Texas Instruments SN74ALS251NS

Overview

SN74ALS251NS from Texas Instruments is an 8-line to 1-line data selector/multiplexer with complementary 3-state outputs, designed for bus-organized digital systems. It features full binary decoding (3 select inputs A/B/C), dual true/inverted outputs (Y/W), and output-enable (OE) control. Operating at 4.5–5.5 V over 0°C to 70°C, it drives TTL-compatible buses directly in parallel-to-serial conversion applications such as address/data routing in legacy microprocessor systems.

For engineers reviewing the SN74ALS251NS datasheet, SN74ALS251NS pinout, SN74ALS251NS application, or SN74ALS251NS equivalent, key selection criteria include its 3-state bus interface capability, complementary output pairing, propagation delay ≤24 ns (Y output), low-level output current of 24 mA, and compatibility with standard 16-pin plastic DIP (N) packaging.

Technical Context

The SN74ALS251NS implements a single 8:1 multiplexer with active-low output-enable (OE) controlling both Y (true) and W (inverted) outputs simultaneously. All eight data inputs (D0–D7) are TTL-compatible, and select lines A/B/C decode binary addresses to route one input to Y/W. When OE is high, both outputs enter high-impedance state-enabling multi-device bus sharing without external gating.

Its ALS (Advanced Low-Power Schottky) logic family delivers improved speed-power trade-offs versus standard LS: typical propagation delays range from 2 ns (D→Y) to 24 ns (A/B/C→Y), with guaranteed 24 mA sink capability at VOL ≤0.4 V. The device requires no external pull-ups or level shifters when interfacing with 5-V TTL buses.

Key Specifications

Pinout & Package

SN74ALS251NS is supplied in a 16-pin plastic dual in-line package (PDIP-N), with 0.3-inch body width and through-hole mounting. Pin spacing conforms to JEDEC MS-001, enabling compatibility with standard PCB footprints and socketing.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 8:1 multiplexer applications.

Compared with SN74ALS251NS, SN74LS251N trades speed for lower cost and wider availability in legacy inventory, while SN74F251N prioritizes minimal propagation delay at the expense of significantly higher supply current and thermal output.

Availability

SN74ALS251NS is available at Aetrix Electronics and suitable for microprocessor address decoding, legacy bus arbitration, parallel-to-serial conversion, and test pattern generation requiring stable component supply across long-lifecycle industrial and embedded programs.

Supply support for SN74ALS251NS 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 SN74ALS251NS belongs to TI's legacy 74ALS logic family, engineered specifically for performance-critical upgrades of 74LS-based systems where speed, power efficiency, and bus compatibility were essential design constraints.

FAQ

What is the maximum operating frequency supported by the SN74ALS251NS?

The SN74ALS251NS does not specify a maximum clock frequency in its datasheet because it is a combinational logic device-not clocked. Its usable data rate depends on propagation delay and system timing margins: with tPHL(D→Y) ≤15 ns and tPZL(OE→Y) ≤15 ns, it supports reliable operation in bus cycles down to ~30 ns period (33 MHz effective), assuming proper setup/hold timing of select and data inputs relative to OE transitions. The SN74ALS251NS is typically deployed in asynchronous or low-to-moderate-speed synchronous systems.

Does the SN74ALS251NS require external pull-up resistors on its outputs?

No, the SN74ALS251NS does not require external pull-up resistors on Y or W outputs. Its totem-pole outputs actively drive both logic HIGH (VOH ≥2.4 V at IOH = −1 mA) and LOW (VOL ≤0.4 V at IOL = 24 mA) states. Pull-ups would interfere with output contention behavior and violate the ALS family's specified drive characteristics. External pull-ups are only needed if interfacing with open-collector systems-a configuration incompatible with the SN74ALS251NS's architecture.

Can the SN74ALS251NS be used as a demultiplexer?

Yes, the SN74ALS251NS can function as a 1-to-8 demultiplexer when the data input is applied to one of the D0–D7 pins and the remaining D inputs are held static (e.g., tied to GND or VCC), while A/B/C select the destination output line. However, it lacks dedicated enable-controlled single-input distribution-so true demux operation requires external gating of D inputs or use of OE to gate the entire output set. The SN74ALS251NS is optimized for multiplexing; dedicated demux ICs like SN74ALS138 offer better routing flexibility.

What is the meaning of "complementary outputs" in the SN74ALS251NS datasheet?

"Complementary outputs" in the SN74ALS251NS refers to the paired Y (true) and W (inverted) outputs that simultaneously reflect the selected D-input: when D3 is routed, Y = D3 and W = !D3. Both outputs share the same OE control and 3-state behavior-enabling simultaneous assertion of true and complement signals without external inversion. This feature simplifies parity generation, differential bus driving, and logic redundancy schemes where matched propagation delay between true/inverted paths is critical.

Is the SN74ALS251NS pin-compatible with the SN74LS251N?

Yes, the SN74ALS251NS is pin-compatible with the SN74LS251N: both use identical 16-pin PDIP (N) packages, share identical pin assignments (including D0–D7, A/B/C, Y/W, OE, VCC, GND), and implement identical truth tables and functional behavior. This allows direct substitution in existing PCB layouts-though designers must verify timing margins, as SN74ALS251NS offers faster propagation (15 ns vs. 25 ns) and lower power (10 mA vs. 19 mA ICC), potentially affecting noise coupling or decoupling requirements.