NEM 651
Six contacts are arranged in a straight row. The decoder may plug in directly, at an angle or via a cable.
Typical For
Gauges N, TT and older compact H0 vehicles.
Identify connectors, contact counts and function ranges reliably
Comparing Decoder Interfaces: NEM, Next18 & PluX
NEM 651, NEM 652, Next18, PluX22 and 21MTC look different and are not electrically interchangeable. This comparison shows which interfaces are used in which gauges and what to consider when installing a decoder.
The mechanical and electrical connection between locomotive and decoder
What Is a Decoder Interface?
A decoder interface connects the vehicle circuit board to a locomotive, sound or function decoder. It carries track voltage, motor connections and lighting outputs and, depending on the version, additional functions, loudspeaker connections, energy storage or data lines.
The interface does not determine the digital protocol. A PluX22 interface, for example, can be fitted with a DCC-only decoder or a multiprotocol decoder. The connector, pin assignment, motor output, installation space and digital format must all be compatible.
In a locomotive with an interface, the analogue blanking plug is removed and the correct decoder is inserted. However, decoders with the same number of contacts are not automatically compatible. Next18, PluX and 21MTC differ fundamentally in design, installation orientation and pin assignment.
A vehicle without an interface can also be digitised. The decoder is then connected using wires. During this conversion, the motor, current pickup and lighting must be fully checked and electrically isolated where required.
Compare the contact arrangement and connector principle
How Do I Identify NEM 651, Next18, PluX and 21MTC?
The schematic illustrations provide an initial guide. The vehicle instructions and markings on the locomotive circuit board remain authoritative.
NEM 652
Eight pins are arranged in two rows. A marking normally indicates the position of pin 1.
Typical For
Older and simply equipped H0 locomotives.
Next18 and Next18S
A very small enclosed connector with contacts on both sides. Next18S accommodates sound applications.
Typical For
Gauges N, TT, H0e and compact H0 vehicles.
PluX12, PluX16 and PluX22
The pins are located on the underside of the decoder. The vehicle circuit board has the matching socket strip.
Typical For
TT and H0 vehicles with different levels of equipment.
21MTC
The pins are normally located on the vehicle circuit board. The decoder is fitted over them using its socket strip.
Typical For
Feature-rich H0 models and many Märklin conversions.
E24
A compact newer interface with 24 electrical contacts and additional guide positions.
Typical For
New compact vehicles in N, TT and small H0 gauges.
Compare design, function range and application
Decoder Interfaces in the Full Comparison Table
| Interface | Contacts | Identifying Feature | Typical Gauges | Strengths | Important Considerations |
|---|---|---|---|---|---|
| Wires | Depending on the decoder | Coloured connecting wires without a vehicle connector | Z to G | Flexible for older vehicles | Correctly isolate the motor and lighting |
| NEM 651 | 6 | One straight row of pins | N, TT and older small H0 | Compact and simple | Distinguish direct, angled and wired versions |
| NEM 652 | 8 | Two rows of four pins | Mainly H0 | Widely used classic standard | Check pin 1 and the available decoder height |
| Next18 | 18 | Small enclosed connector | N, TT, H0e and small H0 | Compact and more versatile than NEM 651 | Check the installation space and output assignment |
| Next18S | 18 | Mechanically closely related to Next18 | N, TT and small H0 sound models | Sound connections in a compact design | Do not replace it with Next18 without checking |
| PluX12 | 12 | Two short rows of pins on the decoder | TT and small H0 | Compact entry into the PluX family | Fewer functions than larger PluX versions |
| PluX16 | 16 | Two rows of pins on the decoder | TT, H0 and sound conversions | Good compromise between size and functions | Distinguish PluX16 and PluX16S precisely |
| PluX22 | 22 positions | Long two-row pin strip on the decoder | Mainly H0 | Numerous lighting, sound and auxiliary functions | Do not confuse it with 21MTC |
| 21MTC | 21 contacts in 22 positions | Pins on the locomotive board, sockets on the decoder | H0 and some Gauge 0 models | Numerous functions, sound and energy-storage connections | Distinguish logic-level and amplified outputs |
| E24 | 24 plus guide positions | Very small enclosed connector | N, TT and small H0 | Compact with numerous possible outputs | Not yet common in every vehicle |
| PluG | Depending on the version, 16 to 44 | Multi-part large pin headers | 0, 1, G and IIm | High currents and numerous large-scale functions | Document the vehicle and decoder pin assignments |
Important:
The current capacity of the contacts is not automatically the current
rating of the decoder. The motor current, function current and total
load must always be taken from the specifications of the specific
decoder.
