FAQ:Can the digital decoder send information back to the command station?

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In Service Mode the digital decoder can acknowledge a packet with a brief pulse of current. This can be used to find out the digital decoder’s address, for example – send a packet asking the digital decoder if it is address 1 and wait for a response. If no response, try address 2, etc. There is currently no defined way to send information back to the command station in normal running mode, although some work is being done in this area to determine what is feasible. See the NMRA Extended Packet Format RP for a description of feedback in service mode.

However, some systems like Digitrax can have limited feedback using their multifunction decoders, which Digitrax calls transponding.

Another technique is RailCom, which can send a limited set of messages regarding speed, load, address and the content of a CV. RailCom requires a compatible decoder and a cutout device to activate it. A typical use is to automatically register a locomotive added to the layout when a compatible command station is used.

LISSY is another method of communication using infrared light.