It is highly desirable to have multiple
printers available to handle different types of print jobs. Standard
black and white laser printers are fast and can produce crisp text
output while ink-jet printers turn out photo-realistic prints on
various media. If multiple printers are to be
connected to a single host PC and the user needs to connect/disconnect
the desired printer on demand, the printers have to connect to
different ports (parallel and USB, multiple USBs, and so on). The
alternative is to use a printer switch.
A printer switch is a piece of hardware that is used to take
data input from one or more devices and route it to one or more
output devices. Also known as an A/B switch or data switch,
these devices allow a host to access two or more output devices
with the "flick of a switch". In addition to printers,
these output devices can be Zip drives, scanners, or CDR drives.
The parallel port is still the standard printer interface port
on the PC. Most PCs are assembled with only a single parallel
port. Though it is possible to install and configure a second
parallel port, the most feasible method of having access to
multiple printers on a single host is through the use of a
parallel printer switch. The switch device connects to the host
parallel port and gives the host access to two (or more depending
on the model of the switch) user-selectable parallel ports. The
port the host connects to is labeled "Input" or
"Host", and the output ports are usually labeled
"A" and "B". Inside the printer switch is
mechanical or digital circuitry that performs the actual
"switching" when the user selects the port connected to
the desired output device. The user selects the device by turning
a knob, flicking a switch, or pressing a button.
A printer switch is a very useful device that is simple to
set up and use, but there are a few rules that should be followed
before installation:

- A printer switch can only output to one device at a time.
- Each printer (or other device connected to the switch) must be
installed and configured properly on the host. The correct printer
and device drivers are still needed for printer operation.
- Be sure to choose the correct printer for each print job and
check to see that the printer switch is pointing to that printer.
- Read the printer switch and printer manual to ensure that they
are compatible. Many printers require precise bi-directional
signaling from the host, so the printer switch must be able to
handle this.
- Consider using electronic rather than mechanical switch boxes
for laser printers because laser printers (because of the way they
work) do not operate well with mechanical switches.