Reading an MFM hard drive with a floppy disc analyser

Philip Pemberton classiccmp at philpem.me.uk
Fri Dec 5 04:12:44 CST 2008


I've just been thinking about the problem of imaging/copying MFM hard discs... 
I've run the numbers, and this seems right to me, but I'd appreciate a sanity 
check.


Let's say you have an ST-412 drive you want to read. You've also got a 
Catweasel or something like it, that has somehow had an extra bunch of I/O 
lines added to it (to control the head select lines, etc. on the HDD).

An ST-412 rotates at 3600 RPM. That's (3600/60)=60 revolutions per second, or 
1/60 = 16.66(recurring) milliseconds per revolution.

The drive's data rate is 5 megabits per second, but could be lower (or indeed 
higher). But the spec says 5Mbps, and for the sake of argument I'm going to 
stick with that...

(5Mbps/1000) = 5Kbits per millisecond.

5Kbits * 16.667 = 83.35 kilobits per track, absolute maximum.

A Catweasel records the data from a disc by measuring the time between flux 
transitions.

If we assume every data bit on the disc is a "1" (MFM coding "01") then you'd 
need to store at most 90,000 timing samples per disc track.

So hypothetically, if your "Catweasel or something like it" had 128Ksamples 
worth of buffer RAM, and enough I/O lines to drive the HDD, you could read an 
MFM hard drive track-by-track and copy it onto another drive of the same type?

Do my calculations look right?
With 304 cylinders and 4 heads, that works out to 304 * 4 * 83.35 = 101353.6 
Kbits, which does seem awfully low to me...

Thanks,
-- 
Phil.
classiccmp at philpem.me.uk
http://www.philpem.me.uk/



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