Well I began to realize that "Coil" could be read and written ok. Well I can read 8 values, and write the bits and read them back and there's no errors.
However, once I try to Read or Write a Holding Register, I get either "Receive Timeout" or "CRC Error", sometimes "Illegal Address". And I do have Receive Timeout upped to 200mS.
Once I've gotten that error, Read or Write to a Coil register becomes unreliable for a few minutes, prone to "Receive Timeout".
I have seen the Holding Registers read successfully a few times.
I'm also getting a problem where- usually after hitting Read or Write a few times in succession, but not rapidly, per se- I get a lockup inside of Mach3 and a Windows msg that Mach3 has crashed. Then Mach3 can't reload again. It brings up the Loader screen and it acts like it's gonna load Mach3 but never does. And Mach3 wasn't hanging around as a zombie process, Windows Task Manager doesn't show Mach3 as still loaded before then.