I have an HP 7470A plotter that i originally bought to go with my HP-85 computer. It worked ok with the HP 85, but wasn’t fully supported by all the software because it was a serial model rather than the HP-IB version.
The HP 150 seemed happy to allocate the plotter to the second serial port and the manual for the HP 150 Graphics package had instructions for the HP 7470A on a serial port. It uses hardware handshaking – I tried without and it gave errors.
The instructions said to use an HP 13242G cable. The cable I had didn’t have enough conductors, so I made a cut-down version with just the signals supported by the plotter (see technical manual) and to give symmetry.
The formatting of the manual is a little messed up, but the HP 150 has to be set up just right. Go to MS-DOS command prompt. Once there, the machine can be put into the built-in configuration.
I stuck with 2400 baud for compatibility with my notes on the HP 85.
It worked really well!