21MTC Multifunction Decoder Interface

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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.

21MTC Logo


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. Decoders designed for the 21MTC connector are intended to fit into defined size, allowing it to be installed in most vehicles without the requiring tools for installation or removal. Connections via a cable are not provided, as an interface cable does not conform to this standard.

The 21MTC interface supports the connections for a motor, a baseline of 12 functions, 2 sensors, and a data bus expansion with options to support 12 function outputs.

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 21MTC 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 21MTC standard.

A decoder with an 21MTC connector

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 backwards: The connector receptacle is mounted on one side of the PCB, 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 achieved 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 during installation to avoid damage caused by incorrect orientation or damage to the mating pins.

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

There is a variant unique to Marklin, which is identified by 21MTC-M.

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

As of December 2020 the NMRA has accepted the 21MTC Decoder Interface under NMRA Standard S- Previously the NMRA implementation was not aligned with the NEM standard. As of 2021, this conflict has been resolved with the NMRA adopting the NEM pinout.

The Standard does require the 21MTC Logo to appear on packaging of locomotives featuring this interface, as well as decoders with this interface.


NMRA Standard S- has redefined the function outputs to align them with the NEM Standard.

From S-

The 21MTC defines output pins 7, 8, 14, and 15 as Open Collector/Drain switched (heretofore indicated as OC) outputs. Variations exist with output pins 3,4,13, and 17, identified as outputs AUX3 to AUX6.
S- requires providing outputs higher than AUX2 as logic level outputs. While AUX3 through AUX6 are always required to support TTL/LVTTL logic-level outputs, an open collector/drain option switched against ground may be provided. The default configuration must support TTL/LVTTL logic level on AUX3 through AUX6.

Technical Issues

These issues only apply to 21MTC Decoders manufactured prior to the NMRA's adoption of S-

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 a Logic Level output on AUX3 and higher. A voltage is always present on these pins.

The "open collector" output 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 not embraced the PluX interface and continue to use the MTC21.

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 eight function outputs as well as two Sensor inputs.

Mechanical Details

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.

Compact Version

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.

Rotated Variant

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.

NEM 21MTC Multifunction Decoder Interface Pinout
Pin # Name Colour Function Group
1 Input 1 Sensor Input 1, alternate AUX7 4
2 Input 2 Sensor Input 2 alternate AUX8 4
3 AUX 6 AUX Output 8
4 AUX 4 3 AUX Output 8
5 ZBCLK Train Clock Bus, alternate AUX9 7
6 ZBDATA Train Data Bus, Rx/Tx, alternate AUX10 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, tapped 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 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 (AUX3 – 6) are Logic Level, the motherboard in the locomotive should contain any additional circuitry required to interface the logic to external functions.

NMRA Exceptions Prior to S-

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.


Decoder Buddy

The Decoder Buddy is an adapter board manufactured by NixTrainz which makes installation of a 21 pin multifunction decoder easier.

It is 56 X 17 X 1 mm PCB that will fit most locomotives and allows easy installation of a 21 pin decoder such as those from TCS, SoundTraxx and ESU. Full details are on their website.

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.


TCS offers two adaptors for 21MTC connections.

The 21HW is a PCB with a female connector meant to be used to adapt a multifunction decoder with a harness to a 21MTC motherboard in a vehicle.

The 21HWM is a small PCB with an 21MTC (male) connector, meant to be used with a 21MTC multifunction decoder.


ESU offers the 51967 21MTC adapter board (LokPilot/LokSound V3.0 and V4.0), which features a male connector to allow you to install a 21MTC multifunction decoder in a vehicle. Solder pads are provided for wiring.

Further Reading

External Sites

Updated NMRA Standards

How to Plug in a Tsunami2 Decoder with a 21PNEM Connector

Proper installation of a NEM 21 pin multifunction decoder.