28 Jul
2007
28 Jul
'07
4:37 a.m.
Ah, the step gremlin. It's an old 765(all!!!)
problem. The problem is when
the step pulse is set to minimum acceptable for the drive it's possible that
due to internal timing of the 765 it can shorten the step timing of the
first pulse by 1 count. If that occurs many drives seek badly and you get
read or write errors because your not where you thought you were.
Note: some old drives due to the lubricants turning to goo will also exhibit
this type of error. There is one solution, step slower (SRT+1).
A possibility - Christial, try using SR= to set the step rate slower.
Another problem is the 765 was designed for 77track
drives (8" floppy)
and there were no 80track minifloppies. So the Recal only issued
77 step pulses and would flag an see error if the drive was 80track.
One fix is to issue a new Recal and see if the drive properly homed
this time. The problem is there are some cores that fixed this and will
issue up to 256 step pulses. The problem can also arise when using
SA400/400L style drives ans some (not all) will jump track (loose
the groove on the positioner disk) if stepped too far past tracks
000 or 39.
Yeah, known problem - I issue a second recal if the first does not
result in track-0. Agreed, with the "right" controller, and failure
mode, this can result in 512 step pulses going to the drive - not much
you can do.
Last gremlin is the 765(and cores) were designed to
Seek/Recal up to 4
drives but PC hardware does not always decode this and may have drive 0
also appearing as drive 2 or 3 which makes it susceptable to being
repositioned by commands not intended for it.
IMD only works on one drive at a time, and only issues one command at a
time.
Thanks for the input - I'll ask Christian to check out a slower step
rate.
Dave
--
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
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