You don't need to read this section, unless you want to use three or more serial devices... (assuming you don't have a multiport board).
Providing you have another spare serial port, yes you can. The number of serial ports you can use is limited by the number of interrupts (IRQs) and port I/O addresses we have to use. This is not a Linux limitation, but a limitation of the PC bus. Each serial devices must be assigned it's own interrupt and address. A serial device can be a serial port, an internal modem, or a multiport serial board.
Multiport serial boards are specially designed to have multiple serial ports that share the same IRQ for all serial ports on the board. Linux gets data from them by using a different I/O address for each port on the board.
Your PC will normally come with ttyS0 and ttyS2 at IRQ 4,
and ttyS1 and ttyS3 at IRQ 3. You can see what IRQs are
in use by looking at /proc/interrupts. To use more than
two serial devices, you will have to
reassign an interrupt. A good choice is to reassign an interrupt
from your parallel port. Your PC normally comes with IRQ 5 and IRQ 7
set up as interrupts for your parallel ports, but few people use two
parallel ports. You can reassign one of the interrupts to a serial
device, and still happily use a parallel port. You will need the
setserial program to do this. In addition, you have to play with
the jumpers on your boards, check the docs for your board. Set the
jumpers to the IRQ you want for each port.
You will need to set things up so that there is one, and only one
interrupt for each serial device. Here is how I set mine up in
/etc/rc.d/rc.local - you should do it upon startup somewhere:
/sbin/setserial /dev/ttyS0 irq 3 # my serial mouse
/sbin/setserial /dev/ttyS1 irq 4 # my Wyse dumb terminal
/sbin/setserial /dev/ttyS2 irq 5 # my Zoom modem
/sbin/setserial /dev/ttyS3 irq 9 # my USR modem
Standard IRQ assignments:
IRQ 0 Timer channel 0
IRQ 1 Keyboard
IRQ 2 Cascade for controller 2
IRQ 3 Serial port 2
IRQ 4 Serial port 1
IRQ 5 Parallel port 2
IRQ 6 Floppy diskette
IRQ 7 Parallel port 1
IRQ 8 Real-time clock
IRQ 9 Redirected to IRQ2
IRQ 10 not assigned
IRQ 11 not assigned
IRQ 12 not assigned
IRQ 13 Math coprocessor
IRQ 14 Hard disk controller 1
IRQ 15 Hard disk controller 2
There is really no Right Thing to do when choosing interrupts. Just make sure it isn't being used by the motherboard, or any other boards. 2, 3, 4, 5, or 7 is a good choice. ``not assigned'' means that currently nothing standard uses these IRQs. Also note that IRQ 2 is the same as IRQ 9. You can call it either 2 or 9, the serial driver is very understanding. If you have a serial board with a 16-bit bus connector, you can also use IRQ 10, 11, 12 or 15.
Just make sure you don't use IRQ 0, 1, 6, 8, 13 or 14! These are
used by your mother board. You will make her very unhappy by taking
her IRQs. When you are done, double-check /proc/interrupts
and make sure there are no conflicts.
Next, you must set the port address. Check the manual on your board for the jumper settings. Like interrupts, there can only be one serial device at each address. Your ports will usually come configured as follows:
ttyS0 address 0x3f8
ttyS1 address 0x2f8
ttyS2 address 0x3e8
ttyS3 address 0x2e8
Choose which address you want each serial device to have and set the
jumpers accordingly. I have my modem on ttyS3, my
mouse on ttyS0, and my terminal on ttyS2.
When you reboot, Linux should see your serial ports at the
address you set them. The IRQ Linux sees may not correspond to
the IRQ you set with the jumpers. Don't worry about this.
Linux does not do any IRQ detection when it boots, because IRQ
detection is dicy and can be fooled. Use setserial to
tell Linux what IRQ the port is using. You can check
/proc/ioports to see what I/O port addresses are in use
after Linux boots.