Section 4.3
Review Questions
Encoders
  1. What does an encoder do?
answer...
It takes in a number of wires and forms a binary number that identifies which one of the inputs has a 1 on it
  1. Is an encoder just a decoder "run backwards"?
answer...
definitely not, their internals are quite different
  1. Suppose that you had a 16x4 encoder. It takes in 16 wires and presents a 4-bit binary number as its output. Draw four OR gates with the output wires, labeled X3, X2, X1, and X0. (Do not draw all the input wires.) Now show what gates the wire labeled 12 would connect to. 12 is 1100 in binary.
answer...
  1. Decoders have the nasty property that as the number of inputs increases, the number of outputs goes up exponentially. If you look at Fig. 4.2.2 you will see that this means the number of AND gates goes up exponentially, too. Does the same sort of explosion in the number of gates happen with encoders?
answer...
No, since there are only OR gates, the number of OR gates increases logarithmically relative to the number of inputs. (If you double the number of inputs, the number of outputs goes up only by 1. Thus the number of output wires is the logarithm (base 2) of the number of inputs.)