Rewrite PC8801-30 / PC Engine CD honoring the expansion bus

[angelosa]

PC Engine PCB flow is:

• main unit has an expansion bus bay area, pinout is here;
• This connects to an IFU-30 bay, which contains an μPD91061GD;
• Finally, the C-CD connector goes to the actual PC8801-30 CD drive, part list.

Practically, both pc8801mc and pce cdsys have limited host SCSI interface:

• BSY, REQ, MSG, I/O, CD as input;
• SEL as output;

On boot they put 0x81 -> 0xff on the data bus then pings the SEL line, this implies that the SCSI bus is configured to select ID devices 0 and 7. Speculation is that '91061 is a SCSI bridge mapped at 7 that is responsible for details towards device 0.

[angelosa]

IFU-30 PCB: