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

From: Nicolas Welte (
Date: 2001-11-20 12:55:26

Hársfalvi Levente wrote:
> I suppose, the highest data density produced by native C= floppy drive
> (not counting the CMD FDDs that are MFM anyway AFAIK) is of the
> SFD1001s. The shortest period, found on the SFD1001 disk was somewhere
> about 3.75 us, and the next one was 5.25 us (please correct me if
> someone has more accurate data). Note that the PC drive runs @ 360rpm
> instead of 300. At track 68, the same periods are 4.55 and 6.55 us long,
> respectively). The pulse width is constant - I mean, the drive produces
> a stream of equally long 0.5us pulses with variable "distance" from each
> other.

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.

Someone once sent me an SFD1001 for repair, and a replacement mechanism that
he said came from Commodore. But the type number didn't match at all, and it
turned out that this was a 96tpi version of the SFD mechanism. With this one
installed, it was possible to read the directory, but it would report read
errors on most files. I ended up using the 100tpi stepper motor from the dead
drive (broken r/w head) in the good drive. BTW, the motors themselves are the
same, but the wheel that drives the r/w head has a different diameter.

> 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.

> The next thing that comes in mind is, if this is all worth the trouble
> to play with at all. I know 5.25" drives aren't manufactured since
> years. ...Does anyone still have 5.25" drives? Would it be a so much
> more convenient way to go that people dusted off their old 5.25" drives
> to reinstall them into their new PCs?

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.


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