19 Jun
1998
19 Jun
'98
2:54 a.m.
On Thu, 18 Jun 1998, Tony Duell wrote:
**** snip ****
The outputs of the index sensors (and the WP switches
for that matter)
are just ORed together. There's no other electronics there
I guess what was lurking in the back of my mind as I wrote the question
was the situation of trying to read a 'normal' flippy disk (with 2 jacket
index holes symmetrical about the centerline) in a two sensor drive.
rotation positional difference. Or whether when
reading the flippy
side it would simply wait longer to read the sector header.
Why would it matter? Remeber you can't read a true DS disk as a flippy as
it's spinning the wrong way on the second side. So the other side of a
flippy was formatted and used only on flippy drives anyway. There's thus the same delay between index sensor
and data for format,
read and write. So there's no problem at all.
That's apart from the fact that index timing is only critical on hard
sectored controllers...
I think that there are some exceptions to that Tony. The following is a
clip from some correspondence with Chuck Guzis at Sydex a while back:
"Another topic that I'd like to see some commentary on is how people
have handled those old diskettes that lack the Index Address Mark
information, such as those used on the Cromemco C10. On a PC
controller, the first sector on a track on such a diskette usually falls
to be seen by the FDC because it falls in the "blind spot" (ostensibly
PLL sync-up time) of the 765-family chip.
"On 5.25" diskettes, you could sometimes tape over the index hole and
with some finagling, get a good read. I've heard of some people
modifying a drive by moving the index-sensing LED/phototransistor
assembly slightly to gain a few microseocnds.
"But 3.5" diskette drives are too difficult to modify. We've had good
read and write results by passing the index signal through a 1-shot
carefully adjusted to trigger slightly ahead of the actual index
position. But this is a very touchy arrangement, though it does work.
I know that I have experienced the problem of the first paragraph on more
than the C10 disks.
- don