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 Mobile 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 doesn't have a locomotive 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 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.

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.

The 4 pin large interface 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

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

21 Pin MTC Interface

21 Pin MTC Logo

The 21 pin MTC interface is a standard adopted by both the NMRA and NEM 660.

This interface is always a direct-connect type where there is no wiring harness. The decoder plugs directly on to the pin 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.

PluX Interfaces

The NMRA and NEM have adopted newer standard interfaces called PluX8, PluX12, PluX16 and PluX22.

The PluX connector consists of two rows of 4, 6, 8 or 11 pins. The smaller interfaces are subsets of the larger interfaces. The pins are meant to interface directly with a female connector, with the preferred method being that the pins pass through holes in the PCB and enter the bottom of the female connector. This would keep the height to a minimum.

The Standard sets maximum dimensions for the decoder. This allows for small, drop in decoders.

Additional Details

The PluX interface is based on the connector used by Marklin. The pins are rated at 1A each.

Eight pins (1index) is the minimum for a HO/N motor/light decoder.

Sixteen pins is for a sound decoder plus 4 functions, SUSI and a storage capacitor.

Twenty two pins is a premium sound decoder with up to 10 functions, and SUSI.

One orientation possible within the locomotive. The Index to be marked with an arrow/triangle adjacent, on the visible side of the decoder, the Locomotive PCB will also have an arrow/triangle index mark. The user just has to align the two marks. The pin will be missing at the index position, and there will be no hole drilled into the locomotive PCB at that position.

You can insert a PluX8 decoder into a PluX22 socket, as it is a subset of the larger connector. This gives the consumer choice as to the type of decoder to be installed while giving the manufacturer a single decoder interface for all decoder types. The index will ensure correct connections.

Additional Reading

For more information, see the NMRA Standard S-9.1.1 for more information on connectors used to create various locomotive interfaces.