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APPENDIX C
COMMUNICATION PORTS AND INTERRUPTS
Most every device on a PC works in conjunction with the microprocessor by using interrupts.
An interrupt stops the microprocessor for a moment to transfer information to or from its data
lines. Each device, keyboard, hard drive, floppy drive, communication port, CD ROM, etc.
must be assigned a unique interrupt number (IRQ) in order to identify itself to the processor.
Many of these devices have standard IRQ numbers assigned to them. For example, the
standard IRQ for a keyboard is 1, the standard IRQ for the floppy drives is 6, the real-time
clock is 8, and so on. In the past, the COM ports had standard, and usually fixed IRQ numbers
associated with them. But, with the advent of internal fax/modems, CD ROM drives, sound
blaster cards, and other add on boards, each needing a unique IRQ, the total number of readily
available IRQs have dwindled. IRQs once reserved solely for serial and parallel ports are now
being used by add on boards.
Many serial ports now let the user configure the COM port number and interrupt value they
need based on their particular hardware arrangement. The COM port number and IRQ value
for each serial port is set automatically by the operating system software (plug and play) or by
using small connectors on the serial card called jumpers. These jumpers can be moved to
change COM port and IRQ settings in accordance with the instructions from board
manufacturer.
The primary reason for serial ports not working properly is because they no longer have a
unique interrupt value; i.e. they share the same interrupt value with another device. The
average PC owner has installed at least one of the previously mentioned add on boards in their
system since they have purchased their computer. Each of these boards now use one of the
interrupt numbers that may have been originally meant for a serial port. If this is true, the user
probably has not noticed any problems, since that particular serial port may not have been in
use until now.
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