4 Jul
2000
4 Jul
'00
8:53 a.m.
The only problem I'd point out is the the 9500
series CPLD's drive only 8 mA
per output and, unless you were planning to pair them up, i.e. use multiple
pins tied together to drive the FD cable, you might have problems.
The biggest problem is with the outputs that'll drive the floppy disk bus.
Typically it's an open-collector output terminated to +5V with 150 ohms, so
you have to be able to sink 30 mA or so. So yes, this may be a concern.
(Or you can cheat and tie the lines high with "only" 220 or 300 or 470 ohms -
shouldn't be a big problem as long as there's only one drive on the bus.)
Do any CPLD's have "real" open-collector outputs, or can you fake it by
sending the output into tristate mode? (I'm asking this as a guy who's
never designed with CPLD's but does know 74XX00 series logic pretty
well.) I don't think it's an issue either way with exactly one controller
and one floppy drive on the bus.
--
Tim Shoppa Email: shoppa(a)trailing-edge.com
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