The 21pin connector is more formally know as the 21MTC where MTC means Marklin Trix Connector. The most obvious and compelling feature of the 21 MTC pin standard is that it offers a Sound Decoder plug and play capability. All previous decoder connector never had support for speaker connection in the connector.
BRIEF HISTORY
The 21MTC connector existence predates the NMRA PluX-22 socket standard which was attempting to do that same thing. Unlike the 21MTC connector, the Plux-22 was designed specifically to meet the needs of the NMRA DCC standards. However, the PluX-22 connector standard came way to late and as a consequence there are very few PluX based decoders if any.
There are two coexisting versions of the 21MTC standard being pushed out into the general DCC market:
1) NMRA 21MTC Where NMRA is the National Model Railroad Association (DCC Standard).
2) NEM 21MTC Where NEM is the National European Model standard. It more formally called the NEM 660 standard.
The 21pinMTC decoder socket/plug has a convoluted history of evolution starting from a proprietary DCC plug for a very specific set of engines for Marklin to one that today is attempting to become the next generation DCC plug. Unfortunately this is an ongoing story of evolution that is not over which means BUYER BEWARE. The good news is DCC manufactures are starting to offer a way to be compatible with BOTH standards through CV configuration programming and hardware changes.
OUTPUT TERMINOLOGY DEFINITIONS
With the NEM standard, a new type of output is defined when compared to the NMRA DCC standard. Here are the definitions.
1) Function Output (F): An output signal that is capable of directly driving" a load such as a Light Bulb or LED as high DCC track voltage (12V).
2) Auxiliary Output (Aux): A 5V logic output signal that indicates "the state" of the output but itself is not capable of directly driving the load.
Physical output and associated name: (Assumes a non sound decoder with NMRA default function mapping.)
ELECRTRICAL OUTPUT COMPATIBLITY
The key difference between the two standards is the electrical output compatibility (voltage and current) above the headlight functions.
1) Plugging in a NMRA 21MTC decoder into a NEM 21MTC socket can do harm if you using more than 4 functions.
2) Plugging in a NEM 21MTC decoder into a NMRA 21MTC socket will do NO harm.
DCC MANUFACTURES SUPPORT
Below is a simplified table showing which DCC manufactures support which standard.
MORE DETAIL BACKGROUND
Bryan Vianco of Streamline Backshop wrote a complete and accurate detailed understanding of the 21 pin connector mess
Go here: 21 Pin Discussion
Below is a my summary
Unfortunately the 21 pin standard is a mess for it started as something not specifically optimized for generic DCC use and split into 4 different directions at the same time.
1) Originally Marklin and ESU developed a specialized decoder plug specifically intended for Marklin's special decoder needs for its own locomotive product line. Many of those special needs are not commonly used and/or not implemented as traditional DCC decoders would do. This is Version 1 of the 21MTC standard.
2) The NMRA adopted the 21MTC pinout with the only change making ALL the outputs to the common types capable of directly driving LEDs and Lamps. A step in the right direction. But several other important changes that should have been made did not happen. Unfortunately, the NMRA still called its version the same 21MTC name! So there are two version of the 21MTC standard which are not a single standard. The version is informally called the NMRA 21MTC connector. This is Version 2 of the 21MTC standard.
3) European Modeling Standards (NEM) organization, which creates standards for the European Modeler, took the 21MTC definition and address different set of the specialized features but did not including the function outputs issues. A step in the right direction. However the changes are still not enough to meet the traditional needs of US DCC decoders. This version became known the NEM660 21MTC connector based on the NEM660 standard. This is informally known as the NEM 21MTC connector. This is Version 3 of the 21MTC standard.
4) Given BOTH the NMRA and NEM did not address all of the problems with the V1 21MTC standard, the US decoder manufactures were stuck without a suitable 21 pin standard. Consequently multiple DCC manufactures have now started defining their own version of the 21 pin standard for the US market.
3/5/2017