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?
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?
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?
Thanks.
--
Phil. | Acorn RiscPC600 SA220 64MB+6GB 100baseT
philpem at philpem.me.uk | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
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