Since computers use the binary numbering system (O
S
and 1s only),
it is most efficient for each pin to correspond to the decimal equivalent
of one bit in an &bit binary number: 1, 2, 4, 8, 16, etc. (see Figure
10-3).
Figure 70-3. Pins labelled uniquely
The hardware makes this the most practical labelling system. Each
pin corresponds to one of the eight data lines from the computer, and
each data line corresponds to one bit in a binary number. The place
values in a binary number are nothing more than the series made up of
powers of two. Figure 10-4 shows how you use a decimal sum-74-
to fire a particular pattern of pins.
If you try adding several sets of pins together, you’ll see that with
this system you get no duplicates. The number 6 represents pins 2 and
4 (since 2 + 4 = 6) and there is no other way to get 6 by adding
powers of two. This means that any combination of the eight pins
adds up to a unique decimal number which falls within the range 0 to
255.
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