system resource assignments used for the serial ports on most PCs. Notice that COM1
shares an IRQ with COM3 and COM2 shares an IRQ with COM4. What this means is that
you must be careful when assigning devices to COM ports so that you don’t end up with
devices competing for the same interrupt. Chapter 13 has more information on system re-
sources. Chapter 20 includes information on configuring a modem to a serial port.
The COM designation of a serial port is its logical device name, which allows the system
and software programs to refer to devices like the serial ports by a common name instead
of its physical address, which vary by PC.
On most PCs and on serial port expansion cards, COM1 is typically a DB-9 connector
and COM2 is a DB-25 connector. However, there are no standards for these assignments,
so you may need to look at the card to see how the connections are labeled. In most in-
stances, COM1 is labeled “COM1” and COM2 is labeled... well, you get it.Troubleshooting a Serial Port
Most serial port problems are caused by a system resource conflict. These problems
show u pas a serial device that fails intermittently or doesn’t work at all, as another device
that stops working when the serial device is installed, or as the PC locking up during the
boot sequence.
To troubleshoot a serial port problem, check the following: Inspect the port for bent pins. Certain pins must be absolutely straight in
order for the device to work properly.
Ensure that the cable is the appropriate cable for the device. Some serial
devices can’t use a straight-through or null modem cable.
Check the Windows Device Manager for system resource conflicts. An IRQ
conflict is the most common error in this area. Remember, only one customer to
an IRQ at a time.
Be sure that the serial cable is not more than 50 feet long. Beyond this distance,
you lose data integrity, which shows up any number of ways, none of which
are good.(^490) PC Hardware: A Beginner’s Guide
Logical Device Name IRQ I/O Address
COM1 IRQ 4 3F8h
COM2 IRQ 3 2F8h
COM3 IRQ 4 3E8h
COM4 IRQ 3 2E8h
Table 19-4. Serial Port System Resource Assignments