MTC21 / NMRA / NEM decoders

This is an interesting subject, as the concept of "logic level" outputs on decoders is still relatively new to US users.

The impetus for this page is a friend, who is an installer, had questions about interfacing to a SoundTraxx decoder, and he said, "I hooked up lights to the AUX outputs, and they all went on and I could not turn them off. I responded that perhaps (since it was a 21 pin interface, which I know originated in Europe, they could be logic level outputs. What's that was the response, and the journey started.

Overview:

The history of this decoder interface goes way back. I don't feel like researching all the dates, but there was an NEM standard 658 interface called "Plus-X" with a dual row 21 pin connector, and it was related to an NMRA standard, which still exists, but has proven to be a Cul de Sac.... going nowhere.

Recently SoundTraxx started making decoders that met the NEM standard (European NMRA sort of) and as of about 2024, the standard has changed, and finally the NMRA standard is harmonized with the NEM standard. Unfortunately, many, no make that MOST decoder manufacturers just say "meets NEM standard". BAD!

I'm concentrating on just ONE standard for 21 pin decoders, the MOROP/NEM 660 standard, also called MTC21, which is harmonized with the NMRA 9.3.1.1 standard. I'm not interested in the older and even more messed up stuff.

Links to these standards:

NMRA 9.1.1.3 MTC21 https://www.nmra.org/sites/default/files/standards/sandrp/DCC/S/s-9.1.1.3_21mtc_decoder_interface_3.pdf

NEM 660: https://www.morop.org/images/NEM_register/NEM_E/nem660_en_2022.pdf

 New concepts:

For most people, the ability to "change" what a decoder pin does is not new, but note that this is becoming more prevalent, so note that as you review.

The biggest "new" thing is that many "outputs" are now "logic-level" that matches TTL computer logic voltages on digital logic chips. You can no longer just hook lights to ANY decoder output.

Outputs: (normal/amplified)

First a little explanation: Decoder "outputs" are usually really an "input" from the sense of electrical conventions. In the old days (pre-digital logic), ON was the presence of positive voltage, and ground and minus were the same. So if you put a voltmeter on an "output", when you turn it on, you got voltage (as reference to ground). The current flows "out of" the decoder.

But, when we started getting in gear with DCC decoders, all of a sudden things were reversed. You used the blue wire which was actually positive, hooked up your lamp/LED, and the other end went to the decoder "output". To turn the lamp on, the decoder actually GROUNDS that pin.

Why? A result of the process of making integrated circuits... turns out that it's cheaper and easier to make the open collector NPN transistor than the "positive switching" PNP transistor... actually pre-dates IC's, actually cheaper to make standalone transistors in NPN.

So all of our "outputs" are actually "inputs" that are grounded when active. So most people are used to this, know the blue wire is common, and some even realize it is positive voltage. Just part of understanding how things work.

Enter logic-level "outputs":

So manufacturers realized early on that sometimes you connected something smarter than just a light bulb to a decoder. Probably the best first example was the SUSI bus, for sound units, when no motor decoders had sound in them... the SUSI bus allowed communication for configuration and data like motor speed or load to be communicated.

"logic level" signals were used, there were variations, but today it means "TTL signal levels", which are the dreaded 1's and 0's of digital communication. a zero is <=0.4 volts, and a one is >= 2.4 volts. So these outputs clearly do not supply "on and off" as a light bulb would want to see it.

Conversion of logic-level "outputs" to the familiar decoder outputs, a.k.a "amplified" outputs.

So since logic-level outputs are defined, there needed to be a term for "normal", connect to ground outputs, and the term is "amplified" outputs. It's really a misnomer, but since it does not belie the radically different characteristic of the output, but "amplified" it is.

How to convert "logic level" to "amplified"? Simple, you use a transistor as a "power switch". This is actually what is already done in most decoders internally. The logic output is connected to the base of a transistor (that is the transistor's "control pin"), and the emitter and the collector are the 2 parts of the "switch", one goes to ground, the other is your new "output" pin.

In fact there are commercial products to do just this for your decoder with logic levels when you want to run a bulb, or LED or actually anything that needs power.

OK, OK, enough lecture, get to the meat!

The following chart shows a "translation" from the decoder pins to the actual function/description on the decoder:

Deviations from the 9.1.1.3 / NEM 660 will be highlighted in yellow

I've selected 4 current/representative decoders:

• TCS Wow121 decoders - https://drupal.tcsdcc.com/pinout#21,  https://52b7f1f3-d14f-4a29-9386-16e084d54015.usrfiles.com/ugd/52b7f1_06948ecc54324a58bcfdc083df8af2bc.pdf
• SoundTraxx BLU-21PNEM8 / TSU-21PNEM8  - https://soundtraxx.com/content/Reference/Manuals/Tsunami2/Installation-Guide.pdf Note: just a pinout, NO descriptions, and NO reference to whether this meets the 9.1.1.3 standards or not, really crappy documentation. The note says this is an NEM decoder, and not an NMRA decoder... this must have been written in 2017 or before. 
• ESU Loksound 5 - 58419, 58429 and 58449 ESU 21MTC decoders (still trying to narrow this down)  58429:  https://www.esu.eu/en/downloads/instruction-manuals/digital-decoders/
• Zimo MS440C / MS440D - 

LL = logic level

Converters / aids for logic-level conversion

 There are a number of solutions, but I'm only going to be specific on one solution, and really the best one in my opinion.

