Re: Reading 8x50-floppies with 1.2 MB-drive?

From: Hársfalvi Levente (
Date: 2001-11-26 14:38:09


"Baltissen, R (Ruud)" wrote:
> Last weekend I was explaining somebody that it is impossible to read
> 4040/1541-floppies with an 8x50-drive. During this conversation I wondered
> how the data is stored: GCR or MFM?
> AFAIK in GCR-mode. If so, why wouldn't it be possible to read 1541-floppies
> in this drive or 8x50-floppies in a 1571 in 2 MHz mode?

Just my 2 cents (maybe euro, soon?...)

The big C= drives are just higher speed GCR AFAIK.

The difference is, the drive mech is 100tpi (as Nicolas has explained it
last week here) while all others are 48tpi / 96 tpi. Thus the drive head
goes off-aligned a little bit from the tracks with every step towards
higher track numbers.

2030, 4040, 15xx ---> 8x50 : may be possible to read reliably, since the
head width is less than half the original, thus it may be possible to
seek to the real track positions 'closely enough'. I won't make a bet if
it's possible to write data back reliably though (at least not if the
disk was formatted in an 15xx etc. before, similarly to the "writing
360k disks in 1.2m drives" problem).

8x50 ---> 1571: 100tpi mech vs. 48 tpi mech, data is written more than
twice as dense than the head of the 1571 could read. Even if the 1571
could read "halftracks" (as AFAIK most 1541s are capable of), problems
would appear because of the same misalignment problem as above.

2030, 4040, 15xx ---> IBM 360k, IBM 1.2M: mechanically fully compatible,
problems arise from the fact that the PC floppy controller is not able
to pass raw bitstream data to and from the PC (at least without tricks).
Also, writing C= disks with the 1.2M drive could be problematic (the
same problem as explained in the rx50 article you mentioned).

8x50 ---> IBM 360k: same as when 8x50 ---> 1571.

8x50 ---> IBM 1.2M: mechanically incompatible (100tpi vs. 96tpi), and
logically incompatible (GCR vs. MFM, like above). Even if the PC could
read the non-MFM data stream (like with the Catweasel), the misalignment
would be a hard problem to solve.

Best regards,


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