Linux-kernel mailing list archive 1997-11,: RE: Sharing IRQs between the first 4 serial ports.

# RE: Sharing IRQs between the first 4 serial ports.

Niels Kristian Bech Jensen (nkbj@image.dk)
Sun, 23 Nov 1997 19:53:26 +0100 (MET)

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On Mon, 24 Nov 1997, Peter T. Waltenberg wrote:

> I suspect the existing code is correct.
>
> On most systems, you can't actually share com's IRQ's in the sense that they
> can't be used by both ports at the same time. You can use one or the other,
> but not both.
>
> The com's ports have logic to tri-state their IRQ's so that one can safely sit
> unused while you use another. But trying to drive one port's interupts while the
> other is also enabled (but not interupting) results in a bus contention.
>
> With the correct hardware, or a few diodes and a resistor, you can make it work
> properly, but the "standard" PC hardware won't work reliably.
>
I won't recommend using two ports on the same irq on standard hardware.
I've tried it on my system (mouse on /dev/ttyS0, irq 4 and modem on
/dev/cua2, irq 4) and it slows both ports down to crawling speed. This
means that if you have two serial ports plus an internal modem, one port
will be almost unusable if the modem shares an irq with one of the other
ports. I made the patch to make it possible to set up serial ports 3 and
4 with different irqs in a clean way to avoid this problem.

    --
    Med venlig hilsen / Regards

    Niels Kristian Bech Jensen              nkbj@image.dk
                               http://www.image.dk/~nkbj/

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