Subject: Re: ISA IRQ signal active how long?
Date: Sat, 05 Oct 2002 04:52:33 -0400
Organization: Arius, Inc
References: <firstname.lastname@example.org> <3D9E3100.FA5E5BE5@yahoo.com> <3D9E6565.4864FBC3@yahoo.com>
NNTP-Posting-Date: 5 Oct 2002 08:52:29 GMT
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> Or you could use single transistors as emitter followers. They
> would need to be able to stand 5v Vbe(reverse). This does the
> wire-oring for +ve levels, but is relatively hard to implement
> with a choice of IRQ lines.
> IRQreq --|
> |\ emitter, NPN
> +----------> IRQ buss
> 8.2 k ohm (for clamped 1.6 ma, 1 TTL load)
> -12 v
> The IRQreq signal will have to swing up to a minimum high TTL
> level plus 1 Vbe(forward). If having -12V is a nuisance you could
> use -5 and a different resistor, or a current source. You can
> implement a safety current limiter with a resistor in the
> collector line, which will limit the fan-in.
You do realize that this circuit will allow the IRQ line to swing to
-0.7 volts??? I don't think that is within the spec of the PIC. Most
chips have a Vin(min) of -0.2 to -0.5. Anything larger runs the risk of
turning on the parasitic SCR and putting the chip into latchup.
Rick "rickman" Collins
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Arius - A Signal Processing Solutions Company
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