The back of a computer can be daunting with all of its pins, sockets and assorted doohickeys. Let's explore each of the plug-ins you will find. You may have more than one of these plugs, and one may be in use by your internal modem.
Data travels through serial ports one bit at a time, so they are pretty slow.
Only one device can be attached to a serial port at a time, but a switch box allows multiple devices to share the port. Still, only one device can be used at a time. Parallel ports Commonly used for printers, scanners and dongles. They are faster than serial ports because data travels 8 bits at a time. A parallel port cable has a DB25 plug on the computer end and a Centronics plug on the printer end.