The secret? In the end? Calling the number on the back of that motor driver. I was talking to an engineer in about 90 seconds, and he was emailing me the datasheet and user manual five minutes after that. He knew which line of motor driver product I was talking about, and keep in mind the manual says that the requirement was for Windows 95. Way to go, Shawn from Faulhaber!
Those are uploaded to the files list now --- honestly, the driver is incredible overkill for the main task of, like, spinning at a constant and adjustable speed. With digital control you can tell it to home, run macros, report the speed and adjust PID loops, compensate for backlash, set external triggers. And with DeviceNet address and coordinate between thousands of motors.
I wanted to make the thing spin at a constant and adjustable speed. And unfortunately, I couldn't get said digital RS232 to work --- maybe eventually, but with all that frustration... why not just take advantage of the analog input for speed, just in case all you want to do is make the thing spin at a constant and adjustable speed?
Analog input: Zero volts, spin in one direction. Five volts, spin in the other direction. 2.5 volts, stop. And then everything in between, at 10 bits of ADC. THANK YOU, SHAWN!