9 Apr
2007
9 Apr
'07
6:28 p.m.
On 9 Apr 2007 at 3:17, Jules Richardson wrote:
Hmm, so maybe the trick is to find someone selling an
old MK3 board, rather
than the MK4...
Is the MK4 real? It's not clear from Jens' web site. If it is, I'll
probably buy one.
Hmm, that's interesting. Does it store fixed
length counts in the buffer, or
does it use variable-length counts with a couple of bits of "header" to say
how many bits the count takes up?
Each count takes up 7 bits of an 8-bit byte. The high-order bit
records the presence of an index pulse (essential for decoding hard-
sectored formats). He gets around the fixed-width issue by having
several (3, IIRC) programmable clock speeds. As I said, brutally
simple.
One thing that I've found to be useful is the construction of a
"buffer board"; i.e., some high-current OC drivers and schmitt
receivers that feed from an DC-37 external drive connector to the
MK3. I've found that the bare MK3 driving long cables on "heirloom"
drives don't seem to have the 'oomph" to get the job done.
I was thinking the same thing with this concept of
over-sampling the entire
track into a buffer; it's going to be a heck of a lot easier to decode the
data while looking at a graphical utility.
One thing that's useful is a histogram of a track. FM will show two
peaks; MFM, three. GCR usually shows more than three. Unformatted
tracks show very broad distributions.
GCR can be very challenging. You have an idea of the way it works
and what the "prohibited" bit combinations probably are, but you
still don't have your mappings. I've taken several tracks and, based
on a suspicion that each sector probably contained some sort of
header information that included a cylinder number, have worked from
that.
I've still got a couple of GCR formats that I haven't cracked yet,
though. The process of decoding a new format that isn't FM or MFM is
usually one of staring at the data for a couple of hours and then not
thinking about it for a week or two. Eventually, there will be an
"Aha!" that usually comes sometime around bedtime. :-)
Cheers,
Chuck