720k 5.25 disks

[John Hood]

(Note the Followup-To:; we're getting rather far from Xenix)

In article <196500030 at trsvax> uhclem at trsvax.UUCP writes:
[stuff about 720k 5.25" disks]
>If you can watch the heads while it operates, stick one of these disks in
>and seek about two-thirds into the capacity (720K remember) and see if the
>head pegs out on the inner stop.  You may even be able to hear it.
>Don't worry (TM).  The IBM startup test does this every time you boot,
>which most clones copied even though it is not very good for the drives.
>If it does peg, then the system is treating the drive as a 1.2 meg reading
>a 360K and is double stepping the head for each cylinder increment.
>(Under XENIX, pull out your uPC765 manual and decode the status returned
> in the read error.  If it is a seek error, this is what you have got.)

Argh...  I should warn you I had a little war with Bryan Headley
[another Tandy person] once over doing this on the 1000TX.  I disagree
with those who would test drives by running them into stops.  The IBM
startup test does not bash heads against stops, at least on the AT; it
only recalibrates and steps to track 34.  The AT clones I've seen (uh,
heard) are similar.  Bashing head assemblies against their stops is
not a Good Thing to do, in my opinion; many 5.25 drives are not built
to accommodate it, and even on things like Apples and Commodores where
it's a feature of the drive design, it's a major cause of drive
problems.

>
>I routinely have to transport data to an older system with 5.25 720K drives
>and my MS-DOS system steadfastly refuses to cooperate.
>
>To get around this, I lie to the CMOS and tell the system that the drive is
>a 3.5" lowcap, which holds 720K.  The BIOS quits double-stepping and I get
>my files across the abyss.  Don't forget to put the CMOS back when you are
>done.  Under XENIX, you should be able to force recognition by using the
>appropriate dev, that is if the XENIX floppy driver was written properly.
>I haven't tried this with the "real" SCO XENIX floppy driver.

This BIOS trick will only work if the machine uses dual-speed 1.2M
drives.  The more common case of single-speed drives and floppy
controllers with three different clock rates won't work with this
trick; the data will get written at the wrong clock rate.  There are,
I believe, other BIOS tricks which will work with most machines.  Just
curious; what machine are you doing this on?  I don't think I've yet
used one that did have double-speed drives.


  --jh

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