24 Nov
2005
24 Nov
'05
11:01 p.m.
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
25 Nov
25 Nov
2:31 p.m.
Allison <ajp166 at bellatlantic.net> wrote:
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.
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 
5:24 p.m.
On 11/25/2005 at 9:31 AM shoppa_classiccmp at trailing-edge.com wrote:
to do the tweaking, as well as field service, as well
as ...
(Of course us hackers don't mind!)
I've got a system here with a rather elaborate analog data separator PLL
(built around a CA3130 op amp). Part of the manufacturing process involves
a calibration diskette, trimpot and scope. It's pretty robust once it's
been set up, but component aging is probably a significant factor in
long-term performance.
It seems hardly any more robust than a plain old digital (9216-type) data
separator. Which makes me wonder why anyone would bother implementing the
data separator in Figure 13 of the 179x app notes (3 trimpots).
Out of the stats on performance that Allison cited, how many of those
errors could be recovered with a simple retry?
When I learned that Jameco was running low on 9216's, I bought their entire
stock, which amounts to about a 2 ft. tube of 8 pin DIPS. So if you're
looking for one, you know who's got 'em. :)
Cheers,
Chuck
6973
days inactive
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2 comments
3 participants
participants (3)
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ajp166＠bellatlantic.net -
cclist＠sydex.com -
shoppa_classiccmp＠trailing-edge.com