26 Dec
2003
26 Dec
'03
3:35 a.m.
On Fri, 26 Dec 2003, tim lindner wrote:
In the data sheet, under the section "1. Non-IBM
Formats" it says:
"Note that the Index Mark is not required by the 179X."
What does this mean exactly? How can a track contain any useful
imformation without a Index Mark?
Those formats have both DATA address marks, which mark the beginning of
each sector, AND and INDEX address mark, which marks the beginning of
the TRACK. It is the INDEX ADDRESS MARK that is not needed.
If the computer is looking for a given sector to read or write, then it
steps to the appropriate track, and starts looking at the sector headers.
If it encounters the right sector header before it gets around to the
index mark, it proceedds with the read or write, without ever having had
any need for the index mark.
Also note: the NEC type chips (765, etc., the ones used in PCs) perform
some sort of reset whenever they encounter the index pulse, and therefore
can not handle any data that starts too soon after INDEX. Because the WD
type chips (NOT used in PCs) do NOT do that reset, they can start their
first sector sooner after index than the NEC type can handle. Therefore,
in reading SOME alien formats on a PC, it may be necessary to mask the
index pulse (interrupt the cable, or opaque tape on the diskette)
--
Fred Cisin cisin(a)xenosoft.com
XenoSoft http://www.xenosoft.com