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From: pritch@osu-eddie.UUCP (Norman Pritchett)
Newsgroups: net.micro.6809
Subject: R.S. Floppy controller -- thanks
Message-ID: <2003@osu-eddie.UUCP>
Date: Sun, 29-Jun-86 23:49:37 EDT
Article-I.D.: osu-eddi.2003
Posted: Sun Jun 29 23:49:37 1986
Date-Received: Wed, 2-Jul-86 06:40:04 EDT
Organization: The Ohio State University
Lines: 36

Many thanks to the people who responded to my question about why three
double-sided drives worked on my CoCo but four didn't.  Also, tnx to all who
suggested this disk driver and that to use double-sided drives with OS-9 but
you missed where I said I use Dave Lewis' NEWDISK for that.  I'm already
doing it -- I was just a little confused why it worked.

The eve before the responses started flooding in I checked through my
technical reference manuals for the TRS-80 Models I and III and noticed the
schematics for the Tandy drives differed from the ones I have on my CoCo
(ok, that's obvious but you know what I mean).  This is how I discovered
that the coco controller was using drive select 4 on what the rest of the
industry used for head select.  That answered the question about whether not
my not being able to get my 4th drive to work was a fluke or not -- all the
responses to my posting confirmed it.

My next question is why Tandy did things this way.  I have a theory and I
would appreciate it if someone could tell me if it is at all accurate.
Looking at the signal assignments for the drive/controller connection I get
the impression that only 3 floppy drives were allowed for initially -- drive
selects 1 through 3 are grouped together but drive select 4 is way off by in
another corner.  When Radio Shack came out with disk systems for the TRS-80
Model I they wanted to support 4 drives but since drive select 4 didn't
exist they picked one from the unused pins.  The industry later assigned the
head select signal (to Radio Shack's drive select 4) and drive select 4 (to
an unused pin) but Radio Shack kept their interface the way it was for the
Model III and for the CoCo (I assume R.S. initially sold the old Model I/III
drives with CoCo disk systems).  So if you use the original R.S. Model I
drives you can support 4 of them; if you use anybody else's you can only
support 3 but you have the option of double-sided operation with the proper
drives and software.  Is this close?

-- 
Norm Pritchett, The Ohio State University
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