21 pin MTC Breakout Board Wiring

[Jack Maney]

There was a topic here a few years ago regarding 21 pin breakout boards and that was a major game changer for me. Hard wired installs are a pain in the butt, especially when it comes to dealing with soldering resistors into the wire paths and then figuring out where to run all the wires. Both of the ones I've done so far turned messy fast and sucked up many hours of time. Finding out I can use a breakout board for a 21 pin connector streamlines the whole process!

I've gotten my hands on a few of those breakout boards and most of it is self-explanatory. I only run into issues figuring out what to do with a couple of the wires, specifically the white Input 1 wire and yellow Negative wire from the 6-pin side of the LM-3S.

On the breakout board there are two pads that I am not totally certain how to use, labelled CAM and GND. CAM looks to lead to pin 1 of the 21MTC, while GND leads to pin 20.  I couldn't find detailed instructions from either the laisdcc site I bought my boards from or from TCS, who makes a nearly identical product. I'm pretty sure the white Input wire would go to CAM, though I don't think it would be used since I don't have any sensor inputs planned. The yellow Negative is a bit of a head scratcher though. If the negative for the Keep Alive goes to ground, where does the Keep Alive need to be hooked up to on the motherboard? Do I just need to bypass the board and wire the keep alive directly to the LM-3S?

[G8B4Life]

Ahhh, the mess that is 21MTC. A decoder manufacturer invented it; the NMRA decided they knew better and changed it and IIRC changed back to the European standard for it. Anyway, one thing you'll need to be careful of is, is the motherboard designed for fully amplified outputs (aux 3, 4, 5 and 6) or logic level outputs for aux 3, 4, 5 and 6.

Other than that, to address the questions, if you not going to use an input then I wouldn't bother hooking that wire up. As for the ground, there's no telling how that's utilised on the motherboard without tracing it out. You may be much better off not hooking the negative from the LM to the breakout board and just connecting straight to the keepalive.

- Tim

[Jack Maney]

Thanks, Tim! In posting this question to several forums, answers have been pretty sparse. Thankfully, this install on this locomotive (the latest run of Athearn Genesis SD60Ms) has contact pins between the shell and frame so there are no wires tethering one to the other. I was able to route the Keep Alive through one of those pins and maintain the separation while just directly running the wire to the LM3S instead of through the breakout board. Might not be quite as successful on subsequent locomotives that lack that feature, but it seems to have gotten me through this one at least.