transmitter or the ProMiniAir used as a transmitter (as discussed in the next section)!
On the LCD, “My Ad: #” is the DCC address of the ProMiniAir itself. The “(L)” means “long” address. Displayed on the second line is the Channel number and whether DCC “filtering” is “off” (Filter: 0, as shown) or “on” (Filter: 1).
Example of output from a ProMiniAir receiver. The yellow signal on the oscilloscope is from the T/R DCC output pin on the ProMiniAir receiver (the green PCB on the left with the red RF transceiver PCB mounted on the left end). The blue trace is the DCC signal produced by the DCC amplifier (the PCB on the right with the blue power/DCC out terminal) from inputs from the ProMiniAir.
The photo below shows the oscilloscope waveforms with no valid RF DCC signal. With filtering off (Filter: 0), the DCC sent to the decoder reproduces the random pulses generated by the receiver.
The ProMiniAir receiver’s outputs when receiving no valid RF DCC. The yellow signal is the RF receiver’s DCC, and the blue signal is one of the DCC outputs from the DCC amplifier that provides input to the onboard DCC decoder.
These two photos show the ProMiniAir’s transceiver and DCC amplifier output when valid RF DCC is received and no valid RF DCC is received. DCC filtering is off, so the PMA outputs DCC Idle messages. The Tam Valley Depot and Gwire receivers simply reproduce the random pulses received by the transceiver.
Valid RF DCC received. The decoder DCC mirrors (blue) the receiver’s DCC (yellow).No valid RF DCC. When DCC filtering is off, the PMA injects DCC IDLE messages (Filter: 0).No valid RF DCC. The random pulses produced by the RF receiver are reproduced by the output DCC, and this is what Gwire and Tam Valley Depot receivers produce.
The user can reconfigure the ProMiniAir receiver using the throttle’s “OPS” mode. Setting the wireless throttle DCC address to 9901 now shows that the Msg address (“Msg Ad: #”) matches the ProMiniAir receiver’s address (“My Add: #”).
Set DCC filtering “on” by selecting the ProMiniAir’s address (9901 in this case). Note that the MSG address now matches ProMiniAir’s address.
Change CV246 to “1” in OPS mode, which will turn “on” the ProMiniAir receiver’s DCC filtering.
In “OPS mode,” setting CV246 to “1.” The display will indicate that you changed CV246.
The display now shows that DCC filtering is “on.”
In “OPS mode,” setting CV246 to “1.” The display will indicate that you changed CV246.
Exiting OPS mode and changing the throttle to the locomotive’s address now shows an updated “Msg Ad: #” with DCC filtering “on.”
Then change the address back to the locomotive’s address. The display now shows DCC filtering is “on.”
Below is the transceiver’s and DCC amplifier’s DCC output when transmitting valid RF DCC.
Again, the receiver and decoder DCC when a valid RF DCC signal is received.
If we turn off the wireless transmitter/throttle sending RF DCC, now the transceiver outputs random pulses (yellow). Since filtering is “on,” the ProMiniAir receiver firmware detects “bad” waveforms that do not appear to represent a valid DCC packet. The ProMiniAir receiver then outputs a constant-level signal that causes the DCC amplifier to output a high level on DCC A (blue) and zero Volts on DCC B (not shown). This behavior is similar the that of the Airwire receivers. However, the detection mechanism for Airwire receivers is simply the lack of a sufficient frequency of DCC “IDLE” packets, not an analysis of the transceiver’s pulse train.
The waveforms when no valid RF DCC signal is received. With filtering on (Filter: 1), DCC A sent to the decoder is positive, and DCC B is zero, assuming that you set CV248 to “1”. If you set CV248 to zero, DCC A is zero, and DCC B is positive.
Repeating the process of changing the wireless throttle’s DCC address to 9901, going into “OPS” mode, changing CV246 to “0”, exiting “OPS” mode, and switching back to the locomotive’s DCC address will now set DCC filtering to “off.”
You can repeat selecting the ProMiniAir’s address and, in OPS mode, set CV246=0 to turn the filtering back off and then set the address back to the locomotive’s.Changing the address back to the locomotive’s address indicates that the DCC filtering is off (Filter: 0).
So, when we turn off the wireless DCC throttle/transmitter, the DCC amplifier’s output (blue) again displays the DCC IDLE messages output by the ProMiniAir receiver.
When no valid RF DCC is received, the ProMiniAir receiver injects DCC IDLE messages amplified by the DCC amplifier and sent to the decoder.
Transmitter Testing
We now focus on testing when using the ProMiniAir as a transmitter.
With the same ProMiniAir, the Pro Mini was re-flashed with the transmitter firmware. The “DCC Converter” PCB (the PCB on the right) converts any throttle’s DCC to Ground, +5V power, and 5V logic DCC for input to the ProMiniAir transmitter (the PCB on the left).
The display will alternate between showing the ProMiniAir transmitter’s DCC address (“My Ad: #”) and the transmitted DCC packet’s DCC address (“Msg Ad: #”). The transmitting Channel (“Ch: #”) and Power Level (“PL: #”) display on the second line.
Note the ProMiniAir transmitter’s ID.The LCD alternately displays the throttle’s address and the ProMiniAir’s address and shows the Channel number and Power Level.
Below is an oscilloscope trace of the input DCC from the throttle (blue) and the DCC transmitted by the RF transceiver on the ProMiniAir transmitter. Since the wireless DCC must keep the Airwire RF receiver “happy” with numerous DCC “IDLE” packets, the ProMiniAir transmitter evaluates the incoming DCC from the throttle. When the throttle outputs frequent, redundant DCC packets, the ProMIni Air transmitter occasionally inserts DCC “IDLE” packets instead of one of the redundant packets. So, the input DCC and the transmitted DCC will not precisely match. Since DCC throttles send many redundant DCC packets, the locomotive will receive sufficient DCC packets to operate correctly.
The DCC sent out (yellow) will not precisely match the throttle DCC because of slight timing delays and the occasional insertion of DCC “IDLE” messages that are required to keep Airwire receivers “happy.”A shorter time scale than the previous photo
You can reconfigure the ProMiniAir transmitter by setting the throttle’s DCC address to 9900 (which can be changed) and then going into the “OPS” mode to set configuration variables (CV) to new values.
Setting the throttle’s address to 9900 allows the throttle to reconfigure the ProMiniAir in OPS mode.
