On Thu, Mar 09, 2000 at 01:37:32AM -0500, David Kramer wrote:
> I need to get another serial port or two on my linux box. I have one for
> the mouse, another for my IR controller, and I would like at least one
> more for an X10 ActiveHome controller. Ideally I would have a fourth for
> a dial-in modem.
> If I get a normal add-on two-port serial card for ~$30-$40, could I use
> that in addition to the two ports I have now, or are they going to
> conflict with IRQ's?
I run 4 serial ports in a number of systems (in addition to using
Computone Multiport boards where I really want a lot of ports). You just
set the additional board to COM3 and COM4 and set the interrupts to something
you're not using, like IRQ5 and (if you're not using the printer interrupt)
IRQ7. If you've got other things that are IRQ expensive (sound cards)
you could run out of interrupts quickly.
> Real 4-port serial cards seem to be very expensive. Are there any that
> are not? I don't need fast. Does each com port need an IRQ? That would
> suck.
Some intelligent mulitport cards, such as the Computone, Digi,
Cyclades, and others, only need one IRQ per board or can even share an
interrupt between mulitple like boards (primarily PCI boards). Those
are the ones that tend to be really pricey, although you can often pick
up a bargain somewhere. Just don't get Computone-I boards. They're
not supported and not going to be supported. We got the Computone-II
drivers into the kernel sources but the older product line is no longer
supported.
"Dumb port" boards <generally> require an interrupt per port.
Some people have had success sharing interrupts between COM ports
on the ISA bus but you are really taking a chance on hardware compatibility.
Unless the card hardware has been specifically designed to share interrupts
on the ISA bus, it will be a crap shoot whether it will work. It will
vary from make to make, model to model, or even board to board what will
work and what won't work. You'll get lots of answers telling you to "sure,
just go ahead and share those interrupts, it works great for us", from
the people who got lucky and it works. You probably won't hear much from
the people who failed, gave up, and got on with other things since they're
probably not reading this thread.
There are also some "dump port" multiport cards out there that
share an interrupt between the ports on the card. They're actually
designed to do this, so you can expect it to work. They're also
cheaper than the intelligent controllers but still quite a bit more
expensive than the simple serial expansion cards.
> Does anyone have experience with a particular make, model, or vendor?
> Thanks in advance.
I'm running some cheap KIA cards (dual 16550A with clock doubling
jumper) in a couple of systems and even have a couple of them jumpered
for "clock doubling" that allows them to go 230.4K. They work well, but
I don't share interrupts using them. I was even running my main ISDN
link at 230.4K through them for a while till we finished integrating the
Computone drivers and I could get the good stuff on line. I think the
price of the KIA cards was about $20-$25 a piece, but I bought them several
years ago so your price may vary.
When you want to go beyond 4 or have serious interrupt limits is
when you need to start looking at some serious hardware.
Mike
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