Digitrax Transponding

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Summary: Transponding is a mechanism where a decoder can report back to the Digital Command Control system.

What is Transponding?

A transponder is a device that, upon receiving a signal, emits a different signal in response. The term is a portmanteau of transmitter and responder.

Digital Command Control and Transponding

One method of transponding is RailCom. The other approach is Digitrax's Transponding. This article deals with Digitrax's implementation and transponding refers to Digitrax Transponding technology.

Digitrax Transponding

Digitrax's Transponding is proprietary and incompatible with RailCom. It is designed to work with LocoNet. Digitrax command stations are compatible with Transponding.


  • Bi-Directional
  • LocoNet compatible
  • Operations mode read back of multifunction decoder CVs is possible when using decoders which support this feature.
  • FIND Function


  • Detectors allow the real time location of trains anywhere on the layout. This can be used by a computer, control a display panel, or allow some operations with a throttle.
  • Speed and other parameters from a specific vehicle can be transmitted back to the operators
  • Staging yards and hidden trackage can be monitored, allowing for automation with computer software.
  • Automation of train operations if needed
  • Sound effects, such as the SurroundTraxx system from SoundTraxx


Typically, when a multifunction decoder capable of transponding gets a packet, it reports back to the transponder which initiated communication. Transponding provides three potential advantages: Letting the system know which signaling block the locomotive (or car) is in, transmitting data about the vehicle back to the system, and ensuring a locomotive with a new command gets it quickly.

Digitrax offers both multifunction and function decoders which support Transponding.

Transponding is used for locomotive and/or car locations. Useful when using computers to control your layout for whatever reason – automated layouts, collision avoidance, passenger service lines, etc.

Currently, transponding is not part of the NMRA standard or RP.

Decoder Features

Movement & speed
Back EMF
Decoder-Assisted Consisting
High Frequency
Kick Start
Loco Momentum
Trim Adjust
Loco Momentum
MU Consisting
Speed Steps
Speed Table
Switching Speed
Three Step Speed Table

Forward/Reverse Trim


Decoder Reset
Programming Lock
User Identifier

Lighting & Effects
Directional Lights
Func Remap
FX Lighting
Master Switch

Thermal Protection

DCC Core components

Command Station
Power Supply

Track work components

Track wiring
Track work

Implementing Digitrax Transponding

Transponding with Digitrax is accomplished with a transponding receiver (RX4) attached to the BDL162 or BDL168 occupancy detectors. When a vehicle equipped with a compatible decoder reports back, only the receiver attached to the block detector will receive that vehicle's message, thereby letting the system know which block it is in.

The DT400 throttle has a button to find a locomotive and will report the locomotive's location in the throttle display. It can be used to implement braking and speed control sections, as well as automating hidden staging yards and other things where knowing the exact location of a locomotive would be handy.

Lenz has recently added Railcom to its Gold line multifunction decoders to communicate to its LH100 and LH90 throttles in a similar fashion. Other decoder manufacturers have also begun offering RailCom compatible devices and multifunction decoders

How Digitrax Transponding Works

When a transponder receives a DCC control packet (which happens many times per second), it responds with a "ping" (or response) on the tracks. This ping occurs during a time, between packets, when the command station allows such things to happen without affecting other packets. According to the SurroundTraxx sales sheet, the decoder receives a packet addressed to it, and upon receipt of the second packet, the headlight is switched on and off at precisely defined intervals between the packet's bits. These minute changes in current flow are detected by the transponding system.

Scope traces show bursts of "noise" appearing on what is probably the preamble of the DCC packet. This would be the decoder sending data back to the receiver. [1]

The occupancy detector (BDL162/BDL168), through the RX4 (see below) transponding receiver, monitors track power to decode the address of all packets sent. Therefore, they always know which address was last sent. When the transponding receiver receives a "ping" back, it notifies the occupancy detector that it received a ping. Since the occupancy detector knows which block the transponding receiver is connected to, it knows which block that pinged address is in, and sends that information out over LocoNet. Any software or hardware that is interested in transponding can then pick up that information for whatever you want.

The BDL162/BDL168 have a certain amount of memory to keep track of which locomotive addresses have been reported for which blocks. Now, when it gets another ping for that same address in the same block (which will happen many times per second), it will not send more messages over LocoNet - doing so would bog the LocoNet down with nothing but transponding messages. The BDL168/162 will only send a LocoNet message when an address is first pinged in each block.

Installing Transponding

To install transponding, you will need the following equipment, some of which you may already have.

