Locomotive Interface

Summary: The NMRA and NEM adopted standard mechanical and electrical interfaces to connect Multifunction Decoders to a locomotive's electrical system. These plugs and sockets make it simpler to install a decoder into a suitably equipped locomotive. In many cases a blanking plug must be removed before installing the decoder. If a locomotive is not DCC-Ready 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.

See the Video.

NMRA and NEM Connectors

DCC Interface Plug Naming Conventions

Name	NMRA	NEM
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

The NMRA is discouraging the use of the 4, 6 and 8 pin interfaces in new models.

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.

Pin Assignments and Wire Colours

Pin Assignments and Colour Coding for Locomotive Interfaces

DCCWiki.com

	6 pin	8 pin	4 Pin	9 Pin JST
	NEM 651	NEM 652	NEM 654
	NMRA Small	NMRA Medium	NMRA Large	NMRA JST
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/C	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

NMRA S-9.1.1 Decoder Interfaces

Wire Colour	Function	Notes
Red	Right-hand rail power pick-up: Center rail, outside third rail, traction/overhead wire to motor or interface	1
Orange	Interface to Motor + (Postive)
Black	Left hand rail power pickup to motor or interface
Grey	Interface to Motor – (Negative)	2
White	Output #1 to Front Headlight
Yellow	Output #2 to Rear Headlight
Brown or Violet	Speaker Connections	3
Green	Output #3 / Auxiliary 1
Brown or Violet	Output #4 / Auxiliary 2
	Output #5 / Auxiliary 3	4
	Output #6 / Auxiliary 4
	Output #7 / Auxiliary 5
	Output #8 / Auxiliary 6
Blue	Common (Anode Connection, Positive) for headlights and outputs
Black w/ White Stripe	Decoder negative rail (Common ground), sink.
Notes
1	Connection to RH, center, third Rail or overhead traction wires
2	Connection to LH rail
3	Either colour could have been used prior to this definition. Either colour can be used but it must be documented.
4	Any colour other than those defined may be used, but must be identifiable and documented.

Reference: S-9.1.1

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.

In December 2020 the NMRA updated their interface standards and has adopted the NEM definitions as the official NMRA standard for the 21MTC connector. In time all 21MTC multifunction decoder manufactures will incorporate the combined NEM/NMRA 21MTC interface. Older models and multifunction decoders may still be available which follow the older (now superseded) NMRA pin assignments. Verify the pinouts before installing a decoder.

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. Verify the alignment with the documentation that came with the decoder, as the connection may not be keyed to prevent incorrect alignment.

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

Next18 Interface

NEM 662 / RCN-118

NMRA S–9.1.1.5

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 Next18–S. Next18 supports motor control, front and rear headlights and 6 auxiliary output functions; two of those function pins may be used to provide an on-board TrainBus serial interface. A Next18–S decoder sacrifices 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 21MTC) 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. If functions work incorrectly, the decoder is installed backwards.

Next18 / Next18–S Pin Assignments
Description
Name	Next18	Pin	Next18–S	Group
Track, Right	Connection to right hand rail	1	Connection to right hand rail	1
Motor +	Motor connection 1, Positive/Forward	2	Motor connection 1, Positive/Forward	2
AUX1	Output 1	3	Output 1	3
AUX3	Output 2	4	Output 2	4
GND	Decoder Ground connection after rectification	5	Decoder Ground connection after rectification
V+	Decoder positive supply, after rectification	6	Decoder positive supply, after rectification
	AUX 6 / Output 6	7	Speaker B	4 or 5
F0f	Headlight forward	8	Headlight forward	3
Track Left	Connection to left hand rail	9	Connection to left hand rail	1
Track Left	Connection to left hand rail	10	Connection to left hand rail	1
Motor –	Motor connection 2, Negative/Reverse	11	Motor connection 2, Negative/Reverse	2
AUX2	Output 2	12	Output 2	3
AUX4 / TBDATA	Output 4	13	Output 4	4
GND	Decoder Ground connection after rectification	14	Decoder Ground connection after rectification
V+	Decoder positive supply, after rectification	15	Decoder positive supply, after rectification
	AUX 5 / Output 5	16	Speaker A	4 or 5
F0r	Headlight Reverse	17	Headlight Reverse	3
Track, Left	Connection to right hand rail	18	Connection to right hand rail	1
S-9.1.1.5 Next18 & Next18–S Multifunction Decoder Interface

