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National Semiconductor |
June 1989
54LS256 DM74LS256
Dual 4-Bit Addressable Latch
General Description
The ’LS256 is a dual 4-bit addressable latch with common
control inputs these include two Address inputs (A0 A1)
an active LOW enable input (E) and an active LOW Clear
input (CL) Each latch has a Data input (D) and four outputs
(Q0 – Q3)
When the Enable (E) is HIGH and the Clear input (CL) is
LOW all outputs (Q0–Q3) are LOW Dual 4-channel demul-
tiplexing occurs when the CL and E are both LOW When
CL is HIGH and E is LOW the selected output (Q0 – Q3)
determined by the Address inputs follows D When the E
goes HIGH the contents of the latch are stored When op-
erating in the addressable latch mode (E e LOW CL e
HIGH) changing more than one bit of the Address (A0 A1)
could impose a transient wrong address Therefore this
should be done only while in the memory mode (E e CL e
HIGH)
Features
Y Serial-to-parallel capability
Y Output from each storage bit available
Y Random (addressable) data entry
Y Easily expandable
Y Active low common clear
Connection Diagram Logic Symbol
Dual-In-Line Package
TL F 9823–1
Order Number 54LS256DMQB
54LS256FMQB or DM74LS256N
See NS Package Number J16A
N16E or W16A
VCC e Pin 16
GND e Pin 8
Pin Names
A0 A1
Da Db
E
CL
Q0a – Q3a
Q0b – Q3b
Description
Common Address Inputs
Data Inputs
Common Enable Input (Active LOW)
Conditional Clear Input (Active LOW)
Side A Latch Outputs
Side B Latch Outputs
TL F 9823 – 2
C1995 National Semiconductor Corporation TL F 9823
RRD-B30M115 Printed in U S A
Truth Table
Inputs
Outputs
CL E A0 A1
Q0
Q1
Q2
Q3
LHX
X
L
L
L
L
LLL
L
D
L
L
L
L LH
L
L
D
L
L
LLL
H
L
L
D
L
L LH H
L
L
L
D
HHX
X
Qtb1
Qtb1
Qtb1
HLL
L
D
Qtb1
Qtb1
HLH
L Qtb1
D
Qtb1
HLL
H
Qtb1
Qtb1
D
H
LH
H
Qtb1
Qtb1
Qtb1
tb1 e Bit time before address change or rising edge of E
H e HIGH Voltage Level
L e LOW Voltage Level
X e Immaterial
Mode Selection
Qtb1
Qtb1
Qtb1
Qtb1
D
E CL
Mode
L H Addressable Latch
H H Memory
L L Active HIGH 4-Channel Demultiplexers
H L Clear
Logic Diagram
Mode
Clear
Demultiplex
Memory
Addressable
Latch
TL F 9823 – 3
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