Avaya Partner 18 Connection Manual page 297

Appendix B: PC Serial Ports
denoted IRQ0, IRQ1, and so on, up through IRQ15), the microprocessor must
take the appropriate action for the device associated with that IRQ. If there is
a mix-up, or if more than one device attempts to use the same IRQ at the
same time, a conflict occurs, and the outcome is often unpredictable and
usually undesirable (for example, the computer may "hang"). Because IRQs
are a limited resource, some newer PCs support IRQ sharing, a hardware
mechanism which permits more than one device to make use of the same
IRQ, but most PCs do not. For example, all PCs which use IBM's
MicroChannel Architecture (MCA) support IRQ sharing, as do most PCs which
use the Enhanced Industry Standard Architecture (EISA) design. However,
most PC's in the marketplace - even new models - are based on the
traditional Industry Standard Architecture (ISA) which generally does not
support IRQ sharing.
The specific I/O port address and IRQ that a particular serial port uses is
determined by the hardware configuration of the serial port. Generally, these
parameters cannot be changed for built-in serial ports, but add-on cards
containing serial ports often provide jumpers or switches which can be used to
configure them to use one of several I/O port addresses and IRQ
combinations. The table below lists the default I/O port addresses and IRQs
used by the serial ports of IBM PC/AT-compatible computers:
Serial Port
Device Name
COM1
COM2
COM3
COM4
The values in this table play an important part in understanding the "wrinkles"
associated with serial ports: although there is provision for up to four serial
ports, with four unique l/O addresses, there are only two unique IRQs
B-4
l/O Port Address
03F8
02F8
03E8
02E8
IRQ
4
3
4
3