The MC54/74HC251 is identical in pinout to the LS251. The deviceinputs are compatible with standard CMOS outputs; with pullup resistors, they are compatible with LSTTL outputs.This device selects one of the eight binary Data Inputs, as determined by the Address Inputs. The Output Enable pin must be a low level for the selected data to appear at the outputs. If Output Enable is high, both the Y and the Y outputs are in the high impedance state. This 3 state feature allows the HC251 to be used in bus oriented systems. The HC251 is similar in function to the HC251 which does not have 3 state outputs.