www.fairchildsemi.com

2

Functional Description

The AC/ACT257 is 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

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 deter-

mined by the logic levels supplied to the Select input. The

logic equations for the outputs are as follows:

When the Output Enable (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 maxi-

mum ratings. Designers should ensure the Output Enable

signals to 3-STATE devices whose outputs are tied

together are designed so there is no overlap.

Truth Table

H

= HIGH Voltage Level

L

= LOW Voltage Level

X

= Immaterial

Z

= High Impedance

Logic Diagram

Please note that this diagram is provided only for the understanding of logic operations and should not be used to estimate propagation delays.

Output

Select

Data

Outputs

Enable

Input

Inputs

OE

Z

HX

X

X

Z

LH

X

L

L

LH

X

H

H

LL

L

X

L

LL

H

X

H