[ILUG] Re: Serial Port IRQ Question

[Niall]

On Wed, Mar 22, 2000 at 08:11:57PM +0000, Peter Ryan wrote:
> 
> I'm building a mini-ISP  (4 internal 33.6K modems) and I want to know if
> I'll take a major performance hit if I share IRQs 3 and 4.  I guess that
> this will be a problem as I will have 4 dial-in users active at the same
> time.

Either very little performance hit, or it won't work. If they're ISA modems,
it won't work. You can share an IRQ between two serial ports, but only one
can use it at a time, which is not what you might expect IRQ sharing to
mean. Unfortunately, because of the electrical design of the ISA interrupt
system, this can't be avoided.

OTOH if you have PCI modems (real modems, not Winmodems) you should be able
to share IRQs as you would like as the PCI design allows for multiple
devices to share interrupts. This can be a performance hit with e.g. SCSI
and network cards but multiple modems shouldn't be an issue.

> If Linux is able to share IRQs then this should be no problem.

As explained, Linux allows you to tell it that two ISA serial devices use
the same IRQ but you can't use both at the same time.

> Comments and suggestions to the list, or to me at

My comment would be that this should be eminently doable, as most cheapish
ISA modems will allow you to choose IO port address and IRQ fairly freely,
so you could use IRQs 3,4,5 & 7 for the modems. (Some just allow you to
choose COM[1234] where the manufacturer has decided what combination of IO
port and IRQ COMX actually means - avoid these like the plague). You'll have
to wiggle your BIOS to either disable the onboard serial ports, or move them
to some other addresses, if supported.  Of course if you're trying to use
some old modems you have which will only allow IRQ 3 or 4 - you're screwed :-(

Kindest regards,


Niall O Broin