DESCRIPTION
The MC74AC257/74ACT257 is a quad 2−input multiplexer with 3−state outputs. It selects four bits of data from two sources under control of a Common Data Select input. When the Select input is LOW, the I0x inputs are selected and when Select is HIGH, the I1x inputs are selected. The data on the selected inputs appears at the outputs in true (noninverted) form. The device is the logic implementation of a 4−pole, 2−position switch where the position of the switch is determined by the logic levels supplied to the Select input. The logic equations for the outputs are shown below: Za = OE●(I1a●S+I0a●S) Zb = OE●(I1b●S+I0b●S) Zc = OE●(I1c●S+I0c●S) Zd = OE●(I1d●S+I0d●S) When the Output Enable input (OE) is HIGH, the outputs are forced to a high impedance state. If the outputs are tied together, all but one device must be in the high impedance state to avoid high currents that would exceed the maximum ratings. Designers should ensure the Output Enable signals to 3−state devices whose outputs are tied together are designed so there is no overlap.
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