21MTC Connector

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21 Pin MTC Logo

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.

A decoder with an MTC connector

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.

Bachmann Branchlines use a slightly different version of the logo; Bachmann Branchlines' Version of the Logo but the connector is the same.

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.

Note Regarding 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 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.

The 21MTC Connector

The 21MTC interface can be used with AC motors (2 field coils), DC motors or Coreless motors. The interface provides up to 8 function outputs as well as two Sensor inputs.

Mechanical Details

The interface on the system board consists of a 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.


The decoder module carries the socket. The socket is located on the flat side of the PCB.

Compact Version

As with the Plux type decoders, there is a smaller version described. It is designed to be the smallest profile possible for the decoder.

Rotated Variant

If space is at a premium, the standard allows for a 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.

Pin # Name Colour Function Group
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
7 F0r Yellow Reverse light 5
8 F0f White Headlight 5
9 LS/A Brown Loudspeaker 6
10 LS/B Brown Loudspeaker 6
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
18 MTR2 Grey Motor, negative2 3
19 MTR1 Orange Motor, positive2 3
20 Ground Decoder ground, tap at rectifier 2
21 Power Black Left Rail pickup 1
21 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 daughter board in the locomotive should contain any additional circuitry required to interface the logic to external functions.

Operation Without a Decoder

With the installation of a suitable plug, operation of the locomotive without a decoder is possible.