Locomotive Interface

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Short Definition
Method of connecting a mobile decoder to a locomotive

The NMRA and NEM have adopted standard mechanical and electrical interfaces to connect Multifunction Decoders to a Locomotive's electrical system. These plugs and sockets make it much simpler to install a decoder into a suitably equipped locomotive. In most cases, a blanking plug must be removed before installing the decoder. If a locomotive is not DCC-Ready then it will lack an interface and must use a Hardwired Decoder or a drop in replacement DCC control board (if available) for that specific model.

Various types of decoder connections, image from ESU.

NMRA and NEM Connectors

DCC Interface Plug Naming Conventions
4 Pin NMRA Large NEM 654
6 Pin NMRA Small NEM 651
8 Pin NMRA Medium NEM 652
21MTC 21MTC NEM 660
Next18/Next18–S Next18/Next18–S NEM 662
PluX PluX8/16/22 NEM 658

NMRA and NEM 6, 8 and 4 Pin Interfaces

Standard Logos for Locomotive Interfaces

These interfaces are designed for basic installations without sound or many additional functions. The various sizes are intended to cope with the various physical size and power requirements for different scales.

The 6 pin small interface is intended for N gauge locomotives due to their restrictive size and lower power requirements. The locomotive side is a 1x6 female socket with 0.05" pitch. It is also called a NEM 651 plug.

The 8 pin medium interface is intended for HO/OO gauge locomotives and allows for slightly higher power and simplifies headlight wiring. The locomotive side is a 2x4 female socket with 0.1" pitch. Also known as the NEM 652 plug.

The 4 pin large interface (NEM 654) is intended for O gauge and above and allows for higher power. The locomotive side is 4 male pins, the decoder side is 4 female sockets. The standards do not define the physical connector layout.

Physical Socket Layout

Pin Assignments and Wire Colours

Pin Assignments and Colour Coding for Locomotive Interfaces DCCWiki.com
6 pin 8 pin 4 Pin 9 Pin JST2
NEM 651 NEM 652 NEM 654
Pin 1 Motor Right Motor Right Motor Left F2
Pin 2 Motor Left Rear Headlight (F0) Motor Right Left Rail
Pin 3 Right Rail N/C1 Left Rail Motor Left
Pin 4 Left Rail Left Rail Right Rail Rear Headlight (F0)
Pin 5 Front Headlight (F0) Motor Left Front Headlight (F0)
Pin 6 Rear Headlight (F0) Front Headlight (F0) Common (V+)
Pin 7 Common (V+) Motor Right
Pin 8 Right Rail Right Rail
Pin 9 F1


1 The standards do not define a use for this pin although it is often used for a function output
2 A JST Connector is not strictly a locomotive interface since it connects a mobile decoder to its harness, not to the locomotive

21MTC Interface

Main article: 21MTC
21 Pin MTC Logo

The 21MTC Connector interface is a standard adopted by both the NMRA and NEM 660. Its name comes from 21 pin Marklin/Trix Connector, developed by Marklin and ESU. It is often found in locomotives with OEM decoders from ESU. There are differences between the NMRA and NEM definitions for this interface.

Update: In December 2020 the NMRA updated their interface standards and has adopted the NEM definitions as the official NMRA standard for the MTC21 connector. In time all MTC21 multifunction decoder manufactures will incorporate the combined NEM/NMRA MTC21 interface.

This interface is always a direct-connect type as there is no wiring harness. The decoder plugs directly on to the male header mounted on the locomotive chassis.

The NMRA standard has discouraged the use of this connector in new locomotive designs from January 2010. It is intended to be replaced by the PluX interface.

The main physical difference between the 21MTC and PluX decoders is the pin arrangement. The 21MTC features a female socket which mates with pins on the locomotive interface. The PluX is the opposite: the pins are on the decoder, which mate with a female connector on the locomotive. The PluX standard defines the length of the pins, as they pass through the locomotive interface board for a lower profile installation.

If a manufacturer states the locomotive comes with a 21-pin decoder, it would indicate a 21MTC interface.

