The MC74HC138A is identical in pinout to the LS138. The device inputs are compatible with standard CMOS outputs; with pullup resistors, they are compatible with LSTTL outputs.
The HC138A decodes a threebit Address to oneofeight activelow outputs. This device features three Chip Select inputs, two activelow and one activehigh to facilitate the demultiplexing, cascading, and chipselecting functions. The demultiplexing function
is accomplished by using the Address inputs to select the desired device output; one of the Chip Selects is used as a data input while the other Chip Selects are held in their active states.