Since 1999, Digitrax has produced function-only decoders with Transponding capabilities. These decoders make it easy to add transponding and additional functions to locomotives with decoders that do not have transponding or enough functions. These are useful for customers with older decoders produced before transponding was incorporated into Digitrax decoders. They are also useful for users of decoders made by manufacturers that do not support transponding.
The TF1 is a single output function decoder with Transponding capabilities. This function decoder adds an additional function along with Transponding ID capabilities to an engine with non-Transponding decoder capabilities or adds a single function along with Transponding capabilities to special pieces of rolling stock.
The TF4 is a stand-alone quad output function decoder with Transponding capabilities. This function decoder added four additional function outputs along with Transponding ID capabilities to locomotives with non-Transponding decoder capabilities or added four functions along with Transponding capabilities to special pieces of rolling stock.
The TD1 (discontinued in December 2003) was a stand-alone transponder. This decoder added Transponding ID capabilities to engines with non-Transponding decoders or added Transponding ID capabilities to special pieces of rolling stock.
The TF2 (discontinued in December 2003) was a stand-alone dual output function decoder with Transponding capabilities. The purpose of this function decoder was to add two additional function outputs along with Transponding ID capabilities to engines with non-Transponding decoder capabilities or to add two functions along with Transponding capabilities to special pieces of rolling stock.