The SG-811 comes with various option – mine didn’t come with the IEEE-488 remote control option. At least, it has a BCD type TTL interface. All the essential functions (band, operation mode, attenuators, and in particular, external frequency control-phase lock input enable) can be controlled via no less than 23 signals, plus ground.
All the signals are available at the rear of the instrument, via a 50-pin Centronics connector (similar to the old-fashioned SCSI connectors).
Several steps were taken to make sure that the ancient but still valuable SG-811 will carefully listen to the commands of a modern area microcontroller:
(1) Fabricate a suitable connector cable. Centronics 50 to D-sub 25. Starting from a pre-assembled D-sub 25 1:1 cable, cut in half, the Centronics connector was soldered on. Quite an effort! Turned out that the 1:1 cable uses pretty thin wire – they are saving on copper, over there, in China!
(2) A little shift register, 3×8 bits (3x 74LS164) – a total of 24 wires that can be controlled. 3 of these wires will be used to select the band of the 1295 receiver (via optocouplers, PC817), the reminder, via direct TTL connection, for the SG-811. The shift registers will later be set by a microcontroller, just using 2 outputs to set 24 wires.