FAQ:Why does Writing an Extended Address with Programming on the Main Fail?

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Many command stations, as well as decoders, prohibit writing an Extended Address while using Operations Mode.

Writing a Primary Address is a simple affair. Writing an Extended Address is a multi-step process, where timing is critical.[1]

To write a Primary Address, only one CV requires changes. For an Extended Address, changes must be written to three CVs. As Operations Mode requires the commands to be directed to a specified address, should that address change during the programming operation, it fails.

To write an Extended Address, CVs 17 and 18 must be written with the appropriate values, then Bit 5 (Extended Address Mode) of CV 29 must be enabled. For this reason, many multifunction decoders will not allow the Extended Address to be changed using Operations Mode. They will only accept Extended Address changes when in Service Mode.

Some command stations also prohibit this for the same reason: When the address changes, it cannot write data to the old address as it no longer exists.

  • Service Mode uses a Broadcast protocol; it does not require the address of the decoder during programming. It will program every decoder on the program track.
  • Operations Mode requires an address to direct the programming commands to the desired decoder. Using address 0 switches the mode to a broadcast protocol, programming every decoder on the layout.
  1. As Opertions Mode uses the track outputs, there is additional traffic related to the operation of the layout present, increasing the possibility of lost packets disrupting the programming operation when multiple CVs are being written.