Re: PET disk drive interface

From: Spiro Trikaliotis (
Date: 2002-11-25 13:00:13

Hello Ruud,

Ruud Baltissen wrote:

> Hallo Spiro,
> > The 1541 and the 2031 layout differ in the length of bytes
> > between end of header and start of sync of the data.
> There is nothing between end of header and start of sync of the data: the
> bytes between the end of the header and the start of the data ARE the
> syncbytes.

No. Between the end of the header and the first sync byte, there is 
a gap. Exactly the length of this gap is what is different between the
2031 and the 1541, they differ in one byte.

> The header and all further syncbytes are only written once: when formatting
> a disk. The syncbytes trigger the hardware in such a way that it doesn't
> matter if there are 2, 3, 4 or even more syncbytes in a row. The minimum is
> 11 1-bits in a row IIRC. The drive starts reading/writing the moment it
> detects the first 0-bit. For more tecnical details please see my site at
> Documents, chapter 1541.

No, this is not true, either. The header is written only once. This is
not true for the data block. When writing to a block, the 1541 reads in
the header. After having read to right one, the 1541 waits exactly the
bytes it is supposed to wait and starts writing the sync, the "data block
ID" ($07), the data data (in GCR, of course), the checksum and the padding.

It cannot be the way you tell me - the SYNC is never written again after
formatting - since the R/W head takes some time to be able to write. In
this time, it writes some garbage on the disc, so the $07 (in GCR) and
the SYNC would not be directly after each other.


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