Pinout for SED9421

Tim Shoppa shoppa_classiccmp at trailing-edge.com
Fri Nov 25 08:31:03 CST 2005


Allison <ajp166 at bellatlantic.net> wrote:

> >Subject: Re: Pinout for SED9421
> >   From: "Chuck Guzis" <cclist at sydex.com>
> >   Date: Thu, 24 Nov 2005 14:19:32 -0800
> >     To: cctalk at classiccmp.org
> >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
people
to do the tweaking, as well as field service, as well as ...
(Of course us hackers don't mind!)

Tim.



More information about the cctech mailing list