HD 5.25 floppy disk drives emulating legacy DSQD floppy disk drives

Andrew Lynch lynchaj at yahoo.com
Sat Dec 29 12:51:17 CST 2007

Hi a quick follow up to my question to the list earlier.  I found that on
the TEAC FD-55GFR more closely emulates the TM100-4M signals if the "E2"
jumper is ON.  It changes the logic of the /INDEX and /READ DATA signals to
be compatible with older PC and XT floppy drive controllers which do not
give the drive adequate time to fully settle.  Apparently the older TM100-4M
drives don't seem to care much about it but the Vector Graphic FDC is
expecting to get those signals whether regardless of seek status.

If anyone has any additional thoughts on the matter, please post.  I am
still trying to figure this out for the Panasonic JU-475-1 drive.  Is anyone
familiar with it so that it's /INDEX and /READ DATA are compatible with PC
and XT FDCs?


Andrew Lynch

Hi,  Sorry for the long post.  I have done a lot of things to make this
work but am still stuck on this problem.  As some of you know, I and
some others have been busily working on restoring some Vector Graphic

One goal is to replace the Tandon TM100-4M 100tpi drives with more
commonly available High Density 96tpi floppy drives.  I realize the
disks between the two types of drives are incompatible and I plan to
retain both types of drives for compatibility with other Vector Graphic
machines.  However, the 100tpi drives are difficult to obtain and I
would like to limit their use to preserve their remaining lifespans.  I
would like to transition my regular usage disks to the HD 96tpi drives
to utilitize commonly available and inexpensive HD floppy drive

I believe it is possible to make an HD drive emulate a legacy DSQD drive
-- at least I am sure it can be done on the PC.  The late Don Maslin was
apparently able to use 720K (DSQD) 5.25" floppy drives on his Vector
Graphic system (see link below).  I would like to do a similar thing but
using the HD drives in DSQD emulation mode.


I have a couple of HD floppy disk drives such as the Panasonic JU-475
and TEAC FD-55GFR.  I have jumpered and modified both drives so that the
spindle motor turns at 300 RPM which I have verified using a fluorescent
lamp and tach wheel.  I have set the jumpers or directly wired to ground
Pin 2 to force Double Density mode.  I have covered Pin 2 on the
interface with tape so that it does not interfere with the FDC on my
Vector Graphic machine and vice versa.  According to the schematic of my
Vector Graphic integrated FD/HD controller (FDC) neither pin 2 (density
select) nor pin 34 (/ready) are used.

After many tests and inspections, I have verified both HD drives do work
in 720K DSQD emulation mode on my floppy test bench PC.  They are
configured as 3.5" 80 track drives.  The motor spins at 300 RPM and they
are both able to format and verify with scandisk under MS-DOS using good
quality 5.25" disks.

The problem comes in when I attach either HD drive to the Vector Graphic
FDC.  Whenever it tries to format under CP/M, the VG is able to write
out all 154 tracks (I have verified it is writing the tracks by
inspecting the disks using my Catweasel) but the VG cannot verify the
tracks correctly and fails the format.  Everytime it tries to format a
disk, it ends with "PERMANENT DISK ERROR" messages.  Using different
CP/M utilities to write to the HD drives results in similar errors such
as VG BACKUP, a disk copy utility, and CPYFSTU, another CP/M disk copy

I do not know for certain whether the disks are have errors when written
or the drive is having problems reading the track.  I know the disks are
good as I can format them on the VG using the TM100-4M drive without
issue.  When I verified the tracks are written using the Catweasel and
the same disk and drive I noticed several read errors and bad sectors
which implies the tracks were written with errors.

Does anyone know why these HD floppy drives are incompatible with the
Vector Graphic FDC?  Is there something I am missing or some setting
which would make them compatible?  The HD drives work just fine at DSQD
drives on the floppy test bench PC so I am fairly sure the drives
themselves are OK.  The VG FDC works fine with the TM100-4M drives so I
think they work OK.  The problem is only when I attach the HD drives to
the VG FDC.

Would anyone please give me some insight on this problem?  

Thank you in advance for any help.

Andrew Lynch

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