12 Apr
2007
12 Apr
'07
9:50 p.m.
From: Fred Cisin <cisin at xenosoft.com
On Tue, 10 Apr 2007, Christian Corti wrote:
Hi
In FM there is always a clock bit and a location for the data bit. Missing
clock is just that, missing clock! No change to the data bits.
MFM may or may not have a clock, depending on the data 1 bit density.
If missing a clock, it means where a clock bit should be related to data.
Dwight
Ok, I was referring to the IBM 5114 maintenance
manual which describes
FM
and MFM encoding that IBM used for their
drives/controllers. But you are
right, e.g. the "Floppy Disk Data Separator Design Guide for the DP8463"
says
that the address mark $A1 has a clock pattern of
$0A. I don't really
know
what IBM meant with "missing clock bits 2, 3
and 4"... I think it
applies
> to the $FE or $F8 address mark in FM.
While you are at it, notice that the 1771 can create some FM data address
marks that the 179x can not. Worse yet, they are used in TRS80 (model 1)
disk formats.
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