Tandon TM-100-1 electronics repair

Fred Cisin cisin at xenosoft.com
Thu Apr 27 15:43:56 CDT 2017

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 (more common, owing to its appearance in the IBM 5150 PC),
> but I'd like to just repair this one and get back to TRS-80 hacking.

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.

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?)

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

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
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.

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

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