Narrow down the interface from the installation situation
Which Decoder Interface Fits My Vehicle?
The finder does not replace a visual inspection:
The same locomotive type may have been supplied with different
interfaces depending on its production year. The circuit board
actually installed must be checked before ordering.
Similar contact counts do not mean interchangeability
The Most Important Differences in Detail
NEM 651 vs Next18
Classic Pin Row or Modern Compact Socket
NEM 651 has six pins and covers the motor, current pickup and driving lights. Next18 provides more contacts for additional functions and requires a matching enclosed socket.
NEM 652 vs PluX22
Basic Functions or Extensive Vehicle Equipment
NEM 652 is a classic eight-pin connection. PluX22 provides considerably more connections for lighting, sound, energy storage and auxiliary functions.
PluX22 vs 21MTC
Completely Reversed Connector Principle
With PluX, the pins are on the decoder. With 21MTC, the vehicle circuit board carries the pins. A mechanically improvised conversion is not permitted without a suitable adapter board.
Next18 vs Next18S
Check the Sound Version Specifically
Both variants use a compact 18-pin connection. Next18S, however, accommodates sound connections and a larger intended installation space.
PluX12 vs PluX16 and PluX22
More Contacts Enable More Functions
The PluX family uses a common basic principle. Smaller decoders cannot, however, provide all functions intended on a larger vehicle socket.
E24 vs Next18
New Compact Interface with More Possibilities
E24 was developed for small installation spaces and a large range of functions. Decoders, adapters and vehicles must be explicitly designed for E24.
Understand older solutions, adapters and auxiliary connections
Special Forms Are Not Automatically Main Interfaces
Direct Decoders
Direct decoders plug straight into a vehicle socket. With NEM 651 in particular, distinguish between straight and angled pin arrangements.
Check
Free space beside and above the socket.
Decoders with a Cable Connector
A cable between decoder and connector makes positioning easier but requires additional space and must not be trapped between the body and chassis.
Check
Cable length, bending radius and secure mounting.
SUSI
SUSI is an auxiliary connection for sound or function modules. It does not replace the main connection between the vehicle circuit board and locomotive decoder.
Check
Support by the decoder and auxiliary module.
Manufacturer-Specific Boards
Some vehicles use proprietary plug-in boards or decoder shapes. A standard decoder can then only be used with an expressly compatible adapter board.
Check
The locomotive product number and production series.
Interface Adapters
Adapters can change the mechanical connection. They do not automatically resolve differences in output type, current rating or function assignment.
Check
The complete pin assignment of the adapter used.
PluG for Large-Scale Railways
PluG is intended for large vehicles with high currents and extensive equipment. Its size can vary according to the required number of contacts.
Check
Motor current, smoke, sound and vehicle documentation.
Vehicle size determines space requirements, not the interface alone
Decoder Interfaces by Gauge
Gauges Z and N
Wires, NEM 651, Next18 and increasingly E24 are particularly space-saving. Decoder height and free space above the circuit board are decisive.
Typical Focus
Micro and nano decoders with low motor output.
Gauges TT, H0e and H0m
NEM 651, Next18, PluX12 and PluX16 are common solutions. Some modern TT vehicles already use PluX22.
Typical Focus
Compact design with several lighting functions.
Gauge H0
NEM 652, Next18, PluX16, PluX22 and 21MTC are widely used. The production year and manufacturer determine the actual version installed.
