21 May
2013
21 May
'13
1:05 p.m.
This might be a silly question...
Since the specs of the MGT +D disk interface are now out there in the
public domain...
http://www.worldofspectrum.org/NotThePlusD/
... but 720kB 3?" floppy drives are getting rather scarce, I was
Most older 3.5" HD drives support 720K disks. Unlike the 5.25" IBM PC
drives, the spindle speed on the 3.5" ones, and the number of cylinders,
is the same for DD and HD drives (yes, Frid, I am misusing those terms a
bit, but I think you know what I am talking about).
wondering... would it be viable to build a version
that supported
1.4MB "HD" drives? I realise this would mean a different FDC chip, but
are there other implications apart from tweaking the DOS?
In most cases the FDC chip can remain the same, what it needs is a higher
data rate, A faster write clock and a modification to the dat separator
clock. If the FDC is campable of handlign 8" drives, then it can handle
HD 3.5" drives.
I know few 8-bits supported HD disks, but I don't
really understand
why. My electronics knowledge is poor. (Cue Tony Duell.)
The problem is that the higehr data rate on the floppy drive leads to a
higher byte transfoer rate on the processor side of the disk controller.
Msot 8 bit machiens (certain of the Spectrom type) didn't use any form of
DMA fro the floppy controller, it relied o nthe processor to transfew the
bytes as neeeded. Doing this at the DD data rate normally involved some
tight code. I think doing it at the HD rate would be very hard/impossible/
I am guessing that an 8-bit would struggled to read/write data fast
enough to keep an HD diskette controller supplied with data, but I was
wondering if that could be overcome with interlacing or adding some
cache RAM?
It would, but the interfce is not trivial. You bascially have to make
some kind of mini-DMA device to access that RAM and transfer bytes
to/from the FDC as needed. Not hard, but not a single-chip solution
either. Well, actually, it could be a single chip soloution, I susepct a
microcontroller would now be fast enough or you could use an FPGA. But
again you have to program them.
This is probably a completely pointless exercise -
people seem more
interested in using CF or SD media these days, but I enjoy the
nostalgic element of actual floppy diskettes.
There are 2 obvious solutions. The first is to get a 1.44M drive that
reliably supports 720K disks and use that [1]. THe other, of course, is
to get a 720K drive that can be repaired nad maintained (and yes, they
did exist) and keep it running.
[1] I don't know if any of the USB floppy drives do that. But contrary
to what has been said here before. some USB floppy drives contain a
normal floppy drive mechanism, often with a 26 wire flexcable as the
interface, linked to a tiny USB interface board. You could extract the
drive and use it with some other controller.
-tony