The Xerox 820 CP/M computer could interface with dual 8" floppy drives (or up to 4 5.25" drives!).
While I did have a working pair of drives, I wanted the ability to run my 820 off of a floppy emulator. I picked a "Gotek" branded floppy emulator with the open source Flash Floppy firmware.
The Xerox has a bespoke 37-pin D-style connector for its floppy drives, so I created a passive adapter PCB.
After configuring Flash Floppy and loading up a CP/M disk image I was able to boot to CP/M without the pair of incredibly loud drives spinning in the background.
The "mod" section was for bodges but it turned out no bodge wires were needed. (it's not a header for a 2nd drive, sadly)
The gotek's default "S1" jumper position is for the CP/M "B" drive and the "S0" position is for the CP/M "A" drive, so most of the time you'll want to move the gotek's selection jumper to the "S0" position.
You can grab the kicad design files from my xerox 820 repository on github. A bootable CP/M image is also in the repo
Here's the content of ff.cfg:
interface=shugart pin02=high # 8/5 select - low is 5.25" pin34=low # nrdy but flashfloppy doesn't assert it early enough display-type=oled128x64-rotate-narrow-inverse side-select-glitch-filter = 50 track-change = instant write-drain = instant index-suppression = no head-settle-ms = 10 motor-delay = ignore chgrst = step ejected-on-startup = no image-on-startup = init display-probe-ms = 0
And here's img.cfg:
[::256256] # Std 8" SS/SD cyls = 77 heads = 1 rpm = 360 gap3 = 42 interleave = 1 bps = 128 secs = 26 mode = fm id = 1 rate = 250
Power the gotek adapter from the 4-pin cable and plug the 34-pin IDC cable into both devices, observing the pin 1 location.
At this time, it's not possible to emulate a 2nd floppy drive with a single gotek with flashfloppy.
It's probably possible to emulate the 5.25" drive as well, but I didn't test this as I didn't have any 5.25" floppy images. As far as I know, 5.25 drives use 500kHz FM recording mode. Note that ready and data lines get different processing for 5.25" vs 8" drives so other changes besides the image.cfg and pin02 state may be needed. (this is done by IC U107 on sheet 5 of the schematic)
This page (Xerox 820 Gotek Adapter) was last updated on June 20, 2024.
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