I tried to connect a SW SynScan HC V3 / latest FW used with a SW Allview mount to connect via HC-cable -> Prolific serial-USB converter to a RPi4 with Stellarmate.
I used the Synscan INDI driver as well as the Synscan legacy INDI driver. If the HC is in "PC direct mode" the connection cannot be established. There is always a serial time out (Synscan INDi driver) or =11.0pt“Synscan mount not responding” error message (Synscan legacy driver).
Solution is to keep the HC in normal mode, not in PC direct mode. This can confuse users and should be noticed somewhere.

Cheers
Hardy