The connector determines the mechanical fit — not the protocol. Overview of NEM651, NEM652, PluX8/12/16/22, 21MTC and Next18.
There is a common misconception: the connector on a decoder does not determine the protocol (DCC, Motorola or mfx). The connector is purely a mechanical interface that joins the loco circuit board to the decoder board. Which protocol the decoder speaks is determined by the decoder itself — independent of the connector.
A NEM652 connector can be a DCC decoder, a Motorola decoder or an mfx decoder. The connector says nothing about the protocol. NEM (Normen Europäischer Modellbahnen) is the European standards body that specifies the mechanical dimensions and pin configurations.
The choice of interface therefore depends on the socket already in your loco — not on the command station or protocol. Want to know which protocol you need? See the loco decoder guide.
| Interface | Pins | Pitch | Scale | Functions |
|---|---|---|---|---|
| NEM651 | 6 (1 row) | 1.27 mm | N, TT, small HO | Motor (2), track (2), front light (1), rear light (1). No aux. |
| NEM652 | 8 (2×4) | 2.54 mm | HO/OO standard | Motor (2), track (2), common return (1), front (1), rear (1), 1 aux. Most common HO interface. |
| NEM658 PluX8 | 8 (2-row) | 1.27 mm | HO and larger | Comparable to NEM652 but more compact. 1 pin = key pin. |
| NEM658 PluX12 | 12 (2-row) | 1.27 mm | HO and larger | Motor, lighting, extra aux outputs. |
| NEM658 PluX16 | 16 (2-row) | 1.27 mm | HO and larger | Motor, lighting, multiple aux, SUSI bus. |
| NEM658 PluX22 | 22 (2×11) | 1.27 mm | HO and larger | 21 active + 1 key pin. Motor, lighting, multiple aux, SUSI, speaker. Dominant for HO sound. |
| NEM660 21MTC | 22 (21 active + 1 key) | 1.27 mm | HO (Märklin/Trix) | Motor, lighting, aux, Hall sensors, SUSI, speaker. Märklin/ESU standard, 3-rail and 2-rail. |
| NEM662 Next18 | 18 (2-row) | 1.27 mm | N, TT, some HO | Motor, front/rear light, 6 aux (2 optionally serial). Compact, more functions than NEM651. |
| Hardwired | — | — | Older/small | Wires soldered directly. No connector, no swapping without soldering. |
NEM651 is a 6-pin connector with a 1.27 mm pitch in a single row. The pin arrangement covers motor (2 pins), track power (2 pins), front light (1) and rear light (1). There is no additional aux output, making it suitable for simple drive decoders without extra functions. Application: scale N (also TT and small HO models with limited space).
NEM652 is the most widely used interface in HO. The 8-pin connector (2.54 mm, 2×4 pins) provides motor (2), track (2), common return (1), front light (1), rear light (1) and one additional function output. Manufacturers such as Roco, Fleischmann and Piko use NEM652 as the standard HO interface. Adapters from NEM652 to NEM651 sockets are common.
PluX is a family of 2-row connectors (1.27 mm) with variants from 8 to 22 pins. One pin is always an anti-rotation key pin; the rest are active. PluX22 (2×11, 21 active) is the dominant sound interface in HO and larger: it provides motor, lighting, multiple aux outputs, SUSI bus (for external sound module) and a speaker connection. PluX8 is functionally comparable to NEM652 but in a smaller housing. Adapters to NEM651 sockets are available.
21MTC (21 functional pins + 1 key pin, 22 positions total, 2-row, 1.27 mm) was designed by Märklin and ESU. The interface is present in HO Märklin and Trix models, both 3-rail and 2-rail. It provides motor, lighting, multiple aux outputs, Hall sensors (for detection), speaker connection and SUSI bus. Not interchangeable with PluX22 — check the socket in your loco.
Next18 has 18 pins (1.27 mm, 2-row) and is designed for scales N and TT where space is limited. It supports motor, front and rear lighting, and 6 aux outputs (2 optionally serial). Next18 offers more functions than NEM651 in a comparably small housing. Sometimes also used in compact HO models.
The interface is in the loco — you choose a decoder that fits the socket already there. Check your loco documentation: NEM652? You need an 8-pin decoder. PluX22 or 21MTC? You need a decoder with that specific connector, or an adapter.
ModelRailPro users: the choice of interface does not change the operation from ModelRailPro — running always goes via the command station, not directly via the decoder connector. More about decoder protocols? See the loco decoder guide. Calculating CV values? Use the DCC tools.
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