CompTIA A+ Certification All-in-One Exam Guide

Imagine that we install a set of 16 light bulbs, 8 inside his box and 8
outside his box. Each of the 8 light bulbs inside the box connects to one of
the 8 bulbs outside the box to form a pair. Each pair of light bulbs is always
either on or off. You can control the 8 pairs of bulbs by using a set of 8
switches outside the box, and the Man in the Box can also control them by
using an identical set of 8 switches inside the box. This light-bulb
communication device is called the external data bus (EDB).
Figure 3-3 shows a cutaway view of the external data bus. When either
you or the Man in the Box flips a switch on, both light bulbs go on, and the
switch on the other side is also flipped to the on position. If you or the Man in
the Box turns a switch off, the light bulbs on both sides are turned off, along
with the other switch for that pair.
Figure 3-3 Cutaway of the external data bus—note that one light bulb pair is
on.
Can you see how this works? By creating on/off patterns with the light
bulbs that represent different pieces of data or commands, you can send that
information to the Man in the Box, and he can send information back in the
same way—assuming that you agree ahead of time on what the different
patterns of lights mean. To accomplish this, you need some sort of codebook
that assigns meanings to the many patterns of lights that the EDB might
display. Keep this thought in mind while we push the analogy a bit more.
Before going any further, make sure you’re clear on the fact that this is an
analogy, not reality. There really is an EDB, but you won’t see any light