Decoder Buddy

There are 2 models that will convert logic-level outputs to "amplified" outputs, the V1 and V5

(Pictures are to follow)

The V1 has "amplification" on 4 pins:

 (the parenthesis refer to the description for NEM 660 / NMRA 9.1.13 specifications)

• 13 (AUX 3)
• 4 (AUX 4)
• 17 (AUX 5)
• 3 (AUX 6)

Note:

Note that I am using the new standards, so if your decoder does not conform, then you need to check which pin has an "amplification" transistor, and use that output

Most decoders are not perfect in following the standard, so they were probably designed to the older NEM / NMRA standards.

SoundTraxx in particular renumbered the AUX "sequence", and the mapping is not easy to find, they have the worst documentation.

this should give a total of 8 "amplified" outputs ( 4 from DB, and 4 stock)

The V5 has "amplification" on 8 pins:

(the parenthesis refer to the description for NEM 660 / NMRA 9.1.13 specifications)

• 1 (AUX7)  (according to the standard, used as AUX 7 is already "amplified")
• 2 (AUX 8) (according to the standard, used as AUX 8 is already "amplified")
• 3 (AUX 6)  
• 4 (AUX 4)
• 5 (AUX 9) 
• 6 (AUX 10)
• 13 (AUX 3)
• 17 (AUX 5)

Note:

note that I am using the new standards, so if your decoder does not conform, then you need to check which pin has the conversion transistor, and use that output

Most decoders are not perfect in following the standard, so they were probably designed to the older NEM / NMRA standards.

Note that pins 1 & 2, if the decoder meets the standard, should already be "amplified", but this does no harm.

apparently stock, there are 12 2.2k resistors, so that covers the 8 amplified (from DB) pins, and the 4 pins that are amplified by the decoder "stock" (pins 7,8,14,15), so properly set up, you have up to 12 "amplified" outputs using a decoder and DB V5

The bottom line:

You need to be SURE of what standard your decoder is following, and double check their literature.

Then relate their "name" to a pin number... not ONE decoder at this time (August 2025) is perfect in matching the standard.

The Decoder Buddy is a great help, saves space and time and frustration.

As time goes on, I would assume everyone will offer decoders that meet the standards, which should reduce the confusion and effort to be sure of what is what.

But, I doubt that ALL the logic level outputs will ever have on-board optional amplification, thus Decoder Buddy is indeed your "buddy"!

 Notes on TCS Wow121 decoders:

Specified only as "21 pin MTC", 8 function, including f/r headlights. Meets standards well, only missing one sensor input, and the optional AUX 7,8,9,10

4 amplified outputs, 4 logic level outputs.

Notes on SoundTraxx Blunami/Tsunami 2 decoders

Oh my God! Absolute worst documentation, and no clue of what pin does what. The installation and wiring guide says:

21-Pin NEM connector interface, no mention of WHAT NEM standard they meet (which turns out to be NEITHER)

Also, "SoundTraxx 21-Pin NEM decoders are designed for use with factory supplied motherboards which support the NEM specifications. Our decoders are not
intended for use with 21-Pin NMRA standard sockets.

Seriously WTF? They cannot update their documentation to reflect NMRA standards, or NEM standards, or what standard IF ANY they meet? The only documentation is reflected in the chart above.

The 2 sensor inputs are NOT SUPPORTED

It is 8 function, BUT for some reason SoundTraxx renumbered F1-F6 to F3-F8 (why?)

No indication on the site which outputs are amplified and which are logic level. I would assume same, F0f, F0r, F3, F4 amplified, F5-F8 logic, but need verification.

There's references that FX5-8 configuration is not on ALL decoders, no telling which!

Terrible documentation, lack of information, renumbering outputs away from standards, and specifying they do not meet NMRA standards.

Notes on ESU decoders:

There is a lot of variation on the 21 pin decoders, 58419, 58429, 58449, look carefully when you choose. There's more differences than just what outputs are logic level.

In any case, for some reason, ESU has weirdly renumbered the outputs, but this must be a leftover from the previous standards.

More flexibility and more outputs than the TCS or SoundTraxx decoders.

Note there are more functions on the decoder solder pads, so if you use just the 21 pin interface, you have up to 12 outputs, 14 if you use the solder pads. AUX 8 & 9 only on the solder pads.

Some of the solder pads are replication of an output pin, but amplified. Pretty nice, but sort of blows up using the socket.

 Notes on Zimo decoders:

Overall, the most compatible of the lot, and the most flexible, with up to 4 inputs, regulated 5 volts available, etc. (I'm a fan, sorry!)