C64 <-> CS/A65 bus adapter

This is the description of an adapter to use most CS/A boards in a C64. 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 into the C64 expansion port. On the other side it provides a socket for a CS/A bus card. Two things have to be considered for the schematics:

• The C64 Video chip releases the bus with rising Phi2. This is too late for the 6522. Thus the rising edge of Phi2 must be delayed to the CS/A bus, to be sure. The CS/A bus is specified to run up to 2 MHz, so there is no speed problem.
• The I/O area in the C64 is 256 byte only. To cover the whole CS/A bus address space (1M memory and 4k I/O) additional address lines have to be generated. This is taken care of with a VIA 6522 on the adapter. More in the description.