An odd issue cropped up here today, and after chasing it down, I thought others might be interested in the solution.
I'm installing a Cruise Commander M and the new ERR RailSounds Commander in a diesel. I'm specifically using the CC-M so I can use the ditch light feature, I used a simple motherboard for the R2LC and just custom cabled it.
All was going just great until it came time to do the first test! I fired it up, programmed it, and flipped the switch to run. Everything seemed fine, sounds worked, couplers worked, so I cracked the throttle. AWAY WE WENT AT FULL SPEED! OK, that's not good. I tried programming it again, and yes I did set the locomotive type! Same issue. Swapped in another CC-M, no change, new R2LC, ditto.
Time to break out the 'scope and see what's going on.
When I plug in the new ERR RS Commander, it really attenuates the serial data signal, it's less than 3V peak. Apparently, the CC-M has an issue with this and doesn't see the serial data. Mystery solved, but what do I do to fix it?
Enter the R2LC Serial Data Buffer. I added an emitter follower buffer to isolate the serial data from the RS Commander, problem solved. I technically didn't need the 470 ohm resistor I suspect, but I have an aversion to putting a potential high current source on an input, and it's cheap enough to add. I tapped the 5V and serial data from the R2LC, and I added the 2.54mm connector for the serial data to the RS Commander. This allows me not to modify the cable itself for the RS Commander, and I can also easily disconnect it.
