Pinout for SED9421 - test-drb@ccmp.vtda.org

26 Nov 2005

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Subject: Pinout for SED9421
   From: lee davison <leeedavison at yahoo.co.uk>
   Date: Sat, 26 Nov 2005 02:50:31 +0000 (GMT)
     To: cctalk <cctalk at classiccmp.org>

  Figures..  However they loose a full sector for
every four
 which is pretty bad considering sync and other overhead that
 floppies incur. 
It's not that clever. One 2K block of data per track, no sectors,
no sync, just the 125Kb/s async bitstream. Wait for the index
pulse and read/write the whole track. At the time it was a lot
cheaper than any other disk controller.

Lee. 
They trade being efficient for a lot. Across 35 tracks (sa400) thats
about 18k.   NS* hard sector was 10x256 (2560bytes) track, and a 
sync chip (one sector per track) will give that much maybe more 
as well. 

I think what they did was very clever and dirt cheap and the hidden feature
of async is the extra two bit times allows a bit more time to read/write the
port 80uS vs 64uS typical. 

Wacky, but soon as you said OSI.. ;)  they were a creative lot.

Allison