Once we have changed the throttle’s DCC address to 9900, note that the message address (“Msg Ad: #”) now matches the ProMiniAir’s address (“My Ad: #”).
The display now indicates that the message address matches ProMiniAir’s address.
For example, changing CV246 to “6” while in OPS mode will reset the ProMiniAir transmitter’s Power Level to 6, as indicated by the below display.
In OPS mode, setting CV254 to 0-10 changes the output power level, as indicated here.
After exiting the “OPS” mode, we see that the display reflects the new Power Level (“PL: #”).
The Power Level is now 6.Note that Msg and My Address are the same.
Changing the throttle’s DCC address back to the locomotive’s address will sometimes show “Msg Ad: 255(S)”, which means that the ProMiniAir transmitter sent out a DCC “IDLE” packet to make the Airwire receiver “happy.”
Changing the throttle’s address back to the locomotive’s allows the ProMiniAir to insert occasional DCC “Idle” messages, indicated by a message address of 255. The IDLE message keeps Airwire receivers “happy.”
A display refresh (every 4 seconds) will most likely display the locomotive’s DCC address, 1654. The “(L)” means “long” address.
The display will alternately show the locomotive address and the ProMiniAir’s address.
Conclusion and Further Information
The ProMiniAir is an inexpensive and hopefully fun introduction to wireless DCC control of your model railroad locomotive!
Please get in touch with the author on this site to purchase the ProMiniAir receiver or transmitter. The ProMiniAir transmitter or receiver (with their additional DCC Converter or DCC amplifier and wiring harness) is only $39.99 + shipping. You can also purchase my offerings on eBay by searching for “ProMiniAir.”
Numerous wireless RF transmitter/receiver (Tx/Rx) options for locomotive control are available in the US and abroad. My discussion is confined to wireless RF transmitter/receiver options that are DCC compatible, which means that the transmitter sends “logic-level” DCC packets. The receiver converts the “logic-level” DCC packets back to “bipolar” DCC packets, as would be transmitted on tracks, that an onboard DCC decoder can “understand.”
Schematic of representative application
Why am I limiting my discussion? Because DCC is a standard, and if you don’t go with solutions that have standards behind them, then you are likely to suffer “vendor lock,” where a single vendor holds you “hostage” with “their” solution. Perhaps that attitude is a bit overblown, but vendors with proprietary solutions tend to lag in innovation for lack of competition, and what happens if the vendor goes out of business?
I know that the NMRA DCC standards have some problems, including the following issues: pending issues under consideration for years; vendors ignoring some parts of the standards; some vagueness in places; and lack of standards for wireless. The DCC standard is imperfect but far better than no standard. Plus, the DCC decoder market is competitive and feature-rich – you can almost assuredly find a DCC decoder that will satisfy your needs.
As a further limitation of this post, I will mostly confine my discussion on DCC-compatible wireless Tx/Rx options to the 902-928 MHz ISM (Industrial, Scientific, and Medical) band because this is where I have direct experience. There is significant and exciting activity in the DCC-compatible 2.4 GHz ISM band (using Bluetooth technology) as well (see BlueRailDCC), but I have no personal experience with this band. Another advantage of the 902-928 MHz ISM band is some interoperability between transmitters and receivers, although there is currently no firm standard behind this interoperability.
DCC-compatible Tx/Rx options are a vast topic that I cannot fully cover in this blog. These options are well-covered in the following links:
Dead Rail Society: This should always be your first stop when looking at topics related to dead-rail. This site is the epicenter of dead-rail. In particular, this page discusses vendors for dead-rail Tx/Rx.
Facebook Dead Rail page: This social media page is a valuable source for the latest announcements and discussions for dead-rail, including Tx/Rx options.
Receivers
Below is my personal experience with 902-928 MHz ISM DCC-compatible receivers.
General Comments
How each of these DCC-compatible wireless receivers handles the loss of valid RF signal from the transmitter is discussed here.
CVP Airwire
A CVP Airwire CONVRTR-60X wireless DCC-compatible RF receiver is mounted to the tender hull’s side using Velcro. The U.FL antenna cable was later connected. The DCC “A/B” output of the CONVTR-60X connects to the “Track Right/Left” inputs of a wiring harness for a LokSound L V4.0 DCC decoder (not yet inserted) on the opposite side of the tender hull.
The company CVP manufactures and supports its Airwire series of products, including hand-held wireless DCC-compliant throttles (such as the T5000 and T1300) and receivers, such as the CONVRTR series that seamlessly connect to DCC decoders onboard the locomotive. As a general comment, CVP provides excellent, detailed installation and operation documentation, partly because they are dominant in some segments of wireless model railroad control. The CONVRTR receiver has some sophisticated features, such as setting its Airwire RF channel purely in software, described in its User Guide.
However, the CONVRTR interacts with the Airwire wireless throttles in ways that make it difficult to impossible to transmit just “garden variety” DCC wirelessly to the CONVRTR for proper operation. The Airwire throttles transmit numerous DCC “Idle” packets as a “keep-alive” message for the CONVRTR. A red LED on the CONVRTR board indicates received signal quality and flickers least when receiving many DCC Idle packets, and the brightness of the LED indicates the received RF power. Typical DCC throttles are not designed with these “keep-alive” concerns in mind and do not output DCC Idle packets often enough to keep the CONVRTR “happy.”
Other than the CVP Airwire transmitters (the T5000 and T1300), the only currently available (the no longer manufactured NCE GWire Cab was also Airwire-compatible) RF transmitter that I am aware of that is capable of communicating with the Airwire CONVRTR is the ProMiniAir, whose open-source software (at GitHub AirMiniTransmitter) intercepts “garden variety” DCC from the throttle and interleaves a sufficient number of DCC Idle packets to communicate correctly with the CONVRTR. This “keep-alive” requirement for the Airwire CONVRTR is challenging to produce, so sometimes a reset of the DCC throttle or the ProMiniAir is required to initially send enough DCC Idle packets to initiate communication with the CONVRTR.
Like the Gwire receiver below, the Airwire CONVRTR “X” versions have a U.FL connector for connecting a shielded antenna cable from the receiver to an externally-mounted antenna. An internal antenna option is also available for CONVRTR mountings that are not surrounded by metal.
QSI Solutions Gwire
Gwire U.FL connector. If using the U.FL connector, detach the wire antenna.
