Pinout for SED9421

Chuck Guzis cclist at
Fri Nov 25 14:58:01 CST 2005

On 11/25/2005 at 2:29 PM Allison wrote:

>Yes, 1meg was way above the usual 190k-380k common for for DDDS drives.
>The highest I got to with MFM was 800k (80tr 2sided).

The FDC board itself has 56 ICs, counting the 1781, and a fair number of
discretes.  The next edition of the machine simply used a 179x and 96 tpi
drive for 720K, but it wasn't nearly as real-estate hungry.  IIRC, the 1781
was a marginal design; it had a nasty tendency to hang unexpectedly, so a
latch was added for the software to toggle the reset pin.  I can't even
find a datasheet for it anymore.

>The disk that Shugart used while innovative was really bad for
>repeatability of position and slooowwww. 

I wondered how well that disc would wear, but then Shugart had lousy drive
electronics too, so it didn't matter in the long run.

We used a Micropolis leadscrew drive as our calibration diskette writer.
We disconnected the stepper on the positioner and hooked a 100:1 reduction
drive onto it and drove it with a separate stepper.  Took forever to get
from cylinder 76 to 0, but it worked just fine.


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