27 Dec
2003
27 Dec
'03
10:11 a.m.
The Index Mark is a single small block of data
recorded on the disk,
that occurs once per track, right after the index hole; it's NOT the
index pulse from the hardware, and NOT a sector header and it is
optional. Lots of controllers don't put it there unless told to.
Given that any sensible controller will see the index pulse from the
drive interface, the Index Mark pattern is unneccessary -- but some
controllers do expect to see it.
Thanks very much, I had not relaized that there was a differens between
the Index Address Mark and the ID Address Mark. Now it all makes perfect
sense.
Now for antoher question:
On a typical 5.25" drive how long does the Index Pules usually last?
The wd177x and wd179x data sheets don't expliciatly say. But it infers
that is lasts about 60 byte times in MFM mode.
Also I see that a single byte time in MFM is about 2 microseconds, that
suggests to me that the index pulse last about 120 microseconds. Is that
correct?
--
tim lindner
tlindner(a)ix.netcom.com