> >Another minor gotcha: machines that used WDC179x disk controller chips
> >were capable of starting their first sector sooner after the index pulse
> >than those using NEC 765 series. Reading those with a 765 required
> >disabling ther index pulse.
On Mon, 21 Oct 2002, Dwight K. Elvey wrote:
> Hi
> I always wondered why some PC's didn't care if the drive had a
> index hole or not to read the disk. It would care for formatting
> but not for read/write.
> Dwight
Well, it's only part of the answer.
Apple and Commodore (and Atari?) didn't use the index hole at all, so they
really don't care. They also didn't CARE when a track started. Since they
were single sided, an additional write-enable notch was all that was
needed to make them into "flippies".
PC, TRS-80, and most systems that used an FDC chip needed the index hole
to declare when to start a track when formatting. [slightly
over-simplified]
To make "flippies" for them required ALSO punching a hole in the jacket
for access to the index hole.
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Once the disk was formatted, most systems no longer needed the index hole.
EXCEPT,...
The NEC 765 chips did a RESET [over-simplified again] whenever they saw
the index pulse, and couldn't read or write for a short period of time
after that. But surely nobody would write sectors so close to the index
that they would fall into that reset period - yep. Some did.
and,
Some drives, such as the Teac 55 series used the index pulse to tell when
the drive was up to speed. No index pulse = no ready.
When using a 765 to read disks that had a substandard index gap, with many
drives (Tandon TM100, etc.), you could cover the index hole on the
disk. A write protect tab would work, IF you could get the user to put it
on solidly, and not let it fall off in the drive. No index = no RESET,
and data could be read immediately after index, or even straddling it.
With the Teac drives, that wouldn't work, but you could interrupt that
signal on the cable. Mike Gingel (TRS-cross) sold a floppy cable with a
SPST switch in it. I used a 34PST inline switch.
That way, the drive could see the index (READY), without the FDC seeing it
(no RESET).
With the index signal interrupted, if a read was successful, fine. But if
anything went wrong, the BIOS would return the wrong error code. 128 (not
ready), instead of 4 (sector not found)
Received on Mon Oct 21 2002 - 19:36:01 BST