21MTC Multifunction Decoder Interface
Summary: The 21MTC connector is another plug and play interface for multifunction decoders. Unlike the PluX Interface, the female connector is on the decoder. It is also a low profile decoder, with the components installed on the underside.
The 21 Pin MTC Connector is a Multifunction 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 on a motherboard 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 multifunction 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 female 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 motherboard. In practice, in many applications, it is unlikely that a decoder would physically fit in the reverse orientation.
This connector is unusual in that it appears to be connected backwards; the connector receptacle is mounted on one side of the PCB but the multifunction decoder is plugged into the locomotive motherboard by inserting the decoder 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.
- When making comparisons between Standards, ensure you have the most recent versions. This article uses the most recent (as of January 2019) NEM and NMRA Standards as references.
- 1 Note Regarding NEM Standards
- 2 NMRA Acceptance of the 21MTC Connector
- 3 Technical Issues
- 4 The 21MTC Connector
- 5 Decoder Buddy
Note Regarding NEM Standards
Many websites published in English will have outdated information regarding the 21MTC Connector. The standard they reference can be up to a decade old, and out of date. Currently Fédération Européenne de Modélisme et Amis des chemins de fer d’Europe does not publish their standards in English, as they have not been translated. The most recent versions are available in French and German.
- This article used the most recent NEM standards in its preparation.
NMRA Acceptance of the 21MTC Connector
The 21MTC connector does appear in NMRA Standard "Electrical Interface and Wire Colour Code for Digital Command Control" S-9.1.1 (May 2015), but contains this footnote:
Starting Jan. 1st 2010, this connector will not be recommended for new locomotive designs. This will not invalidate the conformance of existing designs. Controllers for these connectors will continue to conform, as controllers for this connector will be required as long as locomotives with this connector exist.
The Standard does require the 21MTC Logo to appear on packaging of locomotives featuring this interface.
There are differences in the interface between NEM and NMRA standards. The table of connections below is from the NEM standard, and is consistent with the NMRA Standard.
While the MTC21 connector is considered to be a standard, the NMRA does not recommend its use. Unfortunately, a multifunction decoder which fits may not be the correct multifunction decoder for the application. There are technical issues, in that the decoder may be compliant, while the interface or the model is not compliant to the specification.
The NMRA standard provided for an open collector AUX3, which some manufacturers, such as Zimo, TCS and others follow. The NEM 660 standard requires Logic Leveloutput on AUX3 and higher. A voltage is always present on these pins.
The "open collector" variation performs a switching function, which relies on voltage supplied by the common connection on the decoder.
These issues came about when the MTC21 connector was introduced, and manufacturers made changes to the output functions before the plug became standardized. The interface was declared obsolete in 2009, manufacturers were supposed to cease designing new multifunction decoders or models using the interface. The PluX interface as designed to provide a common standardized interface between the locomotive and the multifunction decoder. Unfortunately many manufacturers have no embraced the PluX interface and continue to use the MTC21.
The 21MTC Connector
The interface on the system (mother) board consists of a male 22-pin, two-row pin header. The dimensions of the decoder are a maximum of 30 (L) x 15.5 (B) x 6.5 (H) mm. On the system board, the installation space must be such that the decoder can be inserted without jamming. Pin and socket strip are arranged in two rows of 11 contacts. Alignment coding is by the omission of the 11th pin and blocking the associated socket.
Multifunction Decoder Interface
The multifunction decoder module carries a female socket. The socket is located on the flat side of the PCB, facing away from the motherboard.
As with the Plux type decoders, there is a smaller version described. It is designed to be the smallest profile possible for a multifunction decoder.
If space is at a premium, the standard allows for a multifunction decoder with the socket facing the locomotive interface, to reduce the vertical clearance.
21MTC Pin Assignments
From the NEM 660 Standard Aug 2015. Note: Many of the NEM Standards available in English are outdated by 10 or more years! This table was taken from the German Version.
|1||Input 1||Sensor Input 1||4|
|2||Input 2||Sensor Input 2||4|
|3||AUX 6||AUX Output||8|
|4||AUX 4 3||AUX Output||8|
|5||ZBCLK||Train Clk Bus||7|
|6||ZBDATA||Train Data Bus, Rx/Tx||7|
|11||INDEX||Alignment Pin, not used|
|12||Vcc||Decoder internal Voltage1||2|
|13||AUX 3 3||AUX Output||8|
|14||AUX 2||Violet||AUX Output||5|
|15||AUX 1||green||AUX Output||5|
|16||V+||Blue||Decoder Positive, connection for capacitor||2|
|17||AUX 5||AUX Output||8|
|20||Ground||Decoder ground, tap at rectifier||2|
|21||Power||Black||Left Rail pickup||1|
|22||Power||Red||Right Rail pickup||1|
1 Vcc is 1.8 to 5.7V
2 Refers to the connections for the forward direction
3 AUX 3 and AUX 4 are logic level outputs. Additional circuitry may be required, i.e. to drive a LED.
1: AC: pin 21 is the power take-off for the wheels and pin 22 is connected to the center conductor.
2: Pin 12 is not mandatory..
3: AC Motors: Pin 19, Field Coil A, 18 is B..
4: Pins 1 and 2: open collector inputs and switched to GND. The input resistance should be approx. 100 kΩ. Sensor input 1 should be used on steam locomotives for wheel synchronization..
5: If forward and reverse lights are wired separately, the rear lights of the driver's cab 1 with pin 15 and that of cab 2 with pin 14 switched..
6: Impedance of the speaker is determined by the manufacturer of the decoder..
7: The processor pins of the train bus are connect through a resistance in series with a maximum impedance of 470 Ω..
8: The outputs may be used with max. 0.5 mA and lead logic level against GND. OFF ≤0.4V at decoder output, ≤ 0.8V at locomotive interface, ON: ≥2.4V at decoder and ≥2.0V at locomotive interface.
Group 8 are Logic Level, the motherboard in the locomotive should contain any additional circuitry required to interface the logic to external functions.
The NMRA Standard does not follow the NEM standard exactly. There are some differences.
For basic operations, the motor and speaker connections are consistent with the NEM standard. The lower Function outputs are also compatible.
Incompatibilities arise with the FX5-8 connections. The NMRA standard allows a direct connection to LEDs or lamps for lighting effects. The NEM standard requires a logic level output with additional components for the lighting circuits.
Most manufacturers have adopted the NEM pinout for their locomotives equipped with an OEM multifunction decoder. Swapping in another decoder will usually work without any issues. Some manufactures such as those from Athearn follow the NMRA standard. Also be aware that not all decoder manufacturers follow the NEM standard, and may offer products for either standard.
Additional Compatibility Notes
A number of manufacturers have also began adding a custom motherboard to their locomotives with a co-processor to control the FX functions. This leads to issues when replacing the multifunction decoder, as they have been designed to work together. Programming can be difficult as a result.
Operation Without a Multifunction Decoder
With the installation of a suitable plug, operation of the locomotive without a decoder is possible.
The Decoder Buddy is an adapter board manufactured by NixTrainz which makes installation of a 21 pin multifunction decoder easier.
Once a Decoder Buddy is installed, it must be used with a decoder. It is not possible to remove the decoder for analog operation. Be aware that the Decoder Buddy must be compatible with the decoder pinout.