Figure 8.25 shows the logic symbol of IC 74154, which is a 4-to-16 line decoder/demultiplexer. The logic symbol is in ANSI/IEEE format. Determine the logic status of all 16 output lines for the following conditions:
(a) D=HIGH, C=HIGH, B=LOW, A=HIGH, \overline{G_{1} } = LOW and \overline{G_{2} } = LOW.
(b) D=HIGH, C=HIGH, B=LOW, A=HIGH,\overline{G_{1} } = HIGH and \overline{G_{2} } = HIGH.
(c) D=HIGH, C=HIGH, B=LOW, A=HIGH, \overline{G_{1} } = HIGH and \overline{G_{1} } = HIGH.
It is clear from the given logic symbol that the device has active HIGH inputs, active LOW outputs
and two active LOW ENABLE inputs. Also, both ENABLE inputs need to be active for the decoder
to function owing to the indicated ANDing of the two ENABLE inputs.
(a) Since both ENABLE inputs are active, the decoder outputs will therefore be active depending upon
the logic status of the input lines. For the given logic status of the input lines, decoder output line 13 will be active and therefore LOW. All other output lines will be inactive and therefore in the logic HIGH state.
(b) Since neither ENABLE input is active, all decoder outputs will be inactive and in the logic HIGH state.
(c) The same as (b).