This is an old post, preserved for reference.
The products and services mentioned within are no longer available.
Here are the details of the USB to IEEE-488 / GPIB adapter I have been using to test the Commodore PET IEEE-488 disk drives.
USB to IEC adapter I build last year for testing Commodore 1541 disk drives, this one is based on xum1541 firmware. That needs to run on an ATmega32U2, and as before I adapted one of my USB keyboard PCBs for the purpose. The latest PCB for the ZX81 adds a sounder to click on each keypress (which makes typing easier), and a mode switch. That leaves a pile of the old style PCBs for tasks like this.