Index hole on 5.25" disks

From: Dwight K. Elvey <dwightk.elvey_at_amd.com>
Date: Mon Oct 21 20:21:00 2002

>From: "Fred Cisin (XenoSoft)" <cisin_at_xenosoft.com>
>
>> >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)
>
>
>

Hi Fred
 You should write a book on "The Realities of the Floppy Disk".
I am always enlightened by the interesting tidbit you come up
with. I've only fiddle with them enough to know that all isn't what
it might seem. It seems that all the rules were broken at one
time or the other.
Take Care,
See you at VCF
Dwight
Received on Mon Oct 21 2002 - 20:21:00 BST