The serial port connects peripheral devices that receive and send moderate amounts of information to and from the CPU. This includes the mouse, keyboard and modem. You can connect one peripheral device to each serial port.

Older PCs have one or two serial ports. Newer PCs can use four. If your PC has an ISA bus, it can only use two serial ports at the same time. Serial ports are often referred to as COM or communication ports. If your PC has two serial ports, these are labeled as COM1 and COM2. If your PC has four serial ports, they are labeled COM1 through COM4.