7 Apr
2007
7 Apr
'07
2:57 p.m.
On 7 Apr 2007 at 13:49, Jules Richardson wrote:
Actually, I wondered this the other day; ESDI got a
lot bigger (around 600MB I
think) but IIRC was pretty much the same interface. So quite what limited the
capacity of ST412 drives I'm not sure. I believe that cylinder seeking was
done by a sequence of single steps (rather than asking the drive: "seek to
this cylinder"), so there's probably an upper limit in the ST412 spec of how
fast step pulses can be sent (and beyond a certain size a drive will either
become slow as mole-asses, or some other timeout will come into play :-)
ESDI uses a substantially different interface from ST412, even though
the cables may resemble each other. The interface itself was much
faster (15Mb/sec vs. 5Mb/sec) and IIRC, the data separator logic was
located on the drive. Up to 7 drives could be addressed by a
controller, though many PC controllers imposed a limitation of 2
drives (I suppose to be compatible with the WD1003-type interface).
Hard-sectoring was very common and the number of sectors per track
was rougly double that of ST412. I believe that the 670MB Miniscribe
that I have uses something like 54 sectors/track.
From a port interface perspective, a WD1007 (and
probably most other
PC ESDI controllers) appears to be a spitting image of an IDE
drive,
complete with support for the IDENTIFY command. As a matter of fact,
many OSes will mistakenly identify a WD1007+ESDI combination as an
IDE drive.
All in all, not a bad gizmo. etc. I think I've got WD and DTC
controllers for ESDI kicking around.
Cheers,
Chuck