74ALS253

The 74ALS253 has two identical 4-input multiplexers with 3-State outputs  which select 2 bits from four sources by using common select input (S0, S1).  When the individual output enable (OEa, OEb) inputs of the 4-input multiplexers are High,  the outputs are forced to a High impedance (Z) state.

The 74ALS253 is the logic implementation of 2-pole, 4-position switch being determined by the logic levels supplied to the common select inputs.

To avoid exceeding the maximum current ratings of the 74ALS253 when the outputs of the 3-State devices are tied together, all but one device must be in the High impedance state. Therefore, only one output enable must be achieved at a time.