Lyndel, thanks for the input. I'm stuck in the process.
I've got a USB to serial adaptor that worked on the XP machine I had.
But the Windows 7 and 10 machines....
I've got a new Thinkpad X1 YOGA with Windows 10. Under PORTS it's got COM 5, labeled Communications Port, has an icon that looks like a serial connector, and has a yellow triangle with ! inside. I changed that port to COM 1 and there's no change.
I've got a Dell Latitude E5400 with Win 7 and it shows no COM ports at all.
Is there a general set of things to try to make these work?