The HP 7475A wakes up with hardware handshaking enabled: DTR starts high and goes low when the internal 1 KB buffer has less than 80 bytes remaining. The plotter also supports XON/XOFF handshaking, a sad software thing you’d use only if you had no other choice.
The color codes over on the left of the top diagram match a prebuilt cable I hoped to repurpose, but it had only five conductors, none of which were DSR or CTS. Pfui!
So I used a hank of gorgeous flexy 9-conductor cable (which came with premolded DE-9 ends of the wrong gender, now amputated into pigtails and back in the GCS), which supported the connections redrawn on the bottom in proper numeric order, used the obvious color sequence (Bn R O Y G Bl V W K), then soldered suitable connectors on each end:
And it worked the first time…