The CY74FCT257T has four identical two-input multiplexers that select four bits of data from two sources under the control of a common data-select (S) input. The I₀ inputs are selected when S is low, and the I₁ inputs are selected when S is high. Data at the output is noninverted.

The CY74FCT257T is a logic implementation of a four-pole, two-position switch, where the position of the switch is determined by the logic levels at S. Outputs are in the high-impedance state when the output-enable (OE\) input is high.

All but one device must be in the high-impedance state to avoid currents exceeding the maximum ratings if outputs are tied together. OE\ inputs must ensure that there is no overlap when outputs of 3-state devices are tied together.