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From: grr@cbmvax.UUCP (George Robbins)
Newsgroups: comp.sys.amiga
Subject: Re: Need help with Homemade 5.25" disk drive for A1000
Message-ID: <2189@cbmvax.UUCP>
Date: Sun, 9-Aug-87 12:27:33 EDT
Article-I.D.: cbmvax.2189
Posted: Sun Aug  9 12:27:33 1987
Date-Received: Sun, 9-Aug-87 22:43:30 EDT
References: <7165@alice.UUCP>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Distribution: world
Organization: Commodore Technology, West Chester, PA
Lines: 44
Keywords: disk drive 5.25

In article <7165@alice.UUCP> sgt@alice.UUCP (Steve Tell) writes:
> Recently, I bought from a friend a standard IBM-PC type 5.25" disk
> drive, case, and power supply to hook up to my Amiga.  I dug out
> a description of the required adaptor that was posted by
> Randy Spencer (spencer@oberon.UUCP) on April 2, 1986.
> I was careful with the wire-wrapping, but now have the following
> two problems on which I seem to be stuck:

There are slight differences between the 5.25 adapter card and the
3.5" adapter card.  There is a mechanism to return a serial drive
type identifier over the RDY line.  In the 3.5" this is all 0's, in
the 5" drive it should be alternate 1's and 0's.

> The circuit, Randy claims, was reverse-engineered from an Amiga 3.5"
> external drive.

There is little need to reverse engineer anything.  Both the 3.5" and
5.25" adapters are included in the A1000 Schematics and Expansion
Specifications document, available from Commodore for $20.

> Also, where can I get a description of the disk drive signals on the Amiga's
> disk connector.

Same place...

> I removed the terminator resistor pack from the drive.  Before doing that,
> things were worse:  It would only step towards track 0,
> and format wouldn't even verify track 0.  Aparently the Amiga can't
> handle the current needed to pull down the line with a 150 ohm resistor,
> at least in the step-direction case.

Right, the 3.5" floppies have permanantly installed 1K pullups on each drive,
for a total of no more than 250 Ohms total.  The A2020 drives are specified
similarly.  Some systems will work with 150 Ohm pullups, but a lower value
is wise.

One other thing to check is that your drive can reliably handle a 3mS step
rate.  There is some software way to adjust the step rate discussed a while
back but I don't recollect the details.  I'm not sure about the precomp
issue, fixing the drive ID code will take care of that I'd expect.
-- 
George Robbins - now working for,	uucp: {ihnp4|seismo|rutgers}!cbmvax!grr
but no way officially representing	arpa: cbmvax!grr@seismo.css.GOV
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)