An ABCD data switch is a small box that lets you hook up to four devices from one PC serial or parallel port on your computer. For example, if you have both a black-and-white laser printer for regular printing and a color inkjet printer for when you want color, you may want to attach them both to your computer. Perhaps you also have a tape backup drive that needs to be attached. You can connect all of them to the ABCD box which is in turn connected to your PC's parallel port. By turning a rotary switch on the box to the A, B, C, or D (if you have a fourth device attached) position, you can select the device you want to be active. (You will need to tell your operating system the kind of device that's attached whenever you switch.)
The data switch can also be used to have up to four computers share the same printer or other device.
This handy little switch costs about $20 (U.S.) and can be purchased in local PC stores.