Sinumeric 810M - Odd RS232 behavior

My first post here...

I am working on an ancient machine that I worked on ages ago and have been a professional photographer for the 15 years (Charles Waugh )
I was called in b/c the machine lost it's battery and died and I'm the only guy in town who can possibly fix it.

Here we go:

Control will LISTEN to only one Win 7 PC.
Won't TALK to it - only allows upload of data to control.

Can't get any other PC to talk OR listen to it.

1200 Baud 7E2 xon/xoff
Existing cable is 3-wire NOT a null-modem, with ground wire as well (this LISTENS to the one PC only)

I'm using the port on the front of the control.

I've tried an old PCMCIA to serial 25-pin adapter on a Win 7 machine
and a USB to 25-pin serial adapter (new this week!) on a Win 10 machine.
No joy on either. Neither will talk or listen to the control.

I'd really like to backup the R-params, etc. before I scrub it and re-commission using an old backup (that I didn't do, but I mostly trust the guy who did it [now retired![)

Any thoughts?

BTW: tomorrow I will try setting the control to hardware control as a last-gasp attempt (I *think* the USB-to-serial can be set to null modem happily, though the documentation is, er, lacking)

Thanks!