I feel like a computer is incomplete without a serial line, so i procured a HP82939A Serial Interface. It requires the I/O ROM for operation.
The serial properties are set by DIP switches inside the package, so it’s a screwdriver job. I used 2400 baud, 8 bits, 1 stop bit, no parity.
Then I set this back to defaults to appease the terminal program which insisted on factory defaults: 7 data bits, 1 stop, 300baud.
I left the plotter at the original settings, so the initialisation program needs to program the serial port via CONTROL statements. (See I/O ROM Manual)
The device number is 10 which is the default.
It worked fine with the plotter.
The Data Communications Pac includes a terminal program, TERMEM. The comms parameters can be changed via the soft keys.