🚀 go-pugleaf

in article <762@mindlink.UUCP>, a186@mindlink.UUCP (Harvey Taylor) says:

>   In every file of harddrive specs I have ever seen there has been an
>  entry for Reduced Write Current and Write Pre-Compensation. I was jawing
>  with a bud tonight & I realized, I didn't have a clue what they were!

In the beginning...
The Seagate ST-506 drive wasn't very bright.  It didn't remeber what cylinder
it was on, and it didn't understand the physical properties of it own disk.
The controller would keep track of what the current cylinder was, prompting 
the drive to step in one direction or the other.

As the target cylinder got closer to the center of the platter, loss of 
physical flying speed over the platter would cause the head to droop ever 
closer to the media.  After some predetermined point, usually half way in, 
the controller would signal RWC, telling the drive to reduce the write current.
The lower current was allowable, because the head was closer to the media, 
and would soon be required, as the larger current would "bleed" on inner 
cylinders.

Newer drives keep track of their own cylinder count, dropping write current
at several different points, maintaining a more optimum rate across the disk.
This allowed RWC, pin 2 of the Daisy cable, to be reassigned for use as 
Head Select Bit 3, raising the maximum head number from 7 to 15.

Meanwhile back at the controller, it was observed that as you got nearer
the center of the disk, the ~10,000 bytes that used to span a circumference of
12" now were packed into a circumference of 8".  Each bit began to have some
effect on its neighboring bits.  In the move from FM to MFM, we no longer
have clock bits as guards, so the spacing of the data is critical.
Certain "runs" of bits would cause the Phase Locked Loop Oscillator to drift 
off frequency, as it tried to stay in synch with the inferred clock bits.

During writes to cylinders beyond some manufacturer-specified "Write PreComp" 
point, the controller would shift the bit cell clock applied to certain bits
in a pattern, compensating for the effect of the read while doing the write,
hence the term Write Pre-compensation.  

Both RWC and WPC were typically specified at the same cylinder.
Modern drives all do their own RWC, leaving the controller to do WPC, 
and the better drives do their own WPC, leaving nothing for the controller
to do except sync on a steady data stream.

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Clarence A Dold - dold@tsmiti.Convergent.COM            (408) 435-5293
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               P.O.Box 6685, San Jose, CA 95150-6685         MS#10-007