The Gwire receiver operates on Airwire RF channels 0-7 that the user must select from a dial on the device itself. A nice feature of this receiver is an onboard U.FL connector (see the Figure above) that allows the user to connect a shielded antenna cable from the receiver to an externally-mounted antenna – useful when the antenna needs to be on the exterior of a metal locomotive or tender shell. See Blueridge Engineering’s website for details on how to interface the Gwire to any onboard DCC decoder. The Gwire presents no difficulties for wireless 902-914 MHz ISM band DCC-compatible transmitters, and you can find it on eBay at relatively low ($20 US or less) prices.
Tam Valley Depot DRS1, MkIII
Tam Valley Depot DRS1, MKIII in an open-cavity install. Note the built-in long-wire antenna.
The Tam Valley Depot DRS1, MkIII receiver operates only on Airwire RF channel 16 (actually 916.49 MHz, which is close enough to Airwire channel 16 at 916.37 MHz) and makes a suitable wireless DCC receiver. This receiver has a long, single-wire antenna that provides efficient RF reception (see the Figure above). However, placing this wire outside any metal shell would be best, which may be inconvenient in some mounting applications. The DRS1, MkIII, presents no difficulties for the 902-914 MHz ISM DCC-compatible transmitters as long as they transmit near 916.49 MHz. The DRS1, MkIV described in the next section supersedes this receiver.
Tam Valley Depot DRS1, MkIV
The recently-released Tam Valley Depot DRS1, MkIV receiver. Note the internal antenna on the right side of the board.
The Tam Valley Depot DRS1, MkIV receiver is a significant upgrade from the DRS1, MkIII, and operates at the original Tam Valley 916.49 MHz frequency, Airwire Channels 0-16, and at 869.85 MHz (for European operation). The DRS1, MkIV presents no difficulties for the 902-928 MHz ISM DCC-compatible transmitters. It is an interesting choice because it changes channels automatically until it finds a sufficient RF signal carrying DCC packets. See the Figure above for the version that employs an internal antenna that is useful when the receiver is not mounted inside a metal shell.
The DRS1, MkIV with a U.FL antenna connector (and a heatsink update) is now available (see picture below), making it very useful for connecting external antennas outside metal shells. This version of the DRS1 makes it highly competitive in capability and quality with the Airwire CONVTR. Perhaps a future version will provide DC output to the onboard DCC decoder when no valid RF signals carrying DCC packets are available, making it possible to program the DCC decoder’s behavior when no DCC signal is available.
Tam Valley Depot DRS1, MkIV receiver with U.FL connector.
OScaleDeadRail ProMiniAir Receiver
ProMiniAir receiver/transmitter
The inexpensive ProMiniAir receiver presents no issues when used with 902-928 MHz ISM DCC-compatible transmitters. It operates on Airwire RF channels 0–16. It requires a separate, low-cost amplifier (e.g., the Cytron MD13S) to convert the ProMiniAir’s unipolar 5V DCC to bipolar DCC that provides sufficient power to the decoder.
The ProMiniAir’s open-source software is available for download at the GitHub site AirMiniTransmitter.
Transmitters
So far as I’m aware, there are four 902-928 MHz ISM DCC-compatible transmitters: the CVP Airwire T5000 and T1300, the Tam Valley Depot DRS1 transmitter, and the OScaleDeadRail ProMiniAir transmitter.
CVP Airwire Transmitters
The CVP Airwire T5000 and T1300 transmitters are excellent choices for operating with 902-928 MHz ISM DCC-compatible receivers, which will properly communicate with these two transmitters. When I am testing wireless receivers, the T5000 is my “go-to” because, in addition to serving as a DCC-compatible throttle, it can program onboard DCC decoders, via the wireless receiver, in either “OPS” (or Programing-on-the-Main, PoM) or “Service” mode. While the T1300 cannot program the onboard DCC decoders, it serves as a typical DCC throttle.
Of course, the Airwire transmitters send sufficient DCC “Idle” packets to keep the Airwire CONVRTR receivers “happy.”
Tam Valley Depot DRS1 Transmitter
The Tam Valley Depot DRS1 transmitter uses DCC packets produced by any DCC throttle or command station that outputs “bipolar” DCC to tracks. The DRS1 transmitter converts the “bipolar” DCC to “logic-level” DCC and transmits it at only 916.49 MHz, which is close enough to Airwire Channel 16 at 916.36 MHz to be received. This frequency limitation means that only the Tam Valley Depot DRS1, MkIII, and MKIV, and the OScaleDeadRail ProMiniAir receivers can operate with this transmitter if they are receiving on 916.48 MHz or Airwire Channel 16.
While the Airwire CONVRTR can operate on Airwire Channel 16, the DRS1 transmitter is not designed to transmit sufficient “Idle” DCC packets to keep the CONVRTR “happy” since it passively sends along only the DCC packets it receives from the DCC throttle or command station.
OScaleDeadRail ProMiniAir Transmitter
The ProMiniAir transmitter with optional LCD. The antenna in the picture was replaced with a high-quality 1/2-wave Linx ANT-916-OC-LG-SMA antenna from either Mouser or Digi-Key for improved transmission.
OScaleDeadRail provides the ProMiniAir transmitter/receiver that uses open-source software at the Github AirMiniTransmitter site. Like the DRS1 transmitter, it is designed to take inputs from any DCC throttle or command station’s “bipolar” DCC output to tracks (via a simple, low-cost optocoupler provided by OScaleDeadRail) and transmit the “logic-level” DCC on Airwire channels 0-16.
The ProMiniAir transmitter inserts a sufficient number of DCC “Idle” packets into the original throttle-produced DCC to keep the Airwire CONVRTR “happy.” This keep-alive capability, coupled with the transmission on Airwire channels 0-16, ensures that the ProMiniAir transmitter can communicate with any of the 902-928 MHz ISM DCC-compatible receivers discussed in this blog.
This transmitter’s settings, like channel number and output power, can be controlled by the DCC throttle or command station in the “OPS” mode by setting the throttle address to that of the ProMiniAir, which is 9000 by default. An optional LDC display can be attached to the ProMiniAir transmitter for status display. More configuration information is available at the GitHub AirMiniTransmitter site.
This post describes my most difficult dead-rail conversion to date: an MTH O scale 2-8-8-8-2 Virginian Triplex (MTH product number 20-3101-1) that I purchased on eBay circa September 2019. Previously, I converted a Sunset 3rd Rail Allegheny with an MTH Proto-Sound 3.0 board to dead-rail. Still, the Triplex was my first complete dead-rail conversion of an MTH locomotive to 2-rail operation, which included lathe turning high-profile wheels to approximate an NMRA RP-25 flange profile (also see NMRA standard S-4.2) so that the locomotive would operate reliably on track meeting NMRA standard S-3.2.
