While the binary numeration system is an interesting
mathematical abstraction, we haven't yet seen its practical application to
electronics. This chapter is devoted to just that: practically applying the
concept of binary bits to circuits. What makes binary numeration so
important to the application of digital electronics is the ease in which
bits may be represented in physical terms. Because a binary bit can only
have one of two different values, either 0 or 1, any physical medium capable
of switching between two saturated states may be used to represent a bit.
Consequently, any physical system capable of representing binary bits is
able to represent numerical quantities, and potentially has the ability to
manipulate those numbers. This is the basic concept underlying digital
computing.
Electronic circuits are physical systems that lend themselves well to the
representation of binary numbers. Transistors, when operated at their bias
limits, may be in one of two different states: either cutoff (no controlled
current) or saturation (maximum controlled current). If a transistor circuit
is designed to maximize the probability of falling into either one of these
states (and not operating in the linear, or *active*, mode), it can
serve as a physical representation of a binary bit. A voltage signal
measured at the output of such a circuit may also serve as a representation
of a single bit, a low voltage representing a binary "0" and a (relatively)
high voltage representing a binary "1." Note the following transistor
circuit:
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