Typical Focus
Wide choice of locomotive, sound and function decoders.
Gauges 0 and 1
21MTC, larger PluX solutions, PluG or free wiring may be used. The motor current must be checked independently of the interface.
Typical Focus
Numerous functions and powerful sound equipment.
Gauges G and IIm
Large-scale decoders are often connected using screw terminals, wires, manufacturer-specific boards or PluG.
Typical Focus
Several motors, smoke generators and high total currents.
Cross-Gauge Rule
The gauge only provides an initial indication. The socket actually installed, locomotive circuit board and mechanically available installation space are always decisive.
Safest Method
Open the locomotive, photograph and measure it, and compare the instructions.
From blanking plug to verified digital operation
Install a Plug-In Decoder in Six Steps
Step 1
Test the Locomotive in Analogue Operation
The motor, current pickup, gearbox and lighting must operate without faults.
Step 2
Open the Body Carefully
Release clips, screws, wires and body components according to the instructions.
Step 3
Document the Interface
Photograph the socket, blanking plug, pin marking and installation space.
Step 4
Insert the Decoder with the Power Off
Never tilt it, press on components or work while the command station is switched on.
Step 5
Use the Programming Track
Read the decoder address and test the motor and lighting first.
Step 6
Test the Functions Individually
Check auxiliary outputs, sound, energy storage and direction-dependent lighting.
Avoid confusion and electrical damage
Common Decoder-Interface Mistakes
Counting Only the Contacts
Equal or similar contact counts do not mean that the interfaces are identical.
Confusing PluX22 with 21MTC
The pin positions and electrical assignments are different.
Using Next18 in Next18S Without Checking
The sound version and intended installation situation must be considered.
Ignoring Pin 1
With connectors that are not polarised, an incorrect orientation can damage functions.
Not Measuring the Decoder Dimensions
The correct interface does not guarantee sufficient installation space.
Treating an Adapter as a Universal Solution
Unassigned or differently implemented functions remain problematic despite an adapter.
27 decoders organised by connector system
Suitable Decoders for NEM, Next18, PluX and 21MTC
The product selection includes locomotive, sound and function decoders for different gauges. Before ordering, compare the interface, decoder dimensions, motor current and function assignment with the vehicle.
Wires and NEM 651: Compact Solutions for N and TT
Direct connectors and flexible wired versions for small installation spaces and older vehicle circuit boards.
NEM 651 Direct
ZIMO MN150N
Extremely compact direct decoder with six pins.
View Product →
Free Wiring
ZIMO MX615
Subminiature version for vehicles without a plug-in socket.
View Product →
NEM 651
Lenz 10311-02 Silver mini
Compact DCC decoder for small locomotives.
View Product →
NEM 651 with Cable
Tillig 66033
Flexible positioning beside the vehicle socket.
View Product →
NEM 651 Function
Tillig 66031
For lighting and auxiliary functions without motor control.
View Product →NEM 652: Classic Eight-Pin Decoders
Locomotive and function decoders for numerous older and simply equipped H0 vehicles.
NEM 652 Entry-Level
PIKO 56122
Classic eight-pin decoder for H0 vehicles.
View Product →
Modern NEM 652
PIKO 56503 PSD XP 5.1
Multiprotocol version for classic eight-pin sockets.
View Product →
NEM 652 DCC
Lenz 10231-02 Standard+
Reliable standard decoder with ABC braking.
View Product →
NEM 652 Multiprotocol
ESU 59210 LokPilot 5 Fx
For control cars and vehicles without a traction motor.
View Product →
NEM 652 ID2
Uhlenbrock 74320
Modern eight-pin decoder with feedback functions.
View Product →
NEM 652 Micro
ESU 59820 LokPilot 5 micro
Small DCC version for limited installation space.
View Product →Next18 and Next18S: Compact Modern Interfaces
Locomotive, function and sound decoders for Gauge N, TT and small H0 vehicles.
