8-68

Functional Diagram

TRUTH TABLE (Note 1)

INPUTS

DATA I/O (Note 2)

OPERATION OR FUNCTION

OE

DIR

CAB

CBA

SAB

SBA

A0 THRU A7

B0 THRU B7

CD74FCT646

X

X

X

X

↑

X

X

↑

X

X

X

X

Input

Not Specified

Not Specified

Input

Store A, B Unspecified

Store B, A Unspecified

H

H

X

X

↑

H or L

↑

H or L

X

X

X

X

Input

Input

Store A and B Data

Isolation, Hold Storage

L

L

L

L

X

X

X

H or L

X

X

L

H

Output

Input

Real Time B Data to A Bus

Stored B Data to A Bus

L

L

H

H

X

H or L

X

X

L

H

X

X

Input

Output

Real Time A Data to B Bus

Stored A Data to B Bus

NOTES:

1. H= HIGH Voltage Level

L = LOW Voltage Level

↑ = Transition from Low to High

X = Immaterial

2. The data output functions may be enabled or disabled by various signals at the OE and DIR inputs. Data input functions are always en-

abled, i.e., data at the bus pins will be stored on every low to high transition of the clock inputs. To prevent excess currents in the high Z

modes, all I/O terminals should be terminated with 10k

Ω resistors.

4

5

6

7

8

9

10

11

20

19

18

17

16

15

14

13

B0

B1

B2

B3

B4

B5

B6

B7

A0

A1

A2

A3

A4

A5

A6

A7

B DATA PORT

A DATA PORT

OE

DIR

CAB CLOCK

CBA CLOCK

21

3

1

23

2

22

SAB SOURCE

SBA SOURCE

DATA SOURCE

SELECTION

INPUTS

FLIP-FLOP

CLOCKS

GND = PIN 12

CD74FCT646