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

[edit] Transponding

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 loco (or car) is in, transmitting data about the loco (or car) back to the system, and making sure a loco with a new command gets it quickly.

Transponding is used for loco 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.

[edit] 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.

[edit] Digitrax Transponding

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

The DT400 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.

[edit] 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.

The occupancy detector (BDL162/BDL168), through the RX4 (see below) transponding reciever, 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 device (such as automation software) 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 loco 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 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.

[edit] 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.

[edit] 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 seperate 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.

[edit] Digitrax wiring tips

Wiring information and tips go here.

[edit] Transponding Equipment

There are 3 componets to a transponding system:

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

[edit] Transmitters

Transmitters only are very 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.
   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

[edit] Receivers

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 BDL16X device, such as the BDL162 or BDL168.

Retail price on the RX4 is $70.21.

[edit] Transponding Compatable Command Station

Any Digitrax command station is compatable with Digitrax Transponding.

[edit] Digitrax Transponding FAQ

[edit] What is detection actually providing me 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 made to 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 engines 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] 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 computers keyboard so transponding is not necessary. It simply can automate the process. All of the detection is then done by the BDL boards and the identity of the train is tracked in the software.

[edit] Tips and hints

• It's been noted that QSI decoders will kill any transponding for that zone.
• In Garden railroads, detection can be unreliable, however, transponding is. Please the transponding section of the DCC in the garden article.