Creating new CP/M disks - Progress on IMSAI

Richard A. Cini rcini at
Fri Jan 11 22:50:27 CST 2008


    I¹ve been busy lately rebuilding the floppy system on my IMSAI. I
scrapped the Frugal Floppy 8² in favor of a CompuPro Disk 1 using 5.25²
disks to emulate 8². This has been a long running project, but with the
assistance from someone on comp.os.cpm who went through the exact same
process with nearly the same hardware, I¹ve been able to create a brand new
CP/M 2.2 master disk. Yeah!

    Anyway, one of the issues I have with the Teac FD-55GFR (1.2mb
high-density; no schematic) drives I¹m using is that the motor runs all of
the time (just like an 8² drive) but these late-model Teac drives don¹t have
head relays, so the heads are engaged all of the time.

    I¹m looking for some advice/whatever on engaging the motor when needed.
The Disk 1 is based on an 8272 controller chip and it has a HDL (head_load)
signal which, through jumpers can be made to appear on the *MOTOR signal pin
of the standard IBM floppy disk interface.  I tested the HDL signal and it
activates at the right time, so its following the 8² sequence, but the pulse
width leads me to believe that it¹s too short to let the motor spin-up and
go ³ready². 

    There is a way to control this with software...and I may go that route
anyway...but has anyone done this sort of thing ‹ used a 5.25² HD drive to
simulate an 8² drive and if so, which drives did you use?

    Any input appreciated. Thanks.


Rich Cini
Collector of Classic Computers
Build Master and lead engineer, Altair32 Emulator

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