Box detailsLocomotive side viewTender side view
The inside view of the tender below demonstrates a significant challenge: space is very tight with the Proto-Sound board and the prominent speaker consuming a large part of the tender’s internal volume where we need to install additional dead-rail components: DCC-compatible RF receiver; 14.8V LiPo battery; and switch, charging, and antenna wiring.
Inside view of the tender: Note the battery cradle, the space-consuming speaker, and the end-of-tender smoke unit.
The strategy starts to emerge:
Replace the Proto-Sound 2.0 (PS2.0) board with a PS3.0 board that can operate in DCC mode.
Remove the original rechargeable battery and its cradle and locate the 14.8V LiPo battery pack there.
Remove the large speaker and replace it with a smaller 4-ohm speaker so we can make room for the 14.8V LiPo battery pack and the Airwire CONVRTR-60X DCC-compatible RF receiver that operates in the 902-928 MHz ISM band on Airwire channels 0-16.
Lathe down the high-rail wheel flanges to approximate an NMRA “RP-25” profile for 2-rail, dead-rail operation.
An advantage of this strategy is retaining almost all of the control the PS2.0/PS3.0 provides, including the directional head/tail lamp, marker lights, cabin lights, flickering firebox, sound, and fan-driven smoke units.
Proto-Sound 3.0 Conversion
The first step of the dead-rail conversion was easy: replacing the Proto-Sound 2.0 board with a Proto-Sound 3.0 (PS3.0) board from Ray’s Electric Trainworks. As I have mentioned in other posts, working with Ray Manley is a great pleasure. I sent my PS2.0 board as a trade-in to Ray, and he took care of the rest, providing me with a fully-functional PS3.0 board, complete with DCC capability.
The heatsink for the PS3.0 board necessitated drilling and tapping a new mount hole with a spacer, as shown in the figure below.
New Proto-Sound 3.0 heatsink mount with a spacer.
The following photos show the original electrical power inputs to the PS2.0 board and their modified connections for the replacement PS3.0 board.
Original power connections to the Proto-Sound 2.0 board.
As you can see below, the AC power from the center rail pick-up (hot) and the outside rails (ground) were disconnected – we will be getting our power from a 14.8V LiPo battery pack in the tender. In this case, there is no Constant Voltage Unit, so no Battery +(14.8V)/Battery -(Ground) connections are required.
Locomotive power connection modifications. There is no Constant Voltage Unit in this locomotive, so the B+/B- wires indicated above are NOT used.
The AC power connections in the tender are also disconnected, and the power inputs to the PS3.0 connect to the switched battery power. The Battery +(14.8V)/Battery -(Ground) connection on the wiring harness was NOT required.
Tender power connection modifications. There is no need for B+/B- power supply to the locomotive.Power input modifications for the PS3.0 board. The DCC A/B input power comes from the connection to the Airwire CONVRTR-60X DCC-compatible RF receiver.Final wiring connections
Locomotive Electrical Modifications
There were two aspects to the electrical modifications in the locomotive:
Headlamp replacement
Electrical power supply
The original headlamp was a power-hungry incandescent bulb. An LED with a polarity-independent plug from Evan Designs was used to eliminate the need to determine the polarity of the original headlamp wiring.
Headlamp LED replacement details
The power-related modifications consisted of removing the center-rail pick-ups, which is very easy on MTH locomotives and disconnecting any wiring to the center-rail pick-up (hot) and the outside rails (ground).
The original center-rail pick-ups. The “Center-Rail Pick-up” is disconnected.The center-rail connection underneath the motor is disconnected
Battery Installation
Battery installation was very challenging since the only practical placement location was the original rechargeable battery and its cradle mounting beside the PS2.0 board. A special-order 2x2x1 14.8V, 2600mAh (38.48 Wh, 5A rate, LxWxT: 133 mm x 40 mm x 25 mm) LiPo battery purchased from Tenergy.com provides the one cell-diameter thickness required to fit the battery pack between the PS3.0 board and the tender hull.
Original rechargeable battery and cradle location. The speaker volume control potentiometer was moved to accommodate the more extended 14.8 V LiPo replacement battery pack.Volume control potentiometer. It was moved from its original location, and UV glue provides stress relief to prevent breakage of the very fine wires.Final location of the volume control potentiometer. UV glue holds the potentiomenter in place.Final battery location. Velcro attaches the battery to the side of the PS3.0 board.
Mechanical Modifications
In MTH steam locomotives, the wheel axles insert into a solid cast chassis frame, so the driver wheels must be pulled off the axle before machining the high-profile wheels to approximate an RP-25 profile that is compatible with two-rail, dead-rail operation.
The driver wheels must be pulled off the axle for machining after the removal of the side rods. The axle and wheel are scribed to maintain proper “quartering.”A wheel puller separates the driver wheel from the axle.Comparison of lathe-cut and unmodified high-profile wheels. The high-profile flanges were lathe-cut to approximate an RP-25 profile for 2-rail, dead-rail operation.
Tender Mechanical Modifications
The tender’s mechanical modifications involve adding a Kadee 740 coupler and accommodating additional dead-rail electronics.
Coupler Modifications
The original coupler pivot, rather than using a frame-fixed mounting, was used to mount a Kadee 740 coupler. This strategy ensured that tight curves would not bind the coupler.
The original coupler assembly
A Kadee 740 coupler was mounted on the original coupler pivot, as shown in the Figure below. The brass screw heads were ground down to provide clearance with the tender frame.
The added dead-rail electronics include the charging plug, the ON/OFF/Charging plug, a smaller speaker, and the antenna mount.
Charging plug (left) and ON/OFF/Charge switch (right) mountingAntenna mounting location. The antenna is a Linx ANT-916-CW-RCS discussed in this blog.
The original speaker was far too large to provide clearance for the additional battery, DCC-compatible RF receiver, and other electrical components needed for the dead-rail installation. So a 16mm x 35mm speaker was placed in the bottom of the original speaker’s cavity, and UV glue holds the speaker in place.
The original speaker. It is far too large to accommodate the additional dead-rail components: battery, DCC-compatible RF receiver, ON/OFF/Charging switch and charging plug.Candidate speaker. The actual 16mm x 35mm speaker was even smaller and mounted into the bottom of the original speaker cavity.
