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SubjectRe: serial ports
Magnus Hiie:
> Why is it impossible for multiple (standard) serial ports (ttyS?) to
> use the same IRQs?

It's a hardware limitation of the ISA bus. Multiple ports on the
same card can share the same IRQ if they can be logically OR-ed.
But multiple ISA cards can't use the same IRQ line at the same time
- they have three-state outputs which can't do wired-OR (if one of
the outputs is high and the other is low, the result is undefined -
might work with some combinations of cards but don't count on it).
It's one of these things IBM screwed up in the original PC and it
is difficult to change it now because of "backward compatibility"
(EISA and PCI finally got it right but ISA is still quite common).

Hope this helps.


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