It's all reasonably straightforward, but I did waste over a day because I had inadvertently connected some of the wires out-by-one.
Oh, how I hate that.
Well, this will make you laugh ...
This time I'm trying to get high-voltage
serial working. Less wires, so less chance of getting them wrong, right? Plus I knew I had to triple-check my wiring.
Still, after hours of testing, no meaningful results. It even seemed like my Uno might be damaged because at least one signal wasn't even making it to the target chip. This was possible because I may have inadvertently put 12v into it through the pull-up resistor on /RESET.
However, no, it wasn't any of those things. One of my hookup wires was
open-circuit! Grrrr. You don't expect a length of wire to "fail".