Aaaaaarrrrrgggggghhhhhh!
Spent 2-3 hours last night trying to get an RS232 connection going to either the HP 6611C and/or the HP 66312A. Much banging of head against wall, several variants of 'null modem' cable tried, undocumented error messages on the HPs found, nothing worked. Gave up and went to bed.
Tried again today, this time getting out scope and putting it in logic analyser mode. Struggle to get decoding working, eventually realise that the USB<->RS232 adaptor I'm using is putting out something that looks like async serial but the contents, except the start and stop bits, bear no relation to what is being send but send the same rubbish consistently. Swap adaptor for another, all works; with the first null modem cable I tried previous no less.
Sometime between its last use (which was completely successful) and now, the first adaptor had failed in a way that was subtle enough to not show up as any of the basics you check first - e.g. voltages, baud rate (it was setting that properly), framing, polarity, bit order.
Not a happy bunny!