Pinout for SED9421

[Allison]

>
>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