Final Demonstration
With the locomotive reassembled, it’s time to test it out! If your locomotive has a smoke unit(s), always ensure sufficient smoke fluid is loaded. Even if you don’t intentionally turn on the smoke unit – sometimes it’s unexpectedly activated.
Note: This post deals with details of various brands of DCC-compatible, wireless RF receivers operating in the 902-928 MHz “ISM” band that connect to onboard DCC decoders. Some aspects of the discussion may apply to other RF bands as well.
Typical application. In some cases, such as the Airwire transmitters, the throttle and transmitter are combined. Also, the receiver and amplifier may combined, such as for Airwire and Tam Valley Depot receivers.
The designers of various DCC-compatible RF receivers have a couple of strategies for what output to provide to the onboard DCC decoders when a valid RF signal is lost:
Output the random pulses that the RF receiver naturally outputs when a valid RF signal is lost. This option will cause most DCC decoders to maintain direction and speed while the DCC decoder “sifts” the random pulses searching for valid DCC packets.
Output a fixed, positive Direct Current (DC) voltage to one of the DCC decoder’s “Track” inputs and a zero voltage DC the other “Track” input when either a) RF signal is lost, or b) when the RF transmitter does not send sufficiently-frequent “keep-alive” DCC packets. The latter is true for the Airwire CONVRTR. How the DCC decoder responds to these DC “Track” inputs depends upon DCC decoder configuration and, unfortunately, DCC decoder manufacturer discretion.
There are several NMRA-specified Configuration Variables (CV’s) that affect how DCC decoders handle the loss of valid DCC packets and are important to understand when the DCC decoder is connected to the DCC output of DCC-compatible RF transmitters because the RF receivers may lose or receive corrupted RF signal from the dead-rail RF transmitter.
The NMRA standard S-9.2.4, section C “Occurrence of Error Conditions” states “Multi Function Digital Decoder shall have a Packet Update time-out value.” Further down on line 60 the standard states “A value of 0 disables the time-out (i.e., the user has chosen not to have a time-out)”. This part of the NMRA standard is not universally-implemented by manufacturers, and it affects how decoders will respond to the loss of RF transmission of DCC packets. To implement this requirement, the NMRA standard S-9.2.2 has defined the recommended (R), but notmandatory (M), CV11, Packet Time-Out Value. A value of CV11=0 is defined to turn off the time-out, but CV11 is frequently not implemented.
However, another CV that is often implemented addresses some aspects of loss of DCC. The optional (O) CV27, Decoder Automatic Stopping Configuration, is under re-evaluation by NMRA, but the NMRA has taken no definite action some time. Here is what the NMRA standard S-9.2.2 currently (as of 2019) states about CV27:
Configuration Variable 27 Decoder Automatic Stopping Configuration Used to configure which actions will cause the decoder to automatically stop.
Bit 0 = Enable/Disable Auto Stop in the presence of an asymmetrical DCC signal which is more positive on the right rail. “0” = Disabled “1” = Enabled
Bit 1 = Enable/Disable Auto Stop in the presence of an asymmetrical DCC signal which is more positive on the left rail. “0” = Disabled “1” = Enabled
Bit 2 = Enable/Disable Auto Stop in the presence of an Signal Controlled Influence cutout signal. “0” = Disabled “1” = Enabled
Bit 3 = Reserved for Future Use.
Bit 4 = Enable/Disable Auto Stop in the presence of reverse polarity DC. “0” = Disabled “1” = Enabled
Bit 5 = Enable/Disable Auto Stop in the presence forward polarity DC. “0” = Disabled “1” = Enabled
Bits 6-7 = Reserved for future use.
Since DCC decoder manufacturers frequently do implement CV27, what electrical output the DCC-compatible RF receiver provides to the DCC decoder upon loss of a valid RF signal will influence how the DCC decoder responds. We will break this down for various brands of DCC-compatible RF receivers in the 902-928 MHz ISM band in the following subsections.
Note that some DCC decoders will not honor CV27=0; i.e., all auto-stopping features disabled. For example, with CV27 set to 0, the Zimo MX-696, and probably other Zimo DCC decoders as well, will continue speed and forward direction if positive DC level is input to the “Right Track” DCC input, and a zero DC level is input to the “Left Track” DCC input. Under these “track voltage” conditions, the locomotive will stop if originally moving backward. Some (but not all)DCC-compatible RF receivers, such as the Airwire CONVRTR, provide these DC inputs, if a valid RF signal is lost, but only if connected correctly.
The “correct” connection relates to how the user connects the DCC output from the RF receiver to the “Track Right” and “Track Left” inputs of the DCC decoder. Under normal circumstances, when there is a valid RF signal, which way the DCC decoder connects to the RF receiver does not matter. Under the exceptional case of DC-only output by the RF receiver, if it loses a valid RF signal, which way the DCC decoder connects to the RF transmitter does matter. The user will likely want the locomotive to continue forward with the loss of a valid RF signal, so some experimentation is required to determine which of the RF transmitter DCC outputs should connect to which of the DCC decoder’s “Track” inputs to achieve the desired behavior.
Example DCC waveform output from a DCC-compatible RF receiver when there is a valid RF signalExample random pulse output from a DCC-compatible RF receiver when there is no valid RF signal. Note the waveform’s superficial similarity to valid DCC output.
As a further complication, the user should probably turn off the decoder’s “analog” mode of operation by setting Bit 2 of CV29 to 0 to force the decoder to use “NMRA Digital Only” control of ”Power Source Conversion” (see the NMRA standard here). If Bit 2 of CV29 is set to 1, and again we emphasize the user should probably not activate this feature, then “Power Source Conversion Enabled” and then CV12 determines the power source; the most common of which is CV12=1, “Analog Power Conversion.”
Airwire CONVRTR Series
CVP Airwire CONVRTR-60X tender installation. The CONVRTR operates on Airwire channels 0-16. Note that the U.FL antenna lead was later connected to the CONVRTR. The LokSound L V4.0 DCC decoder mounting harness can be seen mounted on the tender wall opposite the CONVRTR, and its Track Left/Right inputs are connected to the CONVRTR-60X’s DCC A/B outputs.
When the CVPAirwire CONVRTR loses a valid RF signal or receives insufficiently-frequent DCC Idle packets, it detects these conditions and outputs a fixed DC voltage to the decoder. Consequently, the user should set CV27 according to the description above.
