Tandon TM-100-1 electronics repair

Ethan Dicks ethan.dicks at gmail.com
Thu Apr 27 21:34:00 CDT 2017

On Thu, Apr 27, 2017 at 4:43 PM, Fred Cisin via cctalk
<cctalk at classiccmp.org> wrote:
> On Thu, 27 Apr 2017, Ethan Dicks via cctalk wrote:
>> It's not impossible to find another TM-100-1 or replace it with a
>> TM-100-2
> Actually, the IBM 5150 PC originally (August 1981) mostly came with TM100-1.
> It wasn't until about nine months later, when IBM released PC-DOS 1.10 (May,
> 1982, which supported double sided drives) that they added TM100-2
> availability.

Ah.  I had forgotten that detail.  I only ever worked machines with
double-sided drives.

> Why not use the TM100-2?   It will work for any of the single sided use, and
> some versions of the operating systems supported double sided drives.
> (On the Tandy version of LDOS, double sided support was undocumented,
> because Tandy didn't sell a double sided drive, until the model 4D?)

I know it will work.

> Remember that the PC uses all drives jumpered as if they were :1/DS1/B:

Right.  I well remember how to jumper drive selects.

> Radio Shack used an "unused" pin of the interface for drive :3/DS3/D:,
> Unfortunately, that was the pin that was "reserved" for side select!
> That poses a problem for model 1 (with four drives on one cable), but not
> for III, 4, nor 4P

Yes.  I was reading about that the other day.  I don't have any
4-drive units, just multiple dual-drive units.

> If you have a model 1 drive that has the modification for drive select using
> the side select signal, don't use it if you use any double sided drives in
> the machine, or clip that wire on the drive.

Good to keep in mind.  I borrowed a half-height "Bachelor" FD-104 from
a Model 1 for drive/controller testing.  I cannot find docs for it,
and barely a mention, even in TheRef.  There is one jumper block by
the 34-pin edge connector - marked D.S. / 0 / (1) / 2 / 3 / MX. (the
'1' is damaged/missing because of a PCB via).   It seems the first
jumper is likely to be single/double-sided (there are 2 heads), then 4
drive selects, then the MX one-drive-only-select jumper.

> You can use "half-height" drives in a model 3 or 4.

Yes but at the moment, I only have one half-height 40-track drive,
that Bachelor FD-104.  My only loose drives are 80-track HD units.


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