New controller is now in place after refactoring the Motori code and adding some HPGL commands including escape sequences for querying the input buffer size and free space. The escape sequences can be used by the app sending data to avoid overflowing the input buffer.
I had an issue with the pen up/down font panel button creeping up on me that took some time to resolve, a tantalum capacitor on the front panel PCB had started leaking. It was used for deboucing the switch signal, but had the oppsite effect when leaking - it caused the signal to bounce in odd ways instead. After replacing all four tantals on the board the swiches now performs flawlessly, this includes the jog buttons.
Remaining now is making some pen holders so that the pen change code can be verified.
Original controller. There is a large heatsink on the underside of the plotter for the stepper driver transistors.
The Pi Pico based controller in place, techonlogy has come a long way since the original was made!