You don't need to read this section, unless you want to use 3 or more serial devices...
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 (IRQ) and port I/O addresses we have to use. 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 card.
Your PC will normally come with COM1 and COM3 at IRQ 4, and COM2 and
COM4 at IRQ 3. To use more than 2 serial devices, you will have to
give up an interrupt to use. 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 2
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.local
- you should do it upon startup somewhere:
/etc/setserial /dev/cua0 irq 3 # my mouse
/etc/setserial /dev/cua1 irq 4 # my dumb terminal
/etc/setserial /dev/cua2 irq 5 # my 2400 bps modem
/etc/setserial /dev/cua3 irq 9 # my telebit 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
IRQ 15 not assigned
There is really no Right Thing to do when choosing interrupts. Just make sure it isn't being used. 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 card 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, 8, 13 or 14! These are used by your mother board. You will make her very unhappy by taking her IRQs.
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 port will usually come configured as follows:
/dev/cua0, /dev/ttyS0 address 0x3f8 COM1
/dev/cua1, /dev/ttyS1 address 0x2f8 COM2
/dev/cua2, /dev/ttyS2 address 0x3e8 COM3
/dev/cua3, /dev/ttyS3 address 0x3e8 COM4
Choose which address you want each serial device to have and set the
jumpers accordingly. I have my modem on /dev/cua3
, and my
mouse on /dev/cua1
.
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.