Hi Mark,
I will try to find a good schematic for you. In the meantime I can provide you with the details. It's two Digipots connected to a microcontroller in parallel, via SPI.
1) The micorcontroller sends data to one controller at a time.
2) The line has a 2.2 K pullup resistor.
3) (VDD= +18V, VSS=0 V, VLogic= +5 V)
Thank you for the reply,
JP