This was my first time using an optocoupler in my design, and I'm realizing I should've read the datasheet more carefully. :)

The above scope capture is the /CS (chip select) waveform as seen by the AD5328 DAC on the shim control board. It seems my hasty choice of 12.4k pull-up resistors was a far cry from the 350 ohms recommended in the optocoupler datasheet.

It's anything but a problem, however. The optoisolated capacitive switch bank in PyPPM 2 can be clocked at 125 kHz (fCLK/128) without a problem.