Dell Precision 620 - Manual (Page 47)

If the problem still is not resolved, the system board may be faulty. See " Getting Help " for instructions on obtaining technical assistance.
Troubleshooting a Serial I/O Device
If the information in " Troubleshooting Basic I/O Functions " indicates that the problem is with a device connected to one of the serial ports, perform
the following steps in order until the problem is resolved:
1. Attach the serial device to the other serial port (for example, if it is currently connected to serial port 1, attach the device to serial port 2).
the problem is resolved, the serial port on the system board is faulty. See " Getting Help " for instructions on obtaining technical assistance.
2. If the faulty device has a detachable serial cable, replace the serial cable.
If the problem is resolved, the serial cable was faulty.
3. Replace the faulty serial device.
If the problem is still not resolved, the system board may be faulty. See " Getting Help " for instructions on obtaining technical assistance.