27 Nov
2005
27 Nov
'05
6:19 p.m.
Interesting,
I've never seen a floppy drive with buffered seeks, and
since there's no seek-complete pin on the interface connector, I don't
see how it could really work. Every floppy controller I've ever seen
sends the step pulses at a constant rate, which would have to be slow
enough for the worst case.
READY goes false while seeking. You can issue pulses as fast as you want (I know the drive will take a pulse every 2 msec. and
probably much
faster). In a way, this isn't all that different from the overlapped seek
That reminds me in passing of the Shugart SA4000 hard disk. That thing
has buffered seeks, but you can't issue step pulses at any old rate. You
either have to give it a step pulse and wait for it to give seek_complete
(i.e. totally unbuffered seeks) or give it all the pulse at greater than
a certain rate and then wait for the seek complete (buffered seek). If
you give it the pulses too _slowly_, it will miscount (there is one
up/down counter for the step pulses, if you're not careful the pulses
from the step input get confused with those from the
drive logic when it
moves the heads by one cylinder!).
Sure... I
remember the Sanders dot matrix printers that ramped up (and
down) the step rate of the carriage feed stepper and sent the printhead
pulses at just the right times to put the dots in the right places.
Are you certain that Sanders used a stepper for carriage positioning? If they were like Diablo or any of the other high-speed
printers, it was more
likely a DC motor with encoder. Steppers were just too slow, particularly
for bidirectional "smart" printing. IIRC, one of the engineers who worked
I think that's the reason for a lot of the Sanders electronics (the 700
had 3 micropeocessors in it -- 2 Z8s and a Z80) - -to handle the
acceleration of the carriage motor and put the dots down at the right time.
on the Diablo dot matrix printer (can't recall the
model number, but it
used the Rockwell PPS-8 set) said that the whole business of accelerating
and decellerating a mass like a printhead made for some interesting
programming.
Paper feed, on the other hand, was almost certainly stepper-driven.
It certainly was. The 12/7 had a separate tractor feed with its own
stepper motor that was simply wired in parallel with the platten motor in
the printer. Turning the knob on the tractor feed caused the motor in
that to generate enough voltage to turn the platten roller motor, even
with the machine turned off.
-tony