Texas Instruments SN74ALS155D

Overview

SN74ALS155D from Texas Instruments is a dual 1-line-to-4-line demultiplexer with individual strobes and common binary-address inputs in a 16-pin SOIC (D) package. It supports dual independent 2-line-to-4-line decoding, 3-line-to-8-line decoding via internal inversion on 1C input, operates at 4.5–5.5 V, delivers ±0.4 mA high-level and 8 mA low-level output drive, and is rated for 0°C to 70°C ambient operation.

For engineers reviewing the SN74ALS155D datasheet, SN74ALS155D pinout, SN74ALS155D application, or SN74ALS155D equivalent, key selection considerations include its dual-section enable control, non-inverting (2C) and inverting (1C) data paths, Schottky-clamped inputs for ringing suppression, and compatibility with TTL-level system buses requiring discrete address decoding or signal routing.

Technical Context

The SN74ALS155D integrates two independent 4-output demultiplexer sections sharing address lines A and B, each controlled by dedicated strobe inputs (1G and 2G). Its logic enables cascading without external gates - e.g., connecting 1C and 2C permits 3-line-to-8-line demux operation using the internal inverter on the 1C path.

All inputs feature high-performance Schottky clamping to suppress line ringing and reduce termination complexity. Output drive capability is specified at −0.4 mA (VOH) and +8 mA (VOL), supporting direct interfacing with standard ALS/AS/TTL loads under 4.5–5.5 V supply conditions.

Key Specifications

Pinout & Package

SN74ALS155D uses a 16-pin SOIC (Small Outline Integrated Circuit) package, 3.9 mm wide, with standard D-suffix footprint per JEDEC MS-012.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar dual demultiplexer applications.

Compared with SN74LS155N and SN74F155N, the SN74ALS155D uniquely combines ALS-speed (≤14 ns), low power (13 mA ICC), Schottky input protection, and built-in 1C inversion - making it the optimal choice for space- and noise-sensitive commercial embedded systems requiring reliable dual demux functionality.

Availability

SN74ALS155D is available at Aetrix Electronics and suitable for memory address decoding, peripheral select logic, legacy bus expansion, and test equipment signal routing requiring stable component supply and long-term obsolescence management.

Supply support for SN74ALS155D 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 global semiconductor leader founded in 1930, specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74ALS155D belongs to TI's Advanced Low-Power Schottky (ALS) logic family, designed specifically to replace standard LS TTL in performance-critical commercial systems where speed, power efficiency, and input robustness are jointly prioritized.

FAQ

What logic family does the SN74ALS155D belong to, and how does it differ from standard LS TTL?

The SN74ALS155D belongs to the Advanced Low-Power Schottky (ALS) logic family. Compared to standard LS TTL, it delivers faster propagation delays (≤14 ns vs. ≤25 ns), lower power consumption (13 mA ICC vs. 19 mA), and integrated Schottky clamping on all inputs to suppress ringing. These features make the SN74ALS155D especially suitable for noise-sensitive, high-density PCB layouts where the SN74ALS155D replaces older LS variants without redesign.

Can the SN74ALS155D be used as a 3-line-to-8-line decoder, and if so, how?

Yes, the SN74ALS155D can implement 3-line-to-8-line decoding by connecting the third address bit (C) to both 1C and 2C inputs, tying 1G and 2G together as the enable, and leveraging the internal inverter on the 1C path. This configuration produces complementary outputs across the two 4-bit sections, yielding eight distinct active-high outputs - all without external gates. This capability is explicitly documented in the SN74ALS155D function table and logic diagram.

What is the purpose of the individual strobe inputs (1G and 2G) on the SN74ALS155D?

The individual strobe inputs 1G and 2G provide independent active-low enable control for each of the two demultiplexer sections. When 1G is high, all 1Y outputs are forced high-impedance (disabled); similarly, 2G controls the 2Y outputs. This allows dynamic partitioning of address space - for example, enabling only one section during memory access and the other during I/O cycle - giving system designers precise, software-controllable routing granularity not possible with single-strobe devices like the SN74ALS139.

Does the SN74ALS155D require external pull-up or pull-down resistors on its inputs or outputs?

No, the SN74ALS155D does not require external pull-up or pull-down resistors on its inputs because all inputs include internal Schottky clamping diodes and meet standard TTL input thresholds (VIH = 2 V, VIL = 0.8 V). Outputs are fully buffered and capable of driving standard TTL loads directly. However, proper VCC decoupling (0.1 µF ceramic near each power pin) is required per ALS design guidelines to ensure stable SN74ALS155D operation under switching transients.

Is the SN74ALS155D pin-compatible with other 16-pin dual demultiplexers such as the SN74LS155 or SN74F155?

Yes, the SN74ALS155D is pin-compatible with the SN74LS155N and SN74F155N in the same 16-pin DIP or SOIC package - all share identical pin assignments for 1C, 1G, A, B, 1Y0–1Y3, VCC, GND, 2C, 2G, 2Y0–2Y3. This allows drop-in replacement in existing layouts. However, designers must verify timing margins and power delivery, as the SN74ALS155D's faster edges may expose previously masked signal integrity issues in legacy designs originally using LS or F variants.