On Thu, 13 Aug 1998 12:35:41 -0600 bowman <bowman@montana.com> writes:
>>         How to use the 82C55A to controll the floppy disk drive
>>         5 1/4" and 3 1/2" directly ?
>
>It has been a long time since i've played with the WD controllers, but
>I
>don't think it would be feasible to use the 8255. You would be able to
>step to the desired track, and select a head, but it gets complex at
>that point.

Yes you can certainly turn the motor on and off and move the head, but
specialized hardware is almost certainly needed to encode and decode the
data.

>not sure about the 3 1/2, but for the old (180k) 5 1/4 the
>controller picked up a signal from a photocell looking through a hole
>in
>the media for the sector location,

PC disks are "soft sector".  There is a special code (a violation of MFM
rules) that marks the start of each sector.  There is only one hole in
the disk, which generates a once-around "index" pulse.  The pulse is used
only while formatting the disk, or to see how many times the disk has
turned before giving up on finding a sector.  3.5" drives have no opto
sensor on the disk, instead a pulse from the motor is used.  The metal
plate attached to the disk ensures the disk and the motor turn in phase.

 and then read or wrote a MFM or FM
>analog signal to the disk. The controller was responsible for
>generating
>this signal much like a modem would.

The drive doesn't do anything with the data pulses from the disk other
than amplify them up to TTL-level and pass them through a one-shot.  So
for reading the controller has to deal with a stream of pulses at
approximately 250 Kbps or higher for higher density disks.  It would take
a very fast processor to bit-bang the data directly.  The controller has
to be able to handle varying data rates because the motor speed isn't
real precise.

>
>you might look for information on the very old Apple computers.
>Wozniak
>implemented the disk drive without using a FD controller, essentially
>using it like a tape recorder and firing a bit stream at it. this,
>however, resulted in a disk format unique to apple.

The Apple controller had a PROM state machine to do some of the work.
Also of course the processor was operating constantly in a tight loop to
finish decoding the disk data.  It was a solid design, though not
necessary today because inexpensive single-chip disk controllers are now
made.  They go between your 8-bit data bus and the disk drive and
literally do everything.  Many also include serial and parallel ports.

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