23 Aug
2005
23 Aug
'05
12:11 p.m.
Hi folks,
I've been reading through the uPD765, R6765 and 8272 FDC datasheets. All of
them seem to cover the basics pretty well, but I've got a few quick questions
for anyone who's used a 765-type FDC (or any other FDC that behaves in a
similar way), or who knows a bit about floppy drive technology:
1) How do you vary the read/write data rate? Some 3.5" formats use MFM at
250kbits/sec, others use MFM at 500kbits/sec. What relation does the WRITECLK
input have to the data rate of the MFM-coded signal at the WRITE DATA output?
What about the 8MHz master clock?
From what I rememebr, the master clock is kept at 8MHz,
you change the
WrClk depending on the data rate (I can't remember if it _is_
the data
rate, or some contant multiple of that). You have to change the RdClk
(from the data separator) too, of course).
Since the only diference between DD and HD is the data rate, this should
solve your question (1a)
1a) Can the uPD765 be made to read and write both DSDD and DSHD floppies?
If so, how? I know the WD 177x controllers could do it if the 8MHz master
clock was increased to 16MHz (an old trick used on Acorn A3000s). Does the
same trick work with the 765?
2) How does the FDC decode the data it's been given, based on the data
window and the data read from the disc? In FM mode I guess you'd count the
cycles within the data window - 2 would be a 0, 4 would be a 1. But what
would you do for MFM?
3) What happens when the drive's head picks up a flux transition? Does the
the drive generate a single pulse every time the head hits a transition, or
does the drive's RD DATA output change state?
According to the scehmatics for every floppy drive I've got, there's a
constant-width pulse on the Rd Data line for every transition on the
disk. The pulse width is set by a 1-shot in the drive.
4) When writing data, I'd guess that setting WR DATA high would polarise
the magnetic coating one way, while setting it low would polarise it the
opposite direction. Is this how it works, or does the WR DATA line get pulsed
to make the drive put down a flux transition?
A particular edge (I guess it's a falling edge at the interface pin) on
the Wr Data line causes a flux transition on the disk. The Wr Data line,
suitably buffered, goes to the clock input of a toggle flip-flip (often a
'74 with Q/ linked back to D), the outputs of this go to the write head
driver transistors.
I am suprised there are no floppy drive schematics on bitsavers...
-tony