While it may seem that the user would want the locomotive to stop if its RF receiver loses a valid RF signal, consider what might happen in tunnels or locations remote to the DCC RF transmitter. Getting stuck under these circumstances if a valid RF signal is lost is probably not what the user wants, so we strongly suggest that the user set CV27=0.
The user is cautioned, however, that some DCC decoders, such as the new ESU LokSound 5 L DCC, do not honor the CV27=0 setting unless the “polarity” of the “Track Right/Left” is connected “correctly” to the CONVRTR’s “A/B” output. Experimentation may be required to determine the correct connection, but my experience is: CONVRTR A <–> Decoder Track Right & CONVRTR B <–> Decoder Track Left
QSI Solutions Gwire and Tam Valley Depot DRS1 Series
The QSI Solutions GWire operates on Airwire Channels 0-7. If the U.FL plug (at the upper-left corner of the Linx Transceiver chip) connects to an externally-mounted antenna, the antenna wire at the upper-left corner of the GWire board should be cut off at board level, or better yet, unsoldered.The Tam Valley Depot DRS1, MKIII, operates on Airwire Channel 16The Tam Valley Depot DRS1, MkIV, operates on Airwire Channels 0-16 (as well as other frequencies). Note the internal antenna on the right-hand side of the board.
The QSI SolutionsGwire and Tam Valley DepotDRS1, MkIII and MkIV DCC-compatible RF receivers will output random pulses to the onboard DCC decoder when a valid RF signal is lost, so setting CV27 is probably of no use. On the “plus” side, most DCC decoders will maintain locomotive direction and speed in the presence of these random pulses since the DCC decoder is actively sorting through these pulses for valid DCC packets, which is usually the behavior the user wants.
A Blueridge Engineering webpage describes how to easily modify the GWire for use as an RF receiver for any onboard DCC decoder.
OScaleDeadRail ProMiniAir Receiver
OScaleDeadRail ProMiniAir receiver operates on Airwire channels 0-16. The ProMiniAir can also be configured to operate as a DCC-compatible transmitter that wirelessly transmits DCC from any DCC source on Airwire channels 0-16.
The OScaleDeadRail ProMiniAir receiver has a default long address of 9001. Like the ProMiniAir transmitter, the ProMiniAir receiver’s channel can be reset in “OPS Mode” by setting CV255 to a value in the range of 0–16. The ProMiniAir receiver has the following options when a valid RF signal is lost:
Output random pulses to the onboard DCC decoder: The user can set the ProMiniAir receiver to output the random pulses when it loses a valid RF signal by setting CV246 to 0 in “OPS mode” at the ProMiniAir’s address. In this case, setting CV27 for the onboard DCC decoder is irrelevant because the random pulses from the ProMiniAir receiver will cause the onboard DCC decoder to maintain the speed and direction of the locomotive while it is “sifting” through the random pulses for valid DCC packets.
Output either fixed positive or negative voltage DC to the onboard DCC decoder: In this case, setting CV27 for the onboard DCC decoder at its address is relevant. The user can set the ProMiniAir receiver to output fixed DC voltage when it loses a valid RF signal by setting CV246 to 1 in “OPS mode” at the ProMiniAir’s address. A positive DC voltage is output by setting the ProMiniAir receiver’s CV248 to 1 in “OPS mode” at the ProMiniAir’s address, or a negative DC voltage is output by setting CV248 to 0. If the user does not want the locomotive to stop with the loss of a valid RF signal, then set CV27=0 for the onboard DCC decoder at its address. Of course, setting CV27 to other values (see above) in the DCC decoder will determine how the DCC decoder responds to the fixed DC voltage that the ProMiniAir outputs to the onboard DCC decoder upon loss of a valid RF signal.
Wrap-Up
It’s an unfortunate fact of life that we can lose a valid RF signal from our DCC-compatible transmitter. However, with a little study of DCC decoder documentation, and possibly a bit of experimentation, gracefully coping is definitely possible.
This is a slight modification of a post titled: O Gauge Forum Post on Smoke Units.
It is possible to modify a non-ESU smoke unit to connect to the LokSound L or XL decoders just as an ESU smoke unit does by connecting the smoke unit to the specialized ESU smoke unit terminals: HTR+/-, MOT+/-, and TMP+/-. This capability allows you to take direct advantage of all the LokSound capabilities provided for ESU smoke units. The missing component in some smoke units is a Negative Temperature Coefficient (NTC) thermistor.
What started me down this road was a “dead rail” conversion of a Sunset 3rd Rail Big Boy (3-rail, “Late Version”) originally outfitted with TMCC and a nice Lionel smoke unit with dual output (photo below).
Dual stack, fan-driven smoke unit for retrofit.
I wanted to retain this beauty and use a LokSound L V4.0 decoder that is controlled by an Airwire CONVRTR-60. THOR73’s posts on the O Gauge Form inspired me to work through using this smoke unit with the LokSound L V4.0 decoder. I thought that if I could figure out how the ESU smoke units created their “temperature” inputs to the LokSound decoder, then I could retrofit the Lionel smoke unit so that it would be “input compatible” with an ESU smoke unit. This retrofit turned out to be simple.
I reverse-engineered an ESU 54678 smoke unit by measuring the resistance between the heater resistor leads (HTR+/-): ~23 ohms; motor leads (MOT+/-): ~16 ohms; and thermistor leads (TMP+/-): ~100K ohm at room temperature. Each of these components is electrically isolated from the others. When powered by a 14.8V LiPo battery, the LokSound L V4.0 decoder I had on hand produced the following results on the ESU Profi board using the LokProgrammer (with ground measured at the Profi board’s ground terminal):
Terminal
Smoke off
Smoke on (Throttle=10)
HTR+ (not connected to heater resistor*)
13.4V
13.2V
HTR-
Open
Switched open/ground @500Hz ~30% duty-cycle PWM
Fan+
0V
Pulsed <= 5V (difficult to determine with low Frequency chuffs)
Fan-
0V
0V
TMP+
5.1V
5.1V
TMP-
1.3V
3.7V
* Battery+ (14.8V) connected to heater resistor + input
Cut-away of a 54678 ESU smoke unit showing the thermistor
The difference in TMP- between unheated and heated conditions suggests, but does not prove, that the thermistor’s decrease in resistance with increased temperature is manifested by a voltage increase at TMP- as part of a voltage divider where the thermistor is in series with a fixed resistor resident in the decoder, possibly with a low-side voltage offset:
Probably thermistor circuit
Guesses: RFIXED~1.5K based on probe measurements and derived @ 25C Voffset~1.24
So right off the bat, the ESU smoke unit’s heater resistance (23 ohms) is similar to Lionel’s (27 ohms), and both smoke units use 5V fan motors. The Lionel was missing only the thermistor. Lower resistance smoke units (around 8 ohms) might be problematic to convert unless retrofitted with a heater resistor in the 20-ohm neighborhood or use an externally-supplied, lower HTR+ voltage. The heater and fan motor similarity between the ESU and Lionel smoke units made this particular Lionel smoke unit an excellent surrogate candidate.
Thermistors with 100K ohm resistance at 25 Celsius are commonly-available, usually with a “B” parameter of around 3900 Kelvin. You can Google what this parameter means (simplified Steinhart-Hart Equation: R(T in Kelvin)=R@TRef*(exp(B/T-B/TRef)) ). While I could not verify that the ESU smoke unit used precisely this type of thermistor, the testing described later supports this selection.
The photo below is the Lionel 27 ohm smoke unit PCB, part #610-PCB1-045, Rev C (Lionel replacement part #691PCB1045), that was retrofitted with an “axial,” glass-coated 100K NTC thermistor with a B of 3892 Kelvin. (Well, it’s a Lionel replacement PCB since I cut some traces retrofitting on the original PCB that I regret doing. Interestingly, the original PCB did not have the mangled lettering of the replacement PCB that some have noted.)
The 3-pin power plug on the PCB can power the heater resistor since the outputs from the rectifier/5V converter do not connect to anything after removing the fan motor plug. The ground on the PCB MUST be isolated from the heater unit metal case since the PCB’s “ground” wire will be connected to the LokSound L’s HTR- terminal that regulates the heating resistor’s current path to the electrical ground! Electrical measurements revealed good electrical isolation of the metal case from the heating element.
Lionel fan-driven smoke unit with a 100K thermistor added.
I drilled two holes in the smoke unit’s PCB board, and the thermistor was inserted and soldered to two-wire leads that connect to the LokSound L’s TMP+/- terminals. I used high melting-point solder because conventional solder might melt at the high operating temperatures of the heater resistor and thermistor (max around 250 Celsius according to documentation for the ESU smoke unit).
The two heater wires from the three-pin PCB plug connect to the LokSound L’s HTR+/- terminals. (Pins 1 and 3 are shorted together on the PCB and connect to one side of the heater resistor; pin 2 is ground and connects to the other side of the heater resistor.)
The motor wires directly connect to the MOT+/- terminals. Out of sheer luck, when the red motor lead from the smoke unit is connected to MOT+, and its black lead to MOT-, the fan motor spins in the “correct” direction.
As others have suggested, I replaced the original 27-ohm ceramic resistor with a Lionel 27-ohm replacement #6008141055.
Once you connect the smoke unit’s six outputs to the LokSound L’s ESU smoke unit terminals, some modifications are needed in the ESU sound files and decoder setup since they did not originally activate the ESU smoke unit. First, follow THOR73’s directions regarding the connection between sound and smoke chuffing under the “Smoke unit” menu. The smoke unit’s automatic power-off time should be reset since the default is 0 seconds. I don’t know if 0 means never turning off, but a non-zero setting seemed like a good idea.
LokSound change decoder settings menu
What differs from THOR73’s discussion is the sound file setup for an ESU smoke unit. Editing the sound files reveals that most “nodes” have the option to set the “ESU Smoke Unit” parameters. Frequently these settings are turned off, but there are some useful “presets” you can select and experiment with. An especially interesting preset is the “preheating” preset available in the stopped state.
Mute State:
LokSound Stop state settings.
Here are the other states I modified, but I am no expert or knowledgeable about these settings. Usually, I chose a “Preset” and then selected the “Steam Chuff” checkbox, which preserves the parameters of the preset (unless you change them), but turns off the Preset name.
Stop State:
LokSound Mute state settings
DCX State:
LokSound DCX state settings
Coast State:
After editing these sound nodes, the next step is to set an “F#” to turn the smoke unit on/off on the “Function mappings” menu. The “logical” outputs column provides an “ESU Smoke Unit” selection, so I selected F23 as the ESU Smoke Unit on/off toggle.
TESTING WARNING: The ESU 53900 Profi Decoder Tester does not appear to provide adequate power to an actual ESU Smoke Unit or surrogates described here! In deployed operation, the LokSound L is perfectly capable of delivering sufficient power, but in my experience (or inexperience), the Profi board is not able to do so. I initially thought the culprit was the puny AC to DC converter provided to power the Profi board. But, the power connection to a very hefty 14.8V LiPo battery did not solve the problem. The workaround uses either THOR73’s high-side MOSFET switch mentioned in this thread or the low-side MOSFET switch described in the same topic thread. Either way, you will need to take power (about +14V DC) from the source providing power to the Profi board and use the Profi board’s HTR- output to control the MOSFET switch. In turn, this switch controls the smoke unit’s heater. Using THOR73’s high-side FET switch, you connect the smoke heater as he describes. If you use the low-side FET switch, I presented, the smoke unit’s HTR- output connects to the switch control input, and the switch’s ground connects to the power ground.
Reiterating, YOU ONLY NEED THIS SPECIALIZED MOSFET SWITCH FOR TESTING WITH THE Profi BOARD! In actual operation, the LokSound L adequately powers an ESU smoke unit by direct connection to the decoder’s ESU smoke unit terminals, as is the modified smoke unit described here.
Here’s the “proof in the pudding” video:
Surrogate smoke unit in action
Please forgive the disassembled state. I haven’t finished the dead rail conversion, but this video does demonstrate battery power with the LokSound L V4.0 controlled by an Airwire CONVRTR-60 wireless receiver.
Follow up
To be pretty linear, my guesses on RFIXED and Voffset are 1.5K and 1.24V. The 1.5K came from an “off” measurement of resistance between decoder GROUND and TMP-, which is fraught with potential error.
These values will give you the following approximate curves: the left axis is the voltage at TMP-, and the right axis is the estimated thermistor resistance.
BUT THE VALUES of Rfixed AND Voffset ARE ONLY ENGINEERING JUDGEMENT GUESSES!
