Last night, while watching TV after dinner, I searched out the #Apple #Newton #MessagePad & #eMate PC serial cable pinouts and tested my cable:
https://www.nsbasic.com/newton/info/technotes/cable.html
All seems in order with no shorts or odd resistance. Good.
Next up: testing loopback connections from #OpenBSD and the eMate running #PT100 by shorting the TR/TX pins. Hopefully that'll help identify where something is going wrong with the connection.