14 Jan
2011
14 Jan
'11
10:05 a.m.
On 13 Jan 2011 at 15:34, Peter Coghlan wrote:
The BBC Micro internally generates the ready signal
for the 8271
controller from index, drive select and the 8MHz clock using a
74LS393, 4020 and 4013. I figured out how the circuit works once and
my head still hurts. As far as I remember, it works like your
description above and if an index pulse is a few percent later than
expected, it deasserts ready.
I should mention that there are many other systems using 5.25" drives
that generate their own READY signal. Even the WD1770/1772
controllers do this within the chip. Details from the datasheet are
a bit sketchy, but the controller counts 6 index pulses from MOTOR ON
before it begins command execution and 10 index pulses after the last
command before it deasserts the motor control signal. What I don't
know (and the datasheet doesn't say) is what the timeout period is
between index pulses before the controller concludes that the drive
isn't ready.
I've got a WD1770-based ISA controller if you're interested, I can
experiement and see what the maximum time between index pulses is if
you're curious.
--Chuck