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Short Definition

Transponding is a system where a decoder can report back to the system, or command station.

Transponding is a system where a decoder can report back to the system, or command station.


Typically, when a decoder gets a packet, it reports back that the packet was received. Transponding provides three potential advantages: letting the system know which signaling block the locomotive (or car) is in, transmitting data about the loco (or car) back to the system, and making sure a locomotive with a new command gets it quickly.

Transponding is used for locomotive and/or car locations. This is 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
Loco Trim
Loco Momentum
MU consisting
Speed Steps
Speed Table
Switching Speed
Three Step Speed Table

Forward/Reverse Trim


CV 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

Transponding Uses

(Topic needs expansion.)

  1. Know what is in a detection zone, not only that there is something there. Now you, or your computer, can know what locomotive or rolling stock is in each detection zone.
  2. Location/ID data can be used to activale sound, display information, animations, etc.

A recent application of Digitrax's transcending technique is the SurroundTraxx system from Soundtraxx. Transponding is used to determine where the locomotive is and alter the soundfield as needed.

The SurroundTraxx unit is a LocoNet device.

(Addition equipment may be needed.)

Digitrax Transponding

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

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

Lenz has recently added Railcom to its line, which permits its Gold line decoders to communicate to its LH100 and LH90 throttles in a similar fashion.

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 happens 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 packets 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.

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.

Features and Benefits of Using Digitrax Transponding

  1. Ability to readback CVs from operations mode programming anywhere on the layout which has a transponding reciever.
  2. Location and identification information is available through the LocoNet and is constantly updated.
  3. Ability to automate staging yards and other operations throughout your layout with the use of DCC Software.

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. Keep mind, that you will not need a transponding reciever for each detection block (zone) for effective coverage and reporting functionality.
    • Use either two RX4s with your BDL168 to set up 8 transponding zones or;
    • Use one RX4 if you only need 4 transponding zones.
  • All recent (Year??) Digitrax decoders are transponder equipped.
    • You can add a separate transponder if needed by installing a TL1 or TF4 mobile 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 locos, any rolling stock, or even a caboose to determine the end of a train.

Digitrax Wiring Tips

Wiring information and tips go here.

Transponding Equipment

There are 3 componets to a transponding system:

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


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

  1. TL1
    1. Supports both 2 digit and 4 digit addresses.
    2. The TL1 is a transponding device with a lighting output. It can also be used as a function only decoder. It has a single 125ma (250ma peak) function output for lights or other functions. The configurable strobe feature lets you simulate flashing lights on locos like FRED, Strobes, Mars Lights, etc. It can also be used it for passenger car lighting. It lacks the motor control feature typical of a decoder.
    3. MSRP: $24.33
  1. TF4
    1. Similar to the TL1, but it has 4 function outputs (250ma peak).
      1. It has programmable strobe and lighting effects on the yellow and white leads and standard on/off function operation on green & violet leads.
    2. MSRP: $29.42


The Digitrax RX4 is currently the only transponding reciever. It's a 4 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.

Retail price on the RX4 is $70.21.

Transponding Compatible Command Station

Any Digitrax command station is compatible with Digitrax Transponding.

Digitrax Transponding FAQ

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.

I Don't Need to Install Transponding for Each Section. Why?

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.

Tips and hints

External Links