HP ProBook 6450b Notebook-PC - Manual (Page 109)

Using a serial device (select models only)
Select computer models include a serial port that connects optional devices such as a serial modem,
mouse, or printer.
Some serial devices may require additional support software, which is usually included with the device.
For more information about device-specific software, refer to the manufacturer's instructions.
▲ To connect a serial device, connect the device cable to the serial port on the computer.
Using a serial device (select models only) 99