Serial Port config

[Dave Haefner]

Can someone please tell me why........

I have RedHat 5.0 (upgraded kernel to 2.0.34) running on a PII 233
system with 2 serial ports and a modem. I have had some interesting
experiences setting up the serial ports. Under DOS, the COM ports get
assigned as follows:

COM1 IRQ4 03F8H
COM2 IRQ3 02F8H
COM3 IRQ4 03E8H

During init, Linux sets up the TTYS to match the above:

TTY0 IRQ4 0x03F8
TTY1 IRQ3 0x02F8
TTY2 IRQ4 0x03E8

Now, setserial's automatic config will also detect the same:

/dev/cua0 IRQ4 0x03F8
/dev/cua1 IRQ3 0x02F8
/dev/cua2 IRQ4 0x03E8
/dev/cua3 ???

When I use the modem, there is a long delay before dialing, then the
chat script runs and connects, but none of the apps will work with the
connection. Netscape says it cannot locate the hosts or DNS. If I set
the ports with manual configuration as follows:

/dev/cua0 IRQ4 0x03F8
/dev/cua1 IRQ3 0x02F8
/dev/cua2 IRQ5 0x03E8

everything works correctly. the man page for setserial advised that COM
ports on the same IRQ could show unreliable behavior, but never said
why. It also advised that I could use any of several available IRQs, but
the modem will only work properly if set to IRQ5.

Why is this? Is there a way to use the ports under IRQ3 and IRQ4?

Now the problem is that I want to get sound working, and the docs
for an AWE 64 state that I can ONLY use IRQ5. It would appear that I can
have either a modem or sound, but not both.

Any help???

[Jeff Raven]

IRQs are part of what the system uses to communicate with
your hardware -- specifically, they let software know when
a device has something to say. When two devices (say a mouse
and a modem) share the same IRQ, a program listening for one
device will pick up chatter from both of them -- and so every
time you move your mouse, your modem software will think it's
getting random gibberish from the modem.

Suffice it to say, this does not lead to the best of modem
connections.

Now, IRQs are a two way street. You can't just tell your
computer to use a different IRQ to talk to a device -- you
have to tell the device to use that IRQ, too. How you do this
depends a lot on the device -- which means it's time to dig out
the manuals that came with your devices.

Note : This only holds if this a new system you're setting up
for Linux. If your computer was previously MSDOS-Win95 box, you
shouldn't have to do any hardware mucking -- just find out what
IRQs you were using in MSDOS, and set Linux to use them. In Win95
you can find this out by opening up the System option in the
control panel.

Jeff