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 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!

So to be crystal clear, the NEM 660 standard, also called MTC21, is harmonized with the NMRA 9.3.1.1 standard.

Links:

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

(I have omitted the PLUS-X NEM and NMRA standards, it seems that the newer standards "grew" from these)

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.

Outputs:

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).

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, and the other end went to the decoder "output". To turn the lamp on, the decoder actually GROUNDS that input.

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.

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 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:

Converters / aids for logic-level conversion

Decoder Buddy