Creating new CP/M disks - Progress on IMSAI

Allison ajp166 at
Sat Jan 12 10:47:08 CST 2008

>Subject: Creating new CP/M disks - Progress on IMSAI
>   From: M H Stein <dm561 at>
>   Date: Sat, 12 Jan 2008 09:14:09 -0500
>     To: "'cctalk at'" <cctalk at>
>-----------Original Message:
>Date: Fri, 11 Jan 2008 23:50:27 -0500
>From: "Richard A. Cini" <rcini at>
>Subject: Creating new CP/M disks - Progress on IMSAI
>    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. 

Cannot use that as the HLT parameter is soo short for adaquate spinup time.
You might get away with it though as the FD55GFRs do spinup fast.

The Disk1  Didn't have a direct motor control port like the 1A but it did
allow for using the latch used for serial output to be used as a general 
motor on.  Find MotorENA on the schematic.

>    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?

The DISK1 serial port if not used as boot console is a 1 bit port that 
can be controlled with software.  It's in the manual, switch S1-3. The 
manual covers all the things for 5.25" floppies.

To do a read or write you set motor enable, wait 1sec then read or write
and reset motor enable.  To minimize starts and stops a timer of 10seconds 
or more is desireable.

Its the way I'd do it using the DISK1 and have done it with the DISK1A.

The disk1A has a differnt specific port for that which is part fo the 1 to 
1A differences.


>    Any input appreciated. Thanks.
>There are several different OEM versions of the FD-55GFR with
>various jumper options.
>If yours has one, put a jumper on the ML position and isolate the
>/MOTOR ON signal somehow (tape over pin 16).
>You will also have to change the DC jumper to RY in order to get
>a proper /READY signal.
>Whether it will ultimately actually work will depend on the controller;
>there are more issues with 1.2HD drives than one would expect.

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