21 Pin MTC Connector
The 21 Pin MTC Connector is a Mobile Decoder Locomotive Interface intended for sound applications. This is a direct PCB connector (there is no wiring harness), the decoder is intended to plug directly onto a fixed pin header inside the locomotive. MTC connectors have additional connections compared to regular DCC connectors to allow for loudspeakers and Hall sensors for chuff synchronization. However, while intended for sound, not all 21 pin MTC decoders are sound-capable and not all locomotives equipped with 21 pin MTC are sound-ready.
This connector was not originally defined by the NMRA but was adopted as a de facto standard. It was originally designed by ESU for certain Märklin and Trix locomotives which is presumably where the name comes from (Märklin/Trix Connector).
Confusingly, the accepted standard for the 21 Pin MTC connector is not the same as the original Märklin/Trix connector. For example, Zimo manufactures two decoder types with 21 pin connectors, type C is for Märklin/Trix with C-Sinus and Softdrive motor and type D is for the MTC standard.
Despite the name, the connector used is actually a 22 pin connector with one position blanked off to polarize the connector (to ensure it cannot be connected in the reverse orientation). There should be no corresponding pin on the locomotive's header board. In practice, in many applications, it is unlikely that a decoder would physically fit in the reverse orientation.
This connector is unsual in that it appears to be connected backwards; the connector receptacle is mounted on one side of the PCB but the decoder is plugged into the locomotive header by inserting it so the pins come up through holes in the underside and into the connector. The blanked off position is acheived by the lack of hole in the board at that location. This may not be obvious from first inspection so extra care should be taken when installing them to avoid damage.
The NMRA standard has discouraged the use of this connector in new locomotive designs from January 2010 but as of 2013 there are still many locomotives in production with this connector and obviously there will long be a market for compatible decoders.