Description of Signals, NEXT18 / NEXT18-S
Group 1	Track, Right (pins 1 & 18) and Left (9 & 10) connect to vehicle wheels for its incoming power supply.
Group 2	When a multifunction decoder is not present, Pin 2 (Motor +) normally connects to the right rail and Pin 11 (Motor –) to the left rail
Group 3	Open Collector/Drain outputs which are switched to ground on the decoder's side. Voltage is supplied from pins 6 and 15 (V+) In situations where the taillights are not connected to the headlight circuit, taillights for the forward direction (Cab1) connect to AUX1, reverse direction (Cab2) connect to AUX2. Maximum current draw is 100mA per output.
Group 4	Members of this group are TTL Logic Levels
Group 5	Effective load presented by the speakers to the multifunction decoder must be 4 – 8Ω, and must be documented. Impedance of speakers provided with the decoder must be documented (NEXT18–S)

SUSI Interface

For SUSI, only four signals may be used

1. GND
2. V+
3. Train Bus Clock
4. Train Bus Data

Track connections are not to be used for this purpose. All other connections can provide functions to a SUSI module.

See NMRA T1-9.2.3 for details.

Operations Without a Next18 Multifunction Decoder Installed

For minimal operations, a dummy plug can be used.

The plug must connect pins 1, 2 and 18 together, and the same for 9, 10 and 11.

Bridge Plugs: These can be constructed to power for head and taillights.

See the associated NMRA Technical Notes and Standards for more information.

Usage as a Function Only Decoder

NEXT18 multifunction decoders may be used in an unpowered (unmotored) vehicle for the purposes of controlling functions; in this instance the decoder must have the necessary means of generating the required Service Mode Acknowledgement defined in S-9.2.3.

Web Links

• S-9.1.1.5 Next18 Next18-s Decoder Interface.pdf
• TN-9.1.1.5 Next18 Next18-s Decoder Interface.pdf
• TI-9.1.1 DCC Interface Specifications

These links are in German:

• Normblatt RCN-118 des Verband der Hersteller Digitaler Modellbahnprodukte (VHDM)
• Normblatt 662 der MOROP

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.)

Purpose

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

Description

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.

Choosing the best Decoder Format for your Model

Just like anything else, model trains evolve over the years. The same model made in different years may look completely different on the inside; therefore, the decoder installation process may be completely different. You may even want to use a different decoder format. In this video, George uses three different Atlas 4-axel GE models to demonstrate how the decoder installation process changes for each.

Additional Reading

• See the NMRA Standard S-9.1.1 for more information on connectors used to create various locomotive interfaces.
• Sources for Connectors for DCC Decoders
• MOROP NEM 651 Standard (in German) defining the European 6-pin DCC interface.
• MOROP NEM 652 Standard (in German) defining the European 8-pin DCC interface.
• MOROP NEM 654 Standard (in German) defining the European 4-pin DCC interface.
• MOROP NEM 658 Standard (in German) defining Plux22, Plux16, PluX12, and PluX8. (Google translation to English)
• MOROP NEM 660 Standard (in German) defining the 21MTC Interface. (Google translation to English)
• MOROP NEM 662 Standard (in German) defining the Next18 Interface. (Google translation to English)

Adapters

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.

Features:

• 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

ESU

ESU offers a number of interface adapters which allow modern multifunction decoders to be used in older locomotives:

• 21MTC Interface Part # 51967 has a socket for plugging in a multifunction decoder, with solder points for connecting to the original wiring of the locomotive.
  • 51968 or 51957, adapter board with transistor amplified outputs AUX3 to AUX10.
• 51958, adapter board for PluX22
• 51999, Adapter board Next18 for 6 amplified outputs. With soldering pads and wires

ESU Adapter Boards