Serial port

Updated: 06/30/2020 by Computer Hope
Serial port cable

An asynchronous port on the computer used to connect a serial device to the computer that transmits one bit at a time. Serial ports are usually identified on IBM-compatible computers as COM (communications) ports. For example, a mouse might connect to COM1 and a modem to COM2. The picture shows the DB9 serial connector on a cable.

Where is the serial port on a computer?

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


With the introduction of USB (universal serial bus), FireWire, and other faster solutions, serial ports are rarely used compared to how often they've been used in the past. Also, many new computers and laptops no longer have a serial port.

Identifying a serial port

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.

Serial port pin information

Below is a listing of each of the pins on the DB9 connector, their purpose, and signal name.


With many computers, the serial port is labeled as "10101" which are ones and zeros to represent binary. A user may also interpret the number "1" as the letter "I" and the zeros as the letter "O" and call it the "IOIOI" port.

Computer serial port and pin numbers

As shown above, pin one is in the top-left portion of the port and pin 9 is in the bottom right.

Pin Purpose Signal Name
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

Most of today's computers are eliminating the serial port in favor of USB ports.

Com port, Connection, Data cable, Hardware terms, Motherboard terms, Obsolete, Parallel port, Port, RS-232, Serial