If an extra serial port board is added, you may need to configure the serial ports via the setserial command. This can be done at boot time by putting the setserial commands in /etc/rc.serial. For example:
setserial /dev/ttyS2 port 0xcc00 UART 16550A irq 11 Baud_base 115200
setserial /dev/ttyS3 port 0xd000 UART 16550A irq 11 Baud_base 115200
This would configure two serial ports on a PCI card, using IRQ 11 and attach them the /dev/ttyS2 and /dev/ttyS3.
The IRQ and port address can be gotten by reading /proc/pci or doing lspci on later versions of the OS.