CHAPTER 7. DIGITAL CIRCUITS146BPAEFigure 7.14: Mechanization for E.7.5Multiplexers and DecodersMultiplexers and decoders are used when many lines of information are being gated andpassed from one part of a circuit to another.Multiplexing is when multiple data signals share a common propagation path. Timemultiplexing is when diﬀerent signals travel along the same wire but at diﬀerent times. Thesedevices have data and address lines, and usually include an enable/disable input. When thedevice is disabled the output is locked into some particular state and is not eﬀected by theinputs. Shown in ﬁgure 7.15 is a 4-line to 1-line multiplexer.A0oA1oS3NA0oA1oS2A0oNA1oS1NA0oNA1oS0A1A0S3S2S1S0Figure 7.15: 4-line to 1-line multiplexer.A decoder de-multiplexes the signals back onto several diﬀerent lines. Shown in ﬁgure 7.16is a binary-to-octal decoder (3-line to 8-line decoder).Decoders (octal decoder) can also convert a 3-bit binary number to an output on one ofeight lines. Hexadecimal decoders are 4-line to 16-line devices. When the decoder is disabledthe outputs will be high. A decoder would normally be disabled while the address lines werechanging to avoid glitches on the output lines.