6 Detailed Description

6.1 Overview

Each of these devices selects four bits of data from two sources under the control of a common Select input (S).

The Output Enable (OE) is active LOW. When OE is HIGH, all of the outputs (Y or Y) are in the high-impedance

state regardless of all other input conditions.

Moving data from two groups of registers to four common output buses is a common use of

the ’AC257, ’ACT257, and CD74ACT258. The state of the Select input determines the particular register from

which the data comes. The ’AC257, ’ACT257 and CD74ACT258 can also be used as function generators.

6.2 Functional Block Diagram

6.3 Device Functional Modes

Table 6-1. Truth Table

OUTPUT

ENABLE

SELECT INPUT

DATA INPUTS

257 OUTPUTS

258 OUTPUTS

OE

S

I 0

I 1

Y

Y

H

X

X

X

Z

Z

L

L

L

X

L

H

L

L

H

X

H

L

L

H

X

L

L

H

L

H

X

H

H

L

CD54AC257, CD74AC257, CD54ACT257, CD74ACT257, CD74ACT258

SCHS248C – NOVEMBER 1998 – REVISED APRIL 2025

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