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2

Unit Loading/Fan Out

Truth Table

H

= HIGH Voltage Level

L

= LOW Voltage Level

X

= Immaterial

Functional Description

The F157A is a quad 2-input multiplexer. It selects four bits

of data from two sources under the control of a common

Select input (S). The Enable input (E) is active LOW. When

E is HIGH, all of the outputs (Z) are forced LOW regardless

of all other inputs. The F157A 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:

A common use of the F157A is the moving of data from two

groups of registers to four common output busses. The

particular register from which the data comes is determined

by the state of the Select input. A less obvious use is as a

function generator. The F157A can generate any four of

the 16 different functions of two variables with one variable

common. This is useful for implementing highly irregular

logic.

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.

Pin Names

Description

U.L.

HIGH/LOW

Source 0 Data Inputs

1.0/1.0

20

µA/−0.6 mA

Source 1 Data Inputs

1.0/1.0

20

µA/−0.6 mA

E

Enable Input (Active LOW)

1.0/1.0

20

µA/−0.6 mA

S

Select Input

1.0/1.0

20

µA/−0.6 mA

Outputs

50/33.3

−1 mA/20 mA

Inputs

Output

E

S

Z

HX

X

X

L

LH

X

L

L

LH

X

H

H

LL

L

X

L

LL

H

X

H