This is an old post, preserved for reference.
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I've been working on some Commodore PET disk drives recently. These differ from the Commodore 64 era drives in that they have a parallel interface, conforming to the IEEE-488 / GPIB standard. The later drives have a slower serial version of this on the 6 pin IEC connector.
Commodore PET 8032 connecting to a Commodore 8250 dual disk drive. I haven't had much success here, and it's one of those situations where when it's all put together, it doesn't work. No response from the drive. I tried the standard LOAD "$",8, the DIRECTORY command and shift + run/stop.
4032 I restored last year.
USB-IEC adapter to connect to serial drives from a PC. That was based on xum1541, and that does have the option to connect to GPIB drives. Rather than modify the previous one, I built a new USB to IEEE-4888 adapter.