Dealing with Loss of RF Signal in Dead-Rail for Onboard DCC Decoders

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:

  1. 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.
  2. 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 not mandatory (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 signal
Example 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 CVP Airwire 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 16
The 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 Solutions Gwire and Tam Valley Depot DRS1, 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.

Blueridge Engineering ProMini Air Receiver

Blueridge Engineering ProMini Air receiver operates on Airwire channels 0-16. The ProMini Air 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 Blueridge Engineering ProMini Air receiver has a default long address of 9001. Like the ProMini Air transmitter, the ProMini Air receiver’s channel can be reset in “OPS Mode” by setting CV255 to a value in the range of 0–16. The ProMini Air 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 ProMini Air receiver to output the random pulses when it loses a valid RF signal by setting CV246 to 0 in “OPS mode” at the ProMini Air’s address. In this case, setting CV27 for the onboard DCC decoder is not relevant, because the random pulses from the ProMini Air receiver will cause the onboard DCC decoder to maintain 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 ProMini Air receiver to output fixed DC voltage when it loses a valid RF signal by setting CV246 to 1 in “OPS mode” at the ProMini Air’s address. A positive DC voltage is output by setting the ProMini Air receiver’s CV248 to 1 in “OPS mode” at the ProMini Air’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 ProMini Air 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.

Author: Darrell Lamm

I earned my Doctorate in Physics from the Georgia Institute of Technology in 1982, and before retiring in 2019 I worked for 37 years at the Georgia Tech Research Institute. My last position was Chief Scientist of the Electro-Optical Systems Laboratory. Like many people, my love for model railroading began at an early age, and I rekindled that interest starting in 2017.

2 thoughts on “Dealing with Loss of RF Signal in Dead-Rail for Onboard DCC Decoders”

  1. What about CV11, which I have been told can be used to effectively say, “Continue last instruction until a new instruction is received”. Potentially very useful if, for example, there is a tunnel or a very long run away from the controller, via a return loop.

    1. Simon, this is an insightful question. Yes, CV11 is indeed supposed to handle packet time-out intervals, with CV11=0 representing no time limit. The NMRA standard S-9.2.2 states:
      Configuration Variable 11 Packet time-out Value
      Contains the maximum time period that the decoder will maintain its speed without receiving a valid packet.
      See S 9.2.4 Section C for further information.

      And 9.2.4, Section C states:

      – A value of 0 disables the time-out (i.e., the user has chosen not to have a time-out)
      – A value range of 1 through TIMEOUT_MAX sets the time-out to the chosen value. The minimum value of TIMEOUT_MAX will be 20 seconds. It may be longer at the manufacturer’s discretion.”

      However, CV11 is only a “Recommended,” not “Mandatory” CV, and some decoder manufacturers do NOT implement CV11, including Zimo and ESU.

      So, in summary, like you, I had hopes that CV11 would be a reliable mechanism for setting a DCC time-out (or none), but it is not, sadly, a universal solution.

      Thanks for stopping by!

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