amiga floppy disk reading experience
cclist at sydex.com
Fri Jun 11 16:12:03 CDT 2010
On 11 Jun 2010 at 12:11, Eric Smith wrote:
> Note that the only data separator worth a damn in the WDC app note are
> the two that use PLLs. The most commonly used was the one in Figure
> 14 that uses a 74LS629 VCO and the phase comparator of the WD1691.
> The circuit by MPI in Figure 13 that uses an MC4024 VCO looks OK but I
> haven't seen it used. The rest are crap.
> Figure 11 uses a one-shot, and won't work reliably even for single
> density. It's comparable to the worthless data separator built into
> the 1771. Figure 12 uses a state machine and works approximately the
> same way the Apple II read state machine works. It's probably good
> enough for single density, but won't work reliably for double density.
There seems to have been a general move toward "good enough" in the
early 80s. The first version of the IBM 5150 floppy controller used
an IBM hybrid-packaged design; the second version used conventional
TTL and a 4024 VCO that was good enough, but not as good as the first
version. I think later controllers (as used in the 5170) used the
all-digital WD9216 type of data seaparator. With the all-in-one FDCs
(WD37C65, N82077), the question of data separator design became a
nonstarter. Just plug it in with no adjustments needed.
I suppose "good enough" accurately describes the Apple II scheme;
"gutless wonder" would be another term that comes to mind. GCR with
the only error conrol being a one-byte arithmetic checksum on each
sector. Were there ever any "turbo" mods to run an Apple II at more
than 2MHz without breaking the floppy logic?
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