Kreisen 3 8 6 X / X E - Manual (Page 136)

Modems
External disk drives
Your computer is designed to communicate with peripherals by
way of the connectors on the rear panel of the main system
unit. The connectors are called “ports,” and allow information
to be sent back and forth between the system CPU and a
peripheral device.
Your keyboard and your video monitor connect to the main
system unit by way of two specialized ports. The keyboard is
connected to the “Keyboard port” on the rear panel of the
main system unit. If you have purchased a video monitor
system for your computer, the display monitor is connected to
the “video port” on the rear panel of the video controller card,
which is mounted inside your main system unit.
Since your system CPU must be in constant communication
with both the keyboard and the display monitor, specific
circuits connect them together. Since other peripheral devices
only communicate with the system CPU occasionally, they can
use general purpose ports.
Computerized data is composed of electronic words called
“bytes.” As an example, each letter or number that appears on
your display monitor has been sent from the main system unit
to the monitor itself in the form of a single byte. This byte is
made up of individual “bits.” in the same way that a word is
made up of letters. In order to keep things consistent for the
system CPU, bytes are made up of groups of eight bits. Using
eight-bit bytes results in 256 basic combinations, each of which
corresponds with a specific letter or number.
7-2 Expanding Your System