I read through richjones posts in the thread.
It is very valuable information for anyone wishing to convert their PC50 to linuxcnc!
As far as I understood he is completely skipping all RS485 communication and replacing the card that does the RS485 to/from host, generates the steps and I/O processing. So in essence he built a electrical interface to preserve all driver electronics and using ie. a custom circuit board to interface to a host that is generating steps and pulses to drive the original mill drivers and sensors.