From: Tony Duell <ard_at_p850ug1.demon.co.uk>
>> some don't, but I can't remember which FDCs do/don't (I think WD 37C65
etc
>> need it but NatSemi DP8473 and WD 1770/1772 don't care).
>
>I seem to remember a problem something like : 765s and related
>controllers (by practical experimentm, the 37C65 is one of these) will
>miss a sector header (and thus be unable to read the sector) if it comes
>too soon after the index pulse. The WD177x and 179x, etc don't seem to
care.
Yes, that is a spec'ed item delay after index. The cheap fix, delay
index
about 95% of one revolution, fakes it into thinking it occured earlier.
>The 765 and the 8272 are essentially the same chip. They use an external
No the ARE the same chip, at one point NEC made them for Intel.
>data separator circuit, and in most PC disk controllers this is
>configured for DD operation only. If there's the well-known 9216 8 pin
data
>separator chip, you might be able to get that to run in single-density
>mode (Inverting the MFM select output of the 8272 and feeding it to pin
5
>(cut the track that grounds this pin of course) of the 9216 often works
>for _reading_. Writing involves modifying the write precompensation
circuit.
Depends on the precomp needed. for single density it's fairly lax, for DD
that might require tweeking or not.
>If the data separator is several smaller chips (like the original IBM
FDC
>card), it's a lot more work.
If you have the drawings, it may be easier. Many more parameters are
alterable that way.
>I think the 37C65 should work in single density mode, but it's a long
>time since I read the datasheet.
It will do single and double 8", unless the board compromized and
used a half speed clock.
>And my experience of other disk controller chips (UMC, etc) is that
while
>they may be documented as working in SD mode, they don't. At least not
>reliably.
It's a setup and config issue.
Allison
Received on Sun May 13 2001 - 19:23:47 BST