Re: WTD: tandon 603s or 602s

From: Ethan Dicks (
Date: 2006-07-02 07:49:56

On 7/2/06, Anders Carlsson <> wrote:
> Bill Degnan wrote:
> > Theoretically you do not have to do anything to the drive if you swap
> > a 603s for a 602s.  If you use a 602s, you in effect convert a d9090
> > into a d9060, and the capacity is reduced to 5MB.

The only difference between a D9060 and D9090 is the specific drive
mechanism installed, the position of a jumper (see below), and the
sticker on the outside.  One of the drives I have at home was sold to
me for a pittance because it "wouldn't format" - someone had replaced
the TM603S with a TM602S and failed to move the jumper.  Quick fix.

> The ftp index on this page says:
> "The jumper J14 on the DOS board is open in the D9060, and closed in
>   the D9090 to select a 4-head vs. 6-head drive."
> Maybe that's what you are missing if you tried a 4-head ST-225 with a
> D9090 drive?

That will certainly fail to format.  Try an ST-251 for a 6-head
drive... those are almost as common as the ST-225 (they shipped
standard with the original IBM PC-AT).  Of course, having a 5MB PET
hard disk isn't so bad, especially since they don't have

> Speaking of which, I see Ethan many years ago went about to disassemble
> the ROMs in order to address a disk with different geometry and perhaps
> be able to access a larger part (all?) of a ST-225 disk. How did it go?

There was a bit of a flurry about it for a while, someone did track
down the location of the drive geometry table in the ROMs, patched
them for an ST-225 and, IIRC, got it working to around 16MB (maximum
size possible with the C= DOS 3.0 block allocation/track pointer

My interest was in reverse-engineering the ROMs to the point that I
could try to re-write the SASI command building code and bypass the
SASI<->ST506 board (I have a spare DOS board that started off my
initial interest in these drives), so I could use a modern embedded
SCSI drive (I paid $70 once to have a TM602S factory refurbed - nobody
does that any more, so modernish replacements are a must).  I have
been sidetracked by so many projects since then (like porting Zork to
the PET), that I have no idea where those files are.  I would probably
have to start from scratch, not that it would take that long.  Modern
development tools being what they are, I'd run the ROM code through a
disassembler, then start assigning I/O port names, etc. and using a
cross-compiler and a binary diff stage in the makefile (to catch
inadvertent code drift).  I think the recently mentioned disk drive
source code might be illuminating, even if only for some major
subroutine names and I/O port names.


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