23 Aug
2005
23 Aug
'05
2:47 p.m.
In message <0ILO00FI3MXS1LM0 at vms048.mailsrvcs.net>
Allison <ajp166 at bellatlantic.net> wrote:
Write clock sets the write data rate and should
corrospond to the
rate needed for media. Writeclock is 2x data rate.
Does this apply to both FM and MFM? The datasheet suggests a 500kHz WrClk for
FM and 1MHz for MFM mode, which would give a 250kHz FM data rate and a 500kHz
MFM data rate. Is that right?
And RdWindow from the datasep should cover the read frequency variations,
assuming the datasep can handle the variation in mode and frequency.
the logic does things like Data Clock recovery, Write
precompensation timing
I've heard the term "write precompensation" thrown about a bit - what's
the
purpose of it?
Using a 765 for _all_ data rates and both
3.5"/5.25"/8" floppy interfaces
implemented with LSttl fills a S100 board completly.
That's why SPLDs (GALs) and CPLDs were invented I guess. Squish the address
decoder, interface logic, and the data separator into a few LSTTLs and GALs,
or just throw everything into a single CPLD :)
Most designs implemented a subset for 8" or
5.25"/3.5" only
(jumper selected) with much lower parts cost.
That's what I wanted. I don't have any 8" drives, so adding support for them
is pointless. Ideally, I want a board that has a 6502-type interface on one
side and a 34-pin IDC connector on the other, with support for most common FM
and MFM 3.5" and 5.25" formats.
Yes, it was done on the 1.2MB AT-class machines.
Reminder: every PC with
rare exceptions used a 765 or a derivitive based on the 765 core.
So I've noticed. I managed to get DMA-mode transfers working, but never
managed to get polled-mode or interrupt-mode transfers working. Guess I'll
have to learn fast when I start writing code to drive the FDC...
For MFM is when the transiston occurs rather than how
many. Generally the
Floppy generates what is by standard expected and the FDC external logic
will regenerate data pulses and data window (data window is a read clock).
Right. So with FM the datasep syncs to the high-frequency signal in Gap1,
then every time there's a 0 you'll pick up a 0/1/0/1 sequence for each read
window, and when there's a 1 you'll pick up a 1/1/1/1 sequence?
And with MFM, a 0 after a 0 would be 1/0, a 0 after a 1 would be 0/0 and a 1
would be 0/1?
0/1 taken to mean "first data window was empty, second had a pulse in it".
Thanks.
--
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