RL drives (was: cctalk digest, Vol 1 #319 - 51 msgs)

From: Jeffrey Sharp <jss_at_subatomix.com>
Date: Sat Dec 14 12:51:01 2002

On Saturday, December 14, 2002, Johnny Billquist wrote:
>> During the time that sector notch passes by the sector transducer, the
>> heads detect two servo pulse bursts (S1 and S2) that are prerecorded on
>> the platter. These servo bursts are used by the drive logic for head
>> positioning."
> Seems like there shouldn't be any argument about this. The RL drives have
> the servo information embedded in the normal track.

That shows that the servo bursts are logically associated with a sector, but
it does not say that they are recorded at the same radius as the sector data
itself. Since the servo bursts are not field-recordable and are not a
need-to-know topic (other than the fact that they exist) for the manual you
quoted, it is conceivable that DEC simplified (or obfuscated) the fact that
S1 and S2 occur at different radii.

And having the radially offset servo signals makes more sense to me, anyway.
IANAEE, but it seems that with the burts in the track, the drive electronics
wouldn't know whether to move the heads in or out; it would only know that
it was off-center. With radially (and angularly!) offset bursts as in Tony's
diagram, the drive electronics know which burst (inside or outside) the head
is closest to, and it would thus be able to discern which direction to move
the heads.

Jeffrey Sharp
Received on Sat Dec 14 2002 - 12:51:01 GMT

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