Texas Instruments SN74LS156D

Overview

SN74LS156D from Texas Instruments is a dual 2-line-to-4-line decoder/demultiplexer in a 16-pin SOIC package, operating over 0°C to 70°C, with TTL-compatible inputs and open-collector outputs enabling wired-AND logic implementation in address decoding and data routing applications.

For engineers reviewing the SN74LS156D datasheet, SN74LS156D pinout, SN74LS156D application, or SN74LS156D equivalent, key selection considerations include its dual independent decoder structure, open-collector output drive capability, TTL input thresholds, propagation delay (typ. 25 ns), and compatibility with legacy 74LS logic families in industrial control and instrumentation systems.

Technical Context

This device integrates two identical 2-to-4 line decoders on a single die, each with two active-low address inputs (A₀, A₁), one active-low enable (G̅), and four active-low open-collector outputs (Y₀–Y₃). No internal logic connects the two sections - they operate fully independently.

Each decoder section implements standard binary decoding: when G̅ = LOW, outputs Y₀–Y₃ go LOW corresponding to the binary value of A₁A₀ (00→Y₀, 01→Y₁, 10→Y₂, 11→Y₃); all outputs remain HIGH when G̅ = HIGH. Output sink current rating is 8 mA per channel at V_OL ≤ 0.5 V.

Key Specifications

Pinout & Package

SN74LS156D is housed in a 16-pin Small Outline Integrated Circuit (SOIC) package (Package Drawing D), 7.5 mm wide, 10.3 mm long, 2.0 mm max height, with 1.27 mm pitch gull-wing leads.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar dual 2-to-4 decoder applications.

Compared with SN74LS156D, SN74LS139DR offers identical functionality with enhanced manufacturability and lifecycle support, while SN74HC139DR trades TTL compatibility for CMOS advantages - making the former a drop-in replacement and the latter a functional alternative requiring design validation.

Availability

SN74LS156D is available at Aetrix Electronics and suitable for legacy system repair, industrial control retrofitting, and educational lab equipment requiring stable component supply of discontinued 74LS-series logic.

Supply support for SN74LS156D 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, embedded processing, and logic products with broad industrial and aerospace heritage.

The SN74LS156D belongs to TI's legacy 74LS TTL logic family, designed for reliable, medium-speed digital control in cost-sensitive, non-portable systems of the 1970s–1990s.

FAQ

What is the primary function of the SN74LS156D?

The SN74LS156D is a dual 2-line-to-4-line decoder/demultiplexer with active-low open-collector outputs. Each of its two independent sections accepts two binary address inputs and one active-low enable signal to assert exactly one of four active-low outputs - enabling precise selection of memory banks, peripherals, or display digits in TTL-based digital systems. Its design centers on deterministic, low-latency decoding without internal latching.

Does the SN74LS156D support 3.3 V logic interfacing?

The SN74LS156D is a 5 V-only TTL device with input thresholds defined for 5 V operation (V_IL ≤ 0.8 V, V_IH ≥ 2.0 V), so direct connection to 3.3 V CMOS outputs may result in marginal or unreliable HIGH-level recognition. However, its open-collector outputs can drive 3.3 V buses when pulled up to 3.3 V - provided the output sink current (≤8 mA) and V_OL (≤0.5 V) remain within spec. Level translation is recommended for robust 3.3 V → SN74LS156D input interfacing.

What is the maximum output sink current per pin on the SN74LS156D?

Each output pin of the SN74LS156D is rated for a maximum sink current of 8 mA when V_OL is held at or below 0.5 V. This specification defines the minimum allowable pull-up resistor value for a given supply voltage - e.g., with a 5 V pull-up, R_PULLUP ≥ (5 V − 0.5 V) / 8 mA = 562 Ω. Exceeding this current risks output voltage degradation or long-term reliability issues.

Is the SN74LS156D pin-compatible with the SN74LS139?

Yes - the SN74LS156D and SN74LS139 share identical pin assignments, electrical specifications, and functional behavior. The SN74LS139 is the standardized, currently produced successor to the SN74LS156D; both devices implement dual 2-to-4 decoding with active-low open-collector outputs in a 16-pin SOIC package. TI documentation confirms functional and mechanical equivalence, making SN74LS139DR a direct replacement for SN74LS156D in new and legacy designs.

Why is the SN74LS156D marked as Obsolete in TI's packaging addendum?

The SN74LS156D is designated Obsolete in TI's official packaging documentation because TI discontinued its active production and marketing in favor of the functionally identical and improved SN74LS139. While SN74LS156D remains available through authorized distributors and legacy stock channels, TI no longer manufactures it - meaning long-term supply relies on existing inventory rather than ongoing wafer fabrication. Aetrix Electronics maintains traceable, tested stock to support repair and maintenance of legacy systems dependent on this part.