Looking at the A570 / CDTV and A590 schematics, it is very obvious how the WD33C93 is connected to the expansion connector, but every A570/CDTV SCSI module I've seen also contains a GAL that I have no idea what it is used for.
You might want to try and see if you can get in contact with this guy.
http://xoomer.virgilio.it/fmiapa/cdtvinfopageen.html
