General description

The 74HC257; 74HCT257 are high-speed Si-gate CMOS devices and are pin compatible

with Low-power Schottky TTL (LSTTL).

The 74HC257 and 74HCT257 have four identical 2-input multiplexers with 3-state outputs,

which select 4 bits of data from two sources and are controlled by a common data select

input (S).

The data inputs from source 0 (1I0 to 4I0) are selected when input S is LOW and the data

inputs from source 1 (1I1 to 4I1) are selected when S is HIGH. Data appears at the

outputs (1Y to 4Y) in true (non-inverting) form from the selected inputs.

The 74HC257 and 74HCT257 are the logic implementation of a 4-pole, 2-position switch,

where the position of the switch is determined by the logic levels applied to S. The outputs

are forced to a high-impedance OFF-state when OE is HIGH.

The logic equations for the outputs are: