21MTC Connector

21MTC Interface

​The 21MTC interface allows a decoder with appropriate socket to plug directly onto a compatible header within the model. This eliminates the traditional cable connection, which is convenient for both decoder manufacturer and user, but is not so friendly when installing battery power and radio control. It is reasonable to expect decoders using 21MTC to be less expensive than decoders with wire harnesses. It also requires more headroom than decoders with JST 9-pin connectors, which will be helpful if manufacturers increase clearance above motor mechanism.

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S-CAB Radio Receivers with 21MTC Header

S-CAB's MB-GP-NEM motherboards (both long and short versions) support 21MTC decoders, but are intended for use in HO scale diesel locos. Adding a 21MTC connector to an S-CAB radio receiver provides an option to choose less expensive and/or interchangeable decoders as well as installing battery power in a broader range of models.

Interface Description

NMRA standard S-9.1.1.3 lists pin assignments on page 6, Dec 1, 2020 version. Although it is best if a  user skips the details, 3 pins are used to implement a direct radio connection (DRC) to the decoder.

• Pin 16 is decoder positive (positive common)
• Pin 20 is decoder ground
• Pin 12, which is not mandatory and unlikely to be used by decoder manufacturers, is used for S-CAB radio data connection.

Interchangeable Decoders

Connecting radio data through the NEM socket improves modularity by allowing decoders with DRC to be removed, replaced, or changed without involving the receiver. The following decoders are available with DRC:

• NCE D16MTC
• SoundTraxx ECO-21PNEM
• SoundTraxx TSU-21PNEM

External Connections

The 9-pin JST socket is consistent with NMRA standard. Additional wired connections are provided for speaker, and functions F5 and F6.

Pin Cross-reference

The following table lists MTC connections used by LXR-NEM. "NMRA Name" corresponds with NMRA standard but manufacturers use their own terminology. Most NCE decoders provide four function outputs; front and rear light, F1 and F2. SoundTraxx NEM decoders have six function outputs; front and rear lights, FX3, FX4, FX5, FX6.

Application

The LXR-NEM radio receiver replaces factory-installed circuit boards and provides radio control for models with on-board battery power. Track power must be disconnected from decoder's rail inputs (red and black leads) before connecting battery power. Although there is no benefit, radio control can be used with track power (DC or DCC), in which case, decoder rail inputs remain connected to track pickups.

If 21MTC becomes popular, models with factory-installed circuit boards and 21-pin headers can be anticipated; with or without a decoder. If included, a factory-installed decoder is connected to operate with track DCC power. A decoder using direct radio connect (DRC) will not respond to DCC from the rails. Consequently, a factory-installed circuit board with 21-pin header is not helpful and best if replaced with either LXR-NEM receiver or MB-GP-NEM motherboard when using a decoder with MTC socket.

What happens if a decoder with S-CAB DRC is plugged into a factory-installed 21-pin header?
Short answer: Nothing. The decoder will have power from wheel pickups, but no control.
Will it be damaged? No.

Will it Fit?

S-9.1.1.3 defines maximum dimension of MTC decoders:
Length: 30 mm (1.18"), Width: 15.5 mm (0.61"), Height: 6.5 mm (0.26")
LXR-NEM is 2.4 mm (0.1") thick.
If mounted horizontally, allow 8.9 mm (0.35") vertical clearance.

Will MTC Decoders be Useful?

Pro

• Plug compatible and interchangeable.
• Maximum dimension is defined by standard.
• Potentially less expensive decoder.
• Decoder works well with S-CAB motherboard.

Con

• Vertical clearance may be a problem.
• LXR-NEM offers no benefit if model has factory-installed MTC header.

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