Example thermistor T versus V and T versus R curves
Please don’t jump to conclusions; MTH did NOT manufacture the C&O 2-6-6-6 Allegheny with MTH Proto-Sound 2.0 (PS2.0) I found on eBay. Instead, it’s a brass locomotive produced by Sunset 3rd Rail (Figure 1) and converted to PS2.0 (see MTH PS2.0 Upgrade Manual). See Figure 2. The retrofit replaced the original tender QSI-OEM Digital Soundboards, the wiring harnesses for the tender and locomotive, and the Suethe smoke units and their voltage regulator board in the locomotive.
Figure 1: Sunset 3rd Rail C&O Allegheny box info
Figure 2: Original tender Proto-Sound 2.0 electronics and harness wiring
It was a well-done conversion, so I was very reluctant to tear out the tender PS2.0 control board and the wiring harnesses in the tender and locomotive. The PS2.0 conversion used an MTH smoke unit with fan speed and smoke intensity controls.
Figure 3: MTH Smoke Unit with funnel
The CVP Airwire receiver boards I typically use for dead-rail conversion don’t have this level of smoke unit control. And the PS2.0 board used a speed encoder on the locomotive motor’s flywheel to synchronize the PS2.0 board’s sound. See Figure 5 for the speed encoder reader and flywheel strip and 5b for additional electrical connections.
Figure 4: Original Constant Voltage Unit wiring with dead-rail modifications indicated
Figure 5: Speed encoder, flywheel strip, and electrical connections for the Constant Voltage Unit.
Figure 5b: Locomotive harness wiring after modifications connecting the Constant Voltage Source to Battery +/- and replacing incandescent bulbs with LEDs
These built-in features were excellent, but I still wanted a dead-rail conversion.
Hmm… Looking around, I discovered that Proto-Sound 3.0 (PS3.0) had DCC/DCS control options, and the wiring harnesses are the same for PS2.0 and PS3.0 boards (see, for instance, MTH PS3.0 Upgrade Manual). Things are looking up. Then I found a great site, Ray’s Electric Trainworks, that provides PS3.0 replacement boards and great support.
My thought was this: if I can upgrade the locomotive to PS3.0, then I can take the following steps. Jumper the PS3.0 board to DCC operation, disconnect the original rail power/communication wiring, and re-connect the rail power/communication wiring to the DCC outputs of a CVP Airwire CONVRTR-60X receiver (CONVRTR Users Guide). Easy, right? Not so fast.
Ray at Ray’s Electric Trainworks was a great help: he steered me to the right PS3.0 card I needed for the tender and loaded the Allegheny sound file for me. Otherwise, I would need a bunch of DCS infrastructure to load the sound file. And he gave me a rebate for the old PS2.0 card! Great guy.
OK, I have the PS3.0 card from Ray. The PS3.0 card came mounted in its plastic carriage that is screw-mounts on the tender chassis through pre-existing holes. The heatsink orients a bit differently between the PS2.0 and PS3.0 – no big deal – I just needed to drill a hole in the tender chassis in a slightly different place. The PS3.0 doesn’t use a Ni-MH battery, so out it went. That was a good thing, too, since I needed the real estate for the replacement LiPO battery to supply power to the control boards, lights, smoke unit, and locomotive.
Locomotive Modifications
Here is a diagram of the locomotive wiring harness from the MTH PS3.0 Upgrade Manual with my annotations.
Figure 6: Locomotive PS3.0 wiring harness with modifications indicated
Since I wouldn’t send track power to the tender, I cut the Ground Lead and Pickup Roller Leads wires. I re-purposed them by connecting these harness wires to the Constant Voltage Unit, a heat-shrink blob whose input leads were cut from their original chassis connections (see Figure 5 again).
While I was at it, I removed the incandescent cabin and headlight bulbs to reduce power consumption. I replaced them with Yeloglo LEDs (see Yeloglo description), whose + input was in series with the Yeloglo’s 470-ohm resistor for 10-16 Volt operation. Yeloglo LEDs have an excellent yellowish output reminiscent of incandescent light.
Tender Modifications
The diagram below shows the tender’s wiring harness with my modifications.
Figure 7: Tender PS3.0 wiring harness with modifications indicated
Continuity testing revealed that the locomotive Roller Pickup Leads and the Ground Lead connected to Pin 1 and 3 of the 7-pin connector, respectively.
Note that BOTH locomotive and tender Ground Leads (both are black) that are input to the PS3.0 on pins 3 and 4 of the 7-pin connector, respectively, MUST BE DISCONNECTED FROM THE LOCO/TENDER FRAMES AND CONNECTED to the “B” DCC output of the CONVRTR. This pair of connections was the trick. I initially connected ONLY the Ground Lead from the locomotive (pin 3 of the 7-pin connector) to the CONVRTR, which did NOT work! I don’t know if only connecting pin 4 of the 7-pin connector would work – I didn’t try it.
Similarly, I disconnected the red wire that is input to the PS3.0 on pin 1 of the 7-pin connector (which initially connected to the locomotive Roller Pickup Leads) from the plug bundle that connects the tender to the locomotive. I connected it to the “A” output of the CONVRTR.
The “harness side” of the two cut wires originally going to the Pickup Roller Leads (red) and Ground Lead (black) on the locomotive were connected to switched Battery + and Battery – (ground), respectively, to provide power to the locomotive’s Constant Power Unit.
Note: “Switched B+” means battery power coming from the Battery’s + terminal that is turned on or off with a switch (you want to be able to turn off the power!). “Battery – (ground)” means the connection to the Battery’s negative terminal that is usually grounded to the tender chassis by a battery charging plug.
A picture is worth a thousand words, so studying Figures 6 and 7 will tell you what wiring cuts and re-connections are needed to convert a Proto-Sound 3.0 steam locomotive to dead-rail.
Other Dead-Rail Conversion Details
Of course, other aspects of the dead-rail conversion are required. These aspects include adding battery power and CONVRTR connections and removing center-rail pick-ups and electrical connections that are part of a typical 3-rail to dead-rail conversion for an O-scale steam locomotive. These conversion aspects are discussed in another blog.
Wrap-Up
In summary, if you have a locomotive with the PS3.0 installed, conversion to battery-powered DCC operation and radio control (dead-rail) is straightforward once you know the few wiring cuts and re-connections you need to make. The DCC operations for this particular locomotive can be found in the MTH document “Premier 2-6-6-6 Allegheny Steam Engine .” What you preserve with the PS3.0 is good DCC functionality, the original sound, and coordinated smoke – and that’s a pretty nice combination.