  • Install BDL16 series occupancy detector(s). Remember, BDL162 and BDL168 can be used together on the same layout.
  • Add RX4 transponder receivers to the zones (blocks) you want to set up for transponding. A transponding receiver is not required for each detection block (zone) for effective coverage and reporting functionality.
    • Use either two RX4s with your BDL168 to set up eight transponding zones or;
    • Use one RX4 if you need four transponding zones.
  • Recent (since 2012) Digitrax decoders are transponder equipped.
    • Add a separate transponder if needed by installing a TL1 or TF4 function decoder.
      • Can be added to items which already have decoders as transponding decoders do not interfere with an already installed decoder.
      • Can be installed in locomotives, rolling stock, or even a caboose to determine the end of a train.

Transponding Equipment

There are 3 components to a transponding system:

  1. Transmitter (decoder)
  2. Receiver (RX4)
  3. Command station (any Digitrax command station)


Transmitters are tiny, and do not have motor control outputs.

  1. TL1
    1. Supports both Primary and Extended addresses.
    2. The TL1 is a transponding device with a lighting output. It can be used as a function only decoder. Single 125ma (250ma peak) function output for lights or other functions. The configurable strobe feature simulates flashing lights on locos like FRED, Strobes, Mars Lights, etc. Can also be used it for passenger car lighting. Lacks the motor control feature typical of a decoder.
    3. MSRP: $24.33
  2. TF4
    1. Similar to the TL1, with four function outputs (250ma peak).
      1. Programmable strobe and lighting effects on the yellow and white leads, standard on/off function operation on green & violet leads.
    2. MSRP: $29.42


The Digitrax RX4 is currently the only transponding receiver. It's a four zone (block) transponding receiver. It cannot work alone and must be connected to a block detector such as a BDL16X device, for example, the BDL162 or BDL168. It consists of the transducers needed to detect the signals generated by the decoder or transponder.

Transponding Compatible Command Station

Any Digitrax command station is compatible with Digitrax Transponding.

Digitrax Transponding FAQ

Q: Does every layout block need transponding installed?

Transponding does not need to be used over the entire railroad. Software such as RR&Co's Train Controller need only identify an engine in a start block of a schedule. This can be done at the computer's keyboard so transponding is not necessary. It simply can automate the process. All of the detection is done by the BDL boards and the identity of the train is tracked in the software.

Edit FAQ  Related Manufacturer: Digitrax Categories: Transponding

Q: PM74 Option Switches

The PM74 has several options available via their Option Switches.

  • OpSw 1 – 4 change their respective districts to auto reverse mode
  • OpSw 7 lowers trip current
  • OpSw 8 increases trip current, overriding OpSw 7
  • OpSw 11 disables Block Occupancy detection and messages
  • OpSw 12 disables Transponding
  • OpSw 13 mutes Power messages to LocoNet.
  • OpSw 40 - Factory Reset
  • OpSw 41 controls the behaviour of LED indicators

Over Current Trip Settings

  1. OpSw 7: When Closed, trip point is at 3A, OpSw 8 is ignored
  2. OpSw 8: When Closed, trip point is at 6A, OpSw 7 is ignored
  3. When both OpSw 7 and 8 are Thrown, trip point is at 4A
    1. This is the factory default

See the manuals, KB1070 and KB4071 for details.

Edit FAQ  Related Manufacturer: Digitrax Categories: Occupancy Detection, Power, Transponding, Wiring

Q: What is detection actually providing if I do not Install transponding?

Detection simply detects the presence of a train on a section of track bounded by gaps and fed by a detection system such as the Digitrax BDL boards. Engines are automatically detected (non-transponding) since they draw power whether moving or not. Rolling stock can be detected by using resistance wheel sets, or in the case of passenger cars and cabooses, lights. By using resistor wheels on the last car of a train the block will always show detection when occupied as long as the train is no longer than any single block. Detection is mandatory for computer control. It is how the computer knows what blocks are occupied.

Transponding will display the ID of the train, usually the engine's number or the rolling stock's ID number. A transponder section acts like a detection section since if an engine is transponding, it is obviously occupying a block. Cars, passenger cars and cabooses, unless equipped with a transponder, will not be identified.

Edit FAQ  Related Manufacturer: Digitrax Categories: Transponding

Tips and hints

External Links

  1. Digitrax uses a high frequency carrier superimposed on the DCC signal to send data. The transponder modulates the DCC signal on the track, and the detector senses the changes in the track voltage/current. As a result, it's not as robust as the current loop used by RailCom.