5.25 drive with sector output

[Chuck Guzis]

From: ard at p850ug1.demon.co.uk

> The normal way to separate index form secotr pulses was to have a 
> non-retriggerable monostable that was triggered from the index sensor and
> which had a period a little less than the time between sector holes. It
> was triggered at the end of each hole. Normally it timed out after each
> sector hole, but was still set when the index hole came after the last
> sectore hole. The output of said monostabel sent the output of the index
> hole sesnor to either the 'sector' line or the 'index' line.

The period of the monostable is pretty flexible--anything longer than 
half the time between sectors and less than the time between sectors 
will work.  If you're "back of the envelope-ing" it, you might shoot 
for 75% of the time between sectors.  This would allow a pot-less 
design, as the component tolerances would be well within the "slop" 
allowed in timing.
 
> Some drives, particularly 3.5" ones, have an adjustable monostable in the
> index circuit which lets you delay the index pulse. It's adjusted when the
> drive is aloighed to give the correct timing between the index pulse and
> the data ffrom the ehad -- twiddlign the pot is easier than moving the
> index sensor. Such a circuit could msess up what you're trying to do, but
> this IBM drive doesn't have anything like that.

Most newer 3.5" drives subsume the "ready" circuit into a monolithic 
hunk of silicon with the rest of the drive control.  Index output 
pulses are blocked until the period between indexes satisfies some 
internal "ready" standard.  Also, 3.5" drives can have *very*  wide 
index pulses when compared to 5.25" and 8" drives.

Of course, any 5.25" drive with this logic (and it's very common) 
will view a hard-sector disk as not being anywhere near the correct 
speed, so will not only not come ready, but will block *all*  index 
pulses going out.

This feature has a curious implication if you decide to replace an 
older drive without the "block index until ready" logic with a newer 
drive with that logic.  Many controllers (such as the WD1770) or 
software will count up a few sectors after select or motor on before 
commencing an operation.  The result is that everything still works, 
but the latency after selecting a drive increases significantly.

Note that once the drive been selected and come ready, this is not an 
issue.

For what it's worth,
Chuck