glitch
Veteran Member
writeup here: http://www.glitchwrks.com/2022/06/25/imsai-sio2
Turns out it's fairly easy to disable the command register on the IMSAI SIO2 and allow the SIO2 to coexist with the MITS disk controller (or FDC+). I was able to do it without any cuts or jumps on the actual circuit board, so there are no permanent modifications. This lets one run original IMSAI software which expects default SIO2 addressing, but also use the excellent DeRamp FDC+ board, which is a drop-in replacement for MITS' two board disk controller set. The FDC+ can also run with regular Shugart-style 8" drives, 5.25" HD drives, or disk emulation over a serial port to a PC, so I've been recommending it to anyone with an 8080 CPU who wants a hassle-free disk controller.
Turns out it's fairly easy to disable the command register on the IMSAI SIO2 and allow the SIO2 to coexist with the MITS disk controller (or FDC+). I was able to do it without any cuts or jumps on the actual circuit board, so there are no permanent modifications. This lets one run original IMSAI software which expects default SIO2 addressing, but also use the excellent DeRamp FDC+ board, which is a drop-in replacement for MITS' two board disk controller set. The FDC+ can also run with regular Shugart-style 8" drives, 5.25" HD drives, or disk emulation over a serial port to a PC, so I've been recommending it to anyone with an 8080 CPU who wants a hassle-free disk controller.