Head positioners (was: Orbis
cclist at sydex.com
Sat Sep 24 11:57:25 CDT 2005
On 9/24/2005 at 9:24 AM Fred Cisin wrote:
>The Shugart was with a separate stepper turning the cam.
>I believe that Chuck is referring to a a similar mechanism connected
>to the SPINDLE motor, resulting in a spiral track! instead of the
>usual series of concentric circles for tracks.
Exactly--once the head was started in motion, it traced a spiral and then returned to home position, all as one single operation--there was no way to stop or pause the positioning--or move the positioner independently of the spindle Very much like a tape drive, if you can visualize a strip of tape laid flat and coiled in a helical shape.
Reminds me a bit of those "stringy floppy" units from the 70's with a continuous loop of tape crammed in a housing.
The encoding was very simple--MFM with with sync patterns between sectors and 16 bit CRC summing. A contract entailed looking for latent data on a pile of these things, so we worked up an ISA card with a WD9216 data separator and a 2651 USART with some TTL "glue", using the 2651 facility to look for a SYNC double-byte value to find the lead-in to each sector. The bit clock rate was pretty low--perhaps 125 KHz. It was much easier to work up a board than trying to figure out how to use the hardware on the word processor itself to do the work (I think the SCM WP used an 8051-family chip).
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