I confirmed with a voltmeter that the HV can be turned on/off, enabled/disabled, and various values loaded. So, this setup works now on hacweb5. That's nice.
Next, I tried to transfer this to one of the adaq computers. I tried the serial connection on adaql1,2,4,5. None of them worked.
As a test, I tried using minicom (which I installed and set up) to get one adaq computer to talk to another; here I tried both NULL modem and straight-through connection. No pair of adaq computers talk via /dev/ttyS0.
Will keep trying tomorrow.