Pinout for SED9421

Tim Shoppa shoppa_classiccmp at
Fri Nov 25 08:31:03 CST 2005

Allison <ajp166 at> wrote:

> >Subject: Re: Pinout for SED9421
> >   From: "Chuck Guzis" <cclist at>
> >   Date: Thu, 24 Nov 2005 14:19:32 -0800
> >     To: cctalk at
> >Has anyone noticed a significant performance difference between the PLL
> >type data separators (e.g. the WD1691/2143 combo) and the plain-Jane WD9216
> >digital ones?  I suspect that the PLL type is more tolerant of drive speed
> >variations, but that hardly matters all that much with modern drives.  I
> >don't recall finding a disk that failed to read on a generic 9216-equipped
> >FDC but read fine on the old IBM PC floppy board.
> >
> >Cheers,
> >Chuck
> I've used he 9216 and 9229 and the NEC apnote digital data seperator 
> (prom state machine synthetic PLL) as well as a custom NEC ASIC (with
> the prom data sep inside)  and they all do very well.  Testing against a
> well developed PLL (not the WD animal) showed a small difference after a
> 4 days of continious read and writes (number of fails less than 3).  
> You have to do a very large number of reads without external influences to
> see the difference.  Power blips and the like were a pain.  A small 
> difference was seen in the order of one of one failure per 250,000 reads
> vs 1 in maybe 400-500,000 reads. You need a lot more testing than we did 
> then to get near 1:10^8 stats with any validity.
> FYI:the average PLL is a bear to build and debug, they required clean 
> power and good board layout with ample groundplanes.  The digital ones are
> very good, simple to layout and shift rates with only a mux.
> Allison

Don't neglect: PLL's require analog components of rather tight tolerances
to give consistent behavior from time-to-time and unit-to-unit.

The digital data separator (I heard its designer once refer to it as
a "jerk-locked-loop") has no such tight tolerances and in fact is usually
driven from a crystal oscillator.

The one-shot-with-critical-RC-constant used in early FM data separators
is a good example.  With a little tweaking it really works pretty well.
But imagine mass-producing such a beast and training the assembly line
to do the tweaking, as well as field service, as well as ...
(Of course us hackers don't mind!)


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