Next18 Interface

NEM 662 / RCN-118

MOROP created the Next18 standard for extremely tight applications, such as TT and N scale locomotives. The NMRA has not embraced the Next18 interface, though some manufacturers have. In addition to specifying the electrical interface and connectors, the NEM 662 standard also mandates the maximum physical size of decoders. A fully compliant Next18 decoder is no larger than 15mm x 9.5mm - one would fit on a dime; a Next18S decoder is no larger than 25mm x 10.5mm.

Connection Length Width Height
Next18 15 mm 9.5 mm 2.9 mm
Next18-S 25mm 10.5 mm 4.1 mm

There are two variants: Next18 and Next18S. Next18 supports motor control, front and rear headlights, and 6 auxiliary output functions; two of those function pins may instead be used to provide an on-board TrainBus serial interface. A Next18S decoder adds sound by sacrificing two auxiliary function outputs for the speaker connections.

The interface specifies an extremely small 18-pin "encapsulated" connector. The entire connector is only 8mm long and 5mm wide. Because of their small size, connector pins are only rated for 500 milliamps, so some connections are allocated two pins. The result being that although there are 18 pins, some are redundant, and the 18 pins provide only 12 discrete connections. A feature of the interface is "smokeless symmetry". Pins are assigned in a somewhat symmetric fashion, such that if installed backward, no harm is done to either the locomotive or the decoder; it will not function properly, but nothing will be "smoked".

  • This interface supports SUSI using pins 5, 6, 14 and 15.
  • A multifunction decoder can be used as a function only decoder, as the decoder has internal circuitry to allow this.

Doehler and Haass offers a range Next18 (and MTC21) adaptors. See their website.

Pinouts of the Next18 and Next18-S

The pinouts are symmetrical, so the decoder can be plugged in either way. AUX 3 to 6 are logic level signals

The female socket is on the locomotive motherboard, the decoder has an 18-pin male header. For operation without a decoder, a jumper plug is used.

PIN Name Next18-S Description
1 Track Right Pin 1 and 18 are the same
2 Motor (Pos)
3 AUX1 Function Out 1
4 AUX3 / TBCLK Function Out 3
5 GND Same connection as pin 14
6 U+ Data or voltage
7 AUX6 Speaker (B) Function Out 6
8 F0_f Headlight, Forward
9 Track Left Common to 10
10 Track Left Common to 9
11 Motor (Neg)
12 AUX2 Function Out 2
13 AUX4 / TBDAT Function Out 4 / Data
14 GND Ground, common with pin 5
15 U+ Voltage, Positive
16 AUX5 Speaker (A) Function Out
17 F0r Reverse headlamp
18 Track Right Same as Pin 1

Web Links

These links are in German:

PluX Interfaces

Main article: PluX Interface

The NMRA and MOROP have adopted a newer standard interface called PluX22, and its subsets, PluX16 and PluX8. (MOROP's NEM 658 also defines PluX12, however, NMRA Standard S-9.1.1 does not.)


The purpose is to provide a uniform interface for safe, secure and rapid installation of decoders adhering to the PluX Standard.


The PluX22 connector consists of two rows of 11 pins. PluX16 and PluX8 connectors consist of two rows of 8 pins and 4 pins respectively. The smaller connectors are subsets of the larger interface. Mechanically, the interface defines male pins on the decoder which are meant to interface directly with a female connector on a locomotive's receiving board. The preferred board-to-board orientation being that the decoder pins pass through holes in the receiving board's PCB and enter the bottom of the female connector. This would keep the height to a minimum.

  • PluX8 offers no advantages over the NEM 651 interface, so it not used.
  • The interface is designed for direct plug-in installation. Connection via a cable is not provided for, and are outside the PluX Standard.
  • The Standard sets maximum dimensions for the decoder -- both footprint and thickness. This allows for small, drop-in decoders.
  • The PluX interface supports one motor, up to 9 function outputs and one sensor input.
  • Also defined are the installation space and size of the decoder. Packaging must be labelled indicating a PluX16 or PluX22 device.

Additional Reading

External Links

Decoder Buddy

A 21MTC adapter board is available from NixTrains called the Decoder Buddy.

This interface board works with decoders by Digitrax, ESU, SoundTraxx, and TCS, as well as other 21MTC decoders.

The Decoder Buddy replaces the standard DCC 8-pin loco motherboard.


  • 10-pin socket and plug with wire-outs for all 21-pin decoder functions
  • 10-pin plug fully removable for easy separation of loco shell and chassis