Re: Reading non-MFM disks in PC with a "small adapter"

From: Hársfalvi Levente (
Date: 2001-11-20 15:09:06


> The SFD disk format is not a good reference for a bitstream converter. While
> it should be normal Commodore GCR at a higher speed, the disks are recorded
> with a track density of 100tpi, which makes them impossible to read in a
> 96tpi PC drive.

Thanks for pointing this out, I haven't known about this issue :-(. BTW
what's the matter with other "big" Commodore drives? Do the 4040, the
8050 and similar drives also use 100tpi mechanics? Then, connecting the
drive to the PC is indeed the only real possibility to handle their
disks reliably :-(. I'm somewhat disappointed.

> > Another problem is to read both sides of the 1541 disk - when you insert
> > the disk with the second side, the PC drive provides no index signal,
> > thus the FDD controller won't even believe that the disk is spinning.
> According to the online docs of the Catweasel controller some/most drives
> have a problem on their own when there is no valid index signal: they just
> shut down their complete read circuit, so you have to supply a fake index
> signal _inside_ the drive. They also list some methods to do this.

Whew, then probably the only "easy" way to go is to insert the disk with
its proper side, and catch the data from the head above the disk +
reverse the stream. Problems: I seem to remember of some alignment
problems when trying to read tracks recorded using the opposite head
( it true, BTW? I haven't done measurements regarding this
problem). And probably some pulse timing problems in the write process,
but that's just another issue...

> I wouldn't install a 5,25" drive in my main PC, but I have them in my older
> 486 machines. Personally I don't have a problem with using an external
> 1541/1571 via XP1541 link, but some people wanted an internal drive so much
> that they used a slimline OC118 drive (1541 compatible) and fitted it into a
> 5,25" drive bay of their PC.

I haven't known about this :-O.

Me personally would, at least if I had space for an extra 5.25" unit in
the midi tower. The advantage would be to handle any possible disks in
the same drive, very quickly, with the possibility to even handle
tricked (protected) disks at the lowest level. Though, if you tell me
about some more quirks that make it complicated / unreliable to handle
C= disks this way, I should probably drop the idea.


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