SantaFeFan posted:John,
It may electrically work, however as I mentioned it was not designed for RS4. One item to note is the Diesel RPM function is driven by a VCO in the PIC on the Powered Motherboard, each RS version, (RS4, RS5, RS6, RSL) has their own VCO frequency to Run Level. I can assure you that RS4 run table VCO frequencies are not compatible with the Powered Motherboard (it has the RS5 tables). Steamers should be OK, if you say it works electrically.
I have standardized many things over the past 10 years, and simplified many of the designs. Still many things are not designed in CA, but in China, and the designs drift. That is not all that bad, but you do see that the folks on this forum are looking to understand the variations, which is only natural. What is really interesting is that sometimes our Engineering or Customer Service does not even know what electronics are in the products designed in China! It is a trade-off for the limited resources we can have in the USA.
As always Jon, thanks for the clear and lucid explanation of things Lionel. 
I'm planning on using the powered motherboards in a couple of Station Sounds Diner retrofits, so they should work as intended I would imagine. I just use a magnet and reed switch to trigger motion effects. I will also keep any RS6 board far away from the powered MB. 
The many variations are sometimes a mystery, and as in this example, many of us learn the do's and don'ts by empirical methods of try and fry. 
One curious question still remains.
I did install the RS6 board in a motherboard with a standard Modular power supply and R2LC, (the Station Sounds Diner MB), and it functioned as expected, no smoke. It still works fine in the Legacy T1 Duplex, so it appears it didn't suffer any grievous harm. What is different between the standard TMCC motherboards and the powered motherboard that would cook the RS6 board?