Next18 Locomotive Decoder
ESU 59818 LokPilot 5 micro
Compact multiprotocol decoder.
View Product →
Next18 Function
PIKO 56518 PSD XP 5.1 F
For control cars and lighting circuit boards.
View Product →
Next18 Model-Specific
Tillig 66037
For selected Tillig H0e vehicles.
View Product →
Next18 Sound
ESU 58928 LokSound 5 Nano
Compact sound decoder for small vehicles.
View Product →PluX12, PluX16 and PluX22
Modern plug-in decoders with different function ranges for TT, H0 and compact sound projects.
PluX22 Multiprotocol
PIKO 56500 PSD XP 5.1
Feature-rich decoder for modern H0 locomotives.
View Product →
PluX22 DCC
ESU 59612 LokPilot 5
DCC locomotive decoder for PluX22 vehicle boards.
View Product →
PluX16 Micro
ESU 59814 LokPilot 5 micro
Small PluX version for N and TT.
View Product →
PluX12
Tillig 66035
Compact plug-in version for suitable TT locomotives.
View Product →
PluX12 TT
Tillig 66034
Decoder for compatible TT vehicle circuit boards.
View Product →
PluX12 SmartDecoder
PIKO 46401
PluX12 locomotive decoder for compact vehicles.
View Product →
PluX22 Function
ESU 59212 LokPilot 5 Fx
Numerous outputs for control cars and function models.
View Product →
PluX22 DCC Function
ESU 59222 LokPilot 5 Fx
DCC function decoder with extensive mapping.
View Product →
PluX16 Sound
ESU 58914 LokSound 5 Nano
Compact sound decoder for N and TT.
View Product →21MTC: Socket Decoders for Feature-Rich H0 Models
Decoders with a socket strip for locomotive circuit boards fitted with contact pins.
Explore decoder selection and vehicle conversion in greater depth
Suitable Product Categories and Guides
Answers before ordering and installing a decoder
Frequently Asked Questions about Decoder Interfaces
Which interface does my locomotive have?
Check the instructions, packaging and circuit board actually installed. The interface may differ between production series.
Can I identify the interface solely from the pins?
Contact count is only one clue. Connector shape, pin arrangement, installation orientation and markings must also match.
Are PluX22 and 21MTC compatible?
No. They have different mechanical and electrical assignments and must not be connected directly.
Does a PluX12 decoder fit a PluX22 socket?
This may be mechanically possible with a standards-compliant configuration. However, not all functions provided by the vehicle board will then be available.
What is the difference between Next18 and Next18S?
Next18S accommodates sound connections and a larger intended installation space. The vehicle documentation must specify the actual variant.
What does NEM 651 mean?
NEM 651 is a six-pin electrical interface used especially in N, TT and older small H0 vehicles.
What does NEM 652 mean?
NEM 652 is a classic eight-pin interface carrying motor, track pickup, lighting and common positive connections.
Can I digitise a locomotive without an interface?
Yes. Use a wired decoder. The motor must be electrically isolated from the track pickup and, where applicable, from the chassis.
Is Next18 always used for Gauge N?
No. Next18 is also used in TT, H0e and compact H0 vehicles. Gauge alone does not determine the interface.
What is E24?
E24 is a compact newer vehicle interface for small installation spaces with numerous possible function connections.
What is PluG?
PluG is an interface for large model railway vehicles with higher current requirements and an extensive range of functions.
Can an adapter convert every interface?
No. An adapter can connect connector shapes but cannot automatically compensate for missing contacts, different output types or insufficient current capacity.
Technical Basis
The NEM interfaces NEM 651, NEM 652, NEM 658, NEM 660, NEM 662, NEM 663 and NEM 664 were considered.
Update Date
Content status: 17 July 2026. Check the current vehicle instructions and decoder pin assignment before installation.
Interface Identified – Choose the Correct Decoder
The main guide covers locomotive decoders, sound decoders, digital command stations, digitisation, wiring and layout planning.