Message-ID:	<3E251E0C.3144A6A8@invalid.com>
From:	Ismo Salonen <not DOT valid AT invalid DOT com>
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Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Libnet serial port problem
References:	<f1a1bbde DOT 0301132122 DOT 7672b79f AT posting DOT google DOT com> <3e23e78b$1 AT news DOT infonet DOT ee> <kc410b DOT 4u DOT ln AT jellix DOT jlfencey DOT com>
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Date:	Wed, 15 Jan 2003 08:37:39 GMT
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To:	djgpp AT delorie DOT com
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Vinzent Hoefler wrote:
> 
> Tonu Aas wrote:
> 
> >> I'm having some trouble with some serial port code I'm writing with
> >> Libnet under djgpp. Everything's fine if I only use one serial port,
> >> but if I try to receive from both serial ports at once, information
> >> can only get through from one of them at a time. Each serial port
> >> works fine by itself.
> >
> > If they use the same IRQ(COM1, COM3 or COM2 COM4) then its normal.
> 
> No. Just bad software. There are enough status bits on the UART chip to
> see which one has raised the interrupt. It just slows down a little
> bite, because you have to ask all possible UART that are sharing the
> same IRQ.
> 
> Vinzent.
> 
> --
> According to Kentucky state law, every person must take a bath at least
> once a year.

Actually the problem is hardware related. On older computers the IRQ
line is 
high level active. Each uart drives the irq line it is connected to
via tristate buffer. The buffer is enabled with a bit in the uart. If
two uarts drive the same line then results are unpredictable.
If one drives it up and other it down it might even break the buffers.

So usually you can connect only one (active) uart to an irq line. 
Software could be setup to ignore the interrupts and poll the uarts
if needed. 

Of course COM(1 or 3) and COM(2 or 4) should work same time because they
use different irqs.

ismo

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