[ILUG] Re: Serial Port IRQ Question

[kilmartin mark]

I have seen a site which describes how to hack a serial I/O card so as all
the serial ports can be used at the same time.

http://www.nukem.freeserve.co.uk/contents/computing/serial/index.html

I have not tried this yet but if anybody does try it could you let me know
how you get on.

Mark.




> 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
> 
> 
> 
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