The Regina and Bosch systems both return a 111 code when you test them. I have run into a LOT of Volvo product with bad diagnostic readout units, usually caused by corrosion. While it is possible that there is no signal TO the diagnostic unit, it is just as likely that either the connection in the terminal, or the connection TO the unit from the EFI computer, is bad. You should try cleaning those connections, and if necessary you can jiggle the connections until you DO get proper readings. I've never had it where I wasn't able to at least read codes and reset the computer, although sometimes it was when I was squeezing the connections into the diagnostic readout unit, or jiggling them around. Whenever a Volvo came in with a Check Engine light on, they gave it straight to me because I could get some things working that others couldn't.

Still, with a Bosch unit you might be looking at a bad EFI puter. The 89s were notorious for that but some of the 90s suffered just as badly.
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1992 940 wagon, 77k
as well as others.