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Serial port

serial portAn asynchronous port on the computer used to connect a serial device to the computer and capable of transmitting one bit at a time. Serial ports are typically identified on IBM compatible computers as COM (communications) ports. For example, a mouse might be connected to COM1 and a modem to COM2. The picture shows the DB9 serial port on the back of a computer.

Where is the serial port on a computer?

The serial port is found on the back of the computer and is part of the motherboard.

Note: With the introduction of USB, FireWire, and other faster solutions serial ports are rarely used when compared to how often they've been used in the past.


In the above graphic of a serial port you can notice the DB9 serial port connection is easy to identify. The connection is in the shape of the letter D, is a male connector, and has 9 pins.

What is the serial port used for?

Below is a listing of various hardware components that can be purchased and used with your serial port.

Mouse - One of the most commonly used devices for serial ports, usually used with computers with no PS/2 or USB ports and specialty mice.
Modem - Another commonly used device for serial ports. Used commonly with older computers, however, is also commonly used for its ease of use.
Network - One of the original uses of the serial port, which allowed two computers to connect together and allow large files to be transferred between the two.
Printer - Today, this not a commonly used device for serial ports. However, was frequently used with older printers and plotters.

Pin information

Below is a listing of each of the pins located on the DB9 connector, their purpose, and signal name. As can be seen in the above picture pin one is in the top left and pin 9 is in the bottom right.

1 Data Carrier Detect DCD
2 Received Data RxData
3 Transmitted Data TxData
4 Data Terminal Ready DTR
5 Signal Ground Gnd
6 Data Set Ready DSR
7 Request To Send RTS
8 Clear To Send CTS
9 Ring Indicator RI


The LPT port or parallel port is found on the back of the computer and is part of the motherboard.

Note: Most of today's computers are eliminating the parallel port in favor of USB ports.

Also see: Com port, Connection, Motherboard terms, Parallel port, Port, RS-232, Serial