Commodore PET <-> CS/A65 bus adapter

This is the description of an adapter to use CS/A boards in a Commodore PET. A lot of CS/A boards, including a video card, serial interfaces, floppy controller and an SCSI card can be found on the CS/A65 homepage.

The interface itself is, in fact, pretty simple. It is a board that is plugged underneath the 6502 CPU of the PET. On the other side it provides a socket for a CS/A bus card. Two things have to be considered for the schematics:

• The PET only has two free memory areas, $9*** and $A***, so additional memory address lines have to be generated separately.
• The I/O area is pretty cramped. Especially when you look at the original I/O area address decoding: PIA1 is selected when A4 is high, PIA2 when A5 is high, VIA when A6 is high and CRTC when A7 is high - which means that on some addresses multiple chips are selected. Only $E80* is left open. So this area is used to map the control registers.