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SN74LS138N

3-to-8 decoder/demux, active-low outputs, 3 enables, PDIP-16

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Manufacturer Part：

SN74LS138N

Package：

PDIP-16 (19.3 x 6.35 x 4.57mm, 2.54mm pitch, through-hole)

Brand：

Product Categories：

Integrated Circuits ICs, Interface ICs

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Description

Product Overview

The SN74LS138N from Texas Instruments is a 3-line to 8-line decoder/demultiplexer with three enable inputs (two active-low, one active-high) and active-low outputs in a 16-pin PDIP package.

Key Specifications

Function	3-line to 8-line decoder/demultiplexer
Logic Family	LS (Low-power Schottky)
Supply Voltage	4.75V to 5.25V (5V nominal)
Select Inputs	3 (A, B, C) — C is MSB
Outputs	8 (Y0-Y7, active-low)
Enable Inputs	G1 (active-high), G2A and G2B (active-low)
Propagation Delay	22ns typical (select to output)
Operating Temperature	0°C to +70°C
Package	PDIP-16 (19.3 x 6.35mm)

Features

3-to-8 line decoder/demultiplexer
Active-low outputs for direct chip-select generation
Three enable inputs for cascading
22ns typical propagation delay
Multiple 138s can be combined for larger decoding

Applications

Memory address decoding
I/O port selection
Chip-select generation
Data demultiplexing
Cascaded larger decoders

Product Introduction:SN74LS138N Product Introduction

The SN74LS138N from Texas Instruments is a 3-line to 8-line decoder/demultiplexer that takes a 3-bit binary address (A, B, C) and asserts one of eight active-low outputs (Y0-Y7). It is the most commonly used address decoder in digital systems, generating chip-select signals for memory and peripheral ICs. The three enable inputs—G1 (active-high) and G2A, G2B (both active-low)—must all be properly asserted for any output to be active; if any enable is inactive, all outputs remain HIGH (inactive). This enables cascading: a 4-to-16 decoder is built from two 138s by connecting the MSB address bit to G1 of one and G2A of the other. The 22ns propagation delay is fast enough for microprocessor address decoding in most 8-bit and 16-bit systems. The N suffix denotes the PDIP-16 through-hole package.

Working Principle:SN74LS138N Working Principle

The SN74LS138N decodes a 3-bit binary input (CBA) to assert one of eight active-low outputs. For input CBA = 000, output Y0 goes LOW (all others remain HIGH). For CBA = 101, Y5 goes LOW, etc. The three enable inputs must all be asserted for the device to function: G1 must be HIGH, and both G2A and G2B must be LOW. If any enable is not satisfied, all eight outputs remain HIGH (inactive), regardless of the address inputs. For address decoding, the address lines connect to C, B, A, and the enables are used for higher-order address qualification. For example, in a 64K memory space with eight 8K devices: A0-A12 connect to each device’s address inputs, A13-A15 connect to the 138’s C, B, A inputs, and a master enable (e.g., MREQ) connects to G1. The 138 then selects one of eight devices based on A15-A13. For a 4-to-16 decoder using two 138s: A0-A2 go to both devices’ CBA inputs; A3 goes to G1 of the first and G2A of the second; the first device decodes addresses 0-7, the second decodes 8-15.

Pin Description:SN74LS138N Pin Definition and Functions

Pin	Name	Type	Description
1	A	Input	Address input A (LSB)
2	B	Input	Address input B
3	C	Input	Address input C (MSB)
4	G2A	Input	Enable (active-low)
5	G2B	Input	Enable (active-low)
6	G1	Input	Enable (active-high)
7	Y7	Output	Output 7 (active-low)
8	GND	Power	Ground
9	Y6	Output	Output 6 (active-low)
10	Y5	Output	Output 5 (active-low)
11	Y4	Output	Output 4 (active-low)
12	Y3	Output	Output 3 (active-low)
13	Y2	Output	Output 2 (active-low)
14	Y1	Output	Output 1 (active-low)
15	Y0	Output	Output 0 (active-low)
16	VCC	Power	Supply (4.75V to 5.25V)

SN74LS138N uses the PDIP-16 package: VCC at Pin 16, GND at Pin 8. Three binary select inputs (A=Pin 1, B=Pin 2, C=Pin 3) decode to one of eight active-low outputs (Y0=Pin 15 through Y7=Pin 7). Three enable inputs control the device: G1=Pin 6 (active-high), G2A=Pin 4 (active-low), G2B=Pin 5 (active-low). All enables must be active for any output to go low. This 3-enable scheme allows cascading for 4-to-16 or 5-to-32 decoding without external logic. The active-low outputs interface directly with active-low TTL chip selects. For 2-to-4 decoding, use SN74LS139N (dual 2-to-4 decoder in the same DIP-16).

Application Scenarios:In which products and applications isSN74LS138N used

Memory Decode: A15-A13 → CBA; MREQ → G1; Y0-Y7 → /CS of 8 memory devices; each gets 8K address space
I/O Port Select: Low address bits → CBA; IORQ → G1; Y0-Y7 → /CS of 8 peripheral chips
4-to-16 Decode: Two 138s; A3 selects which chip; A0-A2 selects output within chip; 16 active-low outputs
Demultiplexer: Data signal on G1; A0-A2 select which output gets the data; G2A=G2B=LOW
LED Scanner: Counter on CBA; Y0-Y7 drive LEDs (active-low = sink); sequentially lights 8 LEDs

Alternative Models:What chips are recommended as replacements for SN74LS138N

Model	Manufacturer	Key Difference	Package	Supply
74LS138N	TI/Nexperia	Original TTL version with 5V-only supply, typical propagation delay 10-15ns	DIP-16	5V TTL
74LS138D	TI/Nexperia	Surface-mount SOIC version of LS with identical logic function	SOIC-16	5V TTL
74HC138D	TI/Nexperia	CMOS version with wide 2-6V supply range, lower power consumption and higher noise immunity	SOIC-16	2-6V CMOS
74HC138N	TI/Nexperia	Through-hole DIP version of HC family for prototyping and repair	DIP-16	2-6V CMOS
74HCT138D	TI/Nexperia	CMOS with TTL-compatible input levels, ideal for mixing with LS devices at 5V	SOIC-16	4.5-5.5V TTL-in
74HCT138N	TI/Nexperia	Through-hole DIP version of HCT family for prototyping and repair	DIP-16	4.5-5.5V TTL-in
74AC138D	TI/Nexperia	Advanced CMOS with 2-6V supply and higher output drive current (24mA vs 6mA for HC)	SOIC-16	2-6V CMOS
74AC138N	TI/Nexperia	Through-hole DIP version of AC family for prototyping and repair	DIP-16	2-6V CMOS
74LVC138D	TI/Nexperia	Low-voltage CMOS for 1.65-3.6V modern logic with 24mA output drive and bus-hold inputs	SOIC-16	1.65-3.6V CMOS
74LVC138N	TI/Nexperia	Through-hole DIP version of LVC family for prototyping and repair	DIP-16	1.65-3.6V CMOS

The 74138 is a 3-Line to 8-Line Decoder/Demultiplexer. Family variants span from the original LS TTL (5V only) to modern LVC/AUP (down to 0.8V). HC and HCT versions offer 2-6V CMOS operation with HCT providing TTL-compatible input thresholds for mixed 5V systems. CD4000-series equivalents offer the widest 3-18V supply range at the cost of lower switching speed. DIP packages (N suffix) are through-hole; SOIC (D suffix) and TSSOP are surface-mount.