Open-source floppy reader: the data separator/sync-er works!
dkelvey at hotmail.com
Fri Apr 13 09:41:00 CDT 2007
>From: Christian Corti <cc at corti-net.de>
>On Thu, 12 Apr 2007, dwight elvey wrote:
>>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.
>I know what clock bits are ;-)
>The question was, why three missing clock bits (the MFM address mark has
>only one missing bit at position 5). But I've found out that this applies
>to the FM address mark where the clock bits 2, 3 and 4 are missing. The IBM
>manual just wasn't precise enough on this point.
I suspect that it was the difference in technology. When FM started,
the clock/data separator was usually a couple of one-shots. This was
probably more error prone at spotting missing clocks.
When MFM came along, most were using PLL's. Since these actually
tracked the data on the disk, they were more sensitive to catch
things like a single missing clock.
The other thing is that in MFM, the sequence is already missing
a number of clocks because of the data sequence. Missing the
last one makes it an illegal sequence.
In FM, it may be the same thing. It isn't until the third clock is
missing that it is truly an illegal sequence. You have to look at
the data sequences to verify this.
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