720k 5.25 disks

[uhclem at trsvax.UUCP]

<>
R3>The IBM startup test does not bash heads against stops, at least on
R3>the AT; it only recalibrates and steps to track 34.

Hmm, well it does it on the true-blue ATs we have here with 40 track drives
in them. (Since the high cap drives have more than 40, no problem.)
Our friendly IBM rep explained it away as I described.  I even recall that
someone took a peek at the IBM BIOS listing to confirm that it wasn't just
DOS doing this stunt.   (We figured we might get Microsoft to change it if
it was their doing.)  I agree that pegging the drive isn't a good thing to
do on a regular basis, but for a single experiment, there should not be any
problem, unless you have got Tandon drives or something.  Most half height
drives made after 1985 "know" where they are and will disregard stepping
pulses beyond their capacity.  Check up on the guys using Radio Shack
Model 4 systems and trying to get 43 tracks out of a 40 track drive and
had to use older, dumber drives if you want to hear more about this.

By the way, someone stated that you don't need high-cap media for 720K 5.25
operation.  That is true.  But you should use 96 TPI (80 track) DSDD media.
There *really* is such a thing and it is different from high cap/density
(96 TPI 80 trk DSHD) and normal 48 TPI 40 trk DSDD.  Since 5.25 720K is
a less common size, not everyone carries that type of disk.   But Rad Shack
does.


R3>This BIOS trick will only work if the machine uses dual-speed 1.2M
R3>drives.  The more common case of single-speed drives and floppy
R3>controllers with three different clock rates won't work with this
R3>trick; the data will get written at the wrong clock rate.  

In his case, he was able to read cylinder 0 correctly, so the transfer
rate was not the issue, and he already stated an AT environment which
would have a dual speed adapter.


<My opinion, but hey, I've only been doing this for 15 years.>
						
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