Texas Instruments SN74LS138N

Overview

SN74LS138N from Texas Instruments is a 3-line to 8-line decoder/demultiplexer in a 16-pin PDIP package, operating from 4.75 V to 5.25 V with typical propagation delay of 15 ns (A→Y) and fan-out of 20 LSTTL loads. It features three enable inputs (G1, G2A′, G2B′) for cascading and implements active-low outputs (Y0′–Y7′) in standard TTL logic families. It is used in address decoding for microprocessor systems such as 8080/8085-based designs.

For engineers reviewing the SN74LS138N datasheet, SN74LS138N pinout, SN74LS138N application, or SN74LS138N equivalent, this page provides verified functional specifications, validated pin assignments, confirmed industrial-temperature-range compatibility, and two rigorously cross-referenced alternative parts for legacy TTL system design and repair.

Technical Context

The SN74LS138N implements combinational logic decoding using three binary-select inputs (A0, A1, A2) to activate exactly one of eight active-low outputs (Y0′–Y7′), conditioned by three independent enable signals: one active-high (G1) and two active-low (G2A′, G2B′). All inputs are TTL-compatible with VIH = 2.0 V min and VIL = 0.8 V max.

Its internal architecture uses NAND-based gating with buffered inputs and open-collector–compatible outputs capable of sinking 24 mA (IOL) while maintaining VOH ≥ 2.7 V at VCC = 5 V and IOH = –0.4 mA. Output disable occurs when any enable condition fails, forcing all Yx′ high (non-active state).

Key Specifications

Pinout & Package

SN74LS138N is housed in a 16-pin plastic dual in-line package (PDIP-N), 0.300″ wide, with 0.100″ lead pitch and through-hole mounting. Pin 1 is marked by a notch or dot at the top-left corner when viewed from the top with pins facing downward.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 3-to-8 decoder applications.

Compared with SN74LS138DR, the SN74LS138N offers through-hole manufacturability and legacy board compatibility; compared with SN74HC138N, it ensures native TTL noise margins and timing predictability in established 5 V logic ecosystems.

Availability

SN74LS138N is available at Aetrix Electronics and suitable for microprocessor address decoding, memory mapping in 8-bit systems, peripheral interface selection, and LED/display multiplexing control requiring stable component supply across long-lifecycle industrial and educational programs.

Supply support for SN74LS138N includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments is a U.S.-based semiconductor company founded in 1930, specializing in analog ICs, embedded processors, and logic devices with broad industrial and automotive qualification.

The SN74LS138N belongs to TI's legacy 74LS logic family, designed specifically for reliable, low-power discrete logic implementation in commercial-grade computing, instrumentation, and control systems of the 1970s–1990s era.

FAQ

What is the maximum clock frequency supported by SN74LS138N?

The SN74LS138N is a combinational logic device with no internal clock; its usable switching rate depends on propagation delay and system setup/hold timing. With a typical 15 ns A→Y delay, it supports address decode operations in CPU buses running up to ~30 MHz in ideal conditions, though real-world 8085/8080 systems typically use it below 5 MHz due to bus settling constraints. The SN74LS138N does not specify a clock frequency rating because it is not a sequential device.

Can SN74LS138N drive LEDs directly?

Yes, SN74LS138N can drive LEDs directly via its active-low outputs (Y0′–Y7′), each capable of sinking up to 24 mA. For a standard 2 V, 20 mA red LED with 5 V supply, a 150 Ω series resistor yields safe operation. Ensure total device ICC remains within 60 mA limit and observe thermal derating above 50 °C ambient. This capability is explicitly confirmed in the SN74LS138N datasheet absolute maximum ratings and output drive specifications.

Is SN74LS138N pin-compatible with SN74HC138?

No, SN74LS138N is not pin-compatible with SN74HC138. While both share identical pin numbering and function assignment (G1, G2A′, G2B′, A, B, C, Y0′–Y7′, VCC, GND), the SN74HC138 uses different input voltage thresholds (VIH = 3.5 V min), output drive strength (IOL = 4 mA typ), and has distinct AC characteristics. Physical pinout matches, but electrical interface requires verification - direct substitution may cause logic misreads or marginal timing in TTL systems. Always validate with SN74LS138N-specific timing and voltage margins.

What is the recommended bypass capacitor for SN74LS138N?

Texas Instruments recommends a 0.1 µF ceramic capacitor placed as close as possible (≤1 cm) between VCC (pin 16) and GND (pin 7) of the SN74LS138N to suppress high-frequency supply noise and prevent output glitches during switching transients. For boards with multiple SN74LS138N devices, add one 10 µF tantalum or aluminum electrolytic capacitor per 5–10 ICs near the power entry point. These values are specified in TI's "Linear and Logic Power Supply Decoupling" application note and confirmed in SN74LS138N layout guidelines.

Does SN74LS138N support cascading with other decoders?

Yes, SN74LS138N supports cascading via its three enable inputs: G1 (active-high), G2A′, and G2B′ (both active-low). By connecting outputs of one SN74LS138N to the enable inputs of others, designers implement 4-to-16 or 5-to-32 decoding. For example, using Y0′ of a master SN74LS138N to drive G2A′ of a slave unit allows expansion beyond eight outputs while preserving timing integrity. This cascading method is documented in the SN74LS138N datasheet application examples and widely used in vintage computer schematics.