Rather confusingly, J1 on the main board does not connect to the J-1 ("RS232C") on the back of the unit. Similarly for J2 and J-2 ("GPIB IEEE-488").
In fact J1 on the main board acts as a switch to select EPROM U1 to act as either RAM or ROM.
I'm not sure what J2 on the main board is for, but GPIB (IEEE-488) should have 24 pins.
Looking at your images of the main board, I am undecided whether J3 ("Aux Interface") and J4 ("Interface") or J5 ("Expansion No.2)" and J6 (Expansion No.1") are the ones to dig into for RS-232 and GPIB. I'm guessing J3 and J4.
But it would be nice to get GPIB going, as (presumably) you could then control your RT-200 remotely using the
SCPI "language" (Standard Commands for Programmable Instruments; usually pronounced "skippy").
By the way, I notice that chip numbers are still visible on many of the (smaller) chips, but I cannot quite make them out. If you could list them for us, that would be nice. For instance, U8 and U14 adjacent to J2 should give clues to what J2 is meant for.
