Serial ports are a type of computer interface that complies with the RS-232 standard. They are 9-pin connectors that relay information, incoming or outgoing, one byte at a time. Each byte is broken up into a series of eight bits, hence the term serial port.
These ports are one of the oldest types of interface standards. Before internal modems became commonplace, external modems were connected to computers via serial ports, also known as communication or "COM" ports. Computer mice and even keyboards also used them. Some used 25-pin connectors, but the 9-pin variety was more common. They are controlled by a special chip call a UART (Universal Asynchronous Receiver Transmitter).
Serial ports differ from 25-pin parallel ports in that the parallel ports transmit one byte at a time by using eight parallel wires that each carry one bit. With data traveling in parallel, the transfer rate was greater. A parallel port could support rates up to 100 kilobytes per second, while serial ports only supported 115 kilobits per second (kbps). Later, enhanced technology pushed serial speeds to 460 kbps.
In traditional computers, serial ports were configured as follows:
| Serial Ports | Interrupt | Memory Address |
| COM 1 | IRQ 4 | 0x3f8 |
| COM 2 | IRQ 3 | 0x2f8 |
| COM 3 | IRQ 4 | 0x3e8 |
| COM 4 | IRQ 3 | 0x2e8 |
Devices configured to use ports COM 1 and COM 3 could not be active at the same time, as they shared interrupt IRQ 4. The same was true of COM 2 and COM 4 port devices. Often this led to manually reconfiguring the ports, which frequently caused more trouble than it was worth and posed a special challenge for the dyslexic.
Today, serial ports are mainly used for dial-up modems and current operating systems handle configuration automatically. Newer, faster technologies of USB (Universal Serial Bus) and Firewire have otherwise replaced both serial and parallel ports. USB supports speeds from 1.5 megabits per second to 60 megabytes per second. Firewire boasts transfer rates between 100 and 400 megabits per second.