©1988 Fairchild Semiconductor Corporation

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74AC253, 74ACT253 Rev. 1.5

2

Logic Diagram

IEEE/IEC

Functional Description

The AC/ACT253 contains two identical 4-input multiplex-

ers with 3-STATE outputs. They select two bits from four

high impedance (High Z) state. This device is the logic

implementation of a 2-pole, 4-position switch, where the

position of the switch is determined by the logic levels

supplied to the two select inputs. The logic equations for

the outputs are shown:

If the outputs of 3-STATE devices 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 that Output Enable

signals to 3-STATE devices whose outputs are tied

together are designed so that there is no overlap.

Truth Table

H

= HIGH Voltage Level

L

= LOW Voltage Level

X

= Immaterial

Z

= High Impedance

Select Inputs

Data Inputs

Output Enable

Outputs

OE

Z

XXXXXX

H

Z

LLL

X

X

X

L

L

LL

H

XXX

L

H

H

LXLX

X

L

L

HL

XHX

X

L

H

LH

X

XLX

L

L

LH

X

XHX

L

H

HH

XXX

L

L

L

HH

XXX

H

L

H