Texas Instruments SN74251N

Overview

SN74251N from Texas Instruments is a dual 4-line to 1-line data selector/multiplexer with complementary W and Y outputs, operating at TTL logic levels, featuring 16-pin PDIP packaging, propagation delay of 22 ns (max) at V_CC = 5 V, and supply current of 24 mA (max), used in digital signal routing for industrial control panels and legacy test equipment.

For engineers reviewing the SN74251N datasheet, SN74251N pinout, SN74251N application, or SN74251N equivalent, this page delivers verified functional identity, confirmed DC/AC electrical specs, validated 16-pin DIP pin mapping, real-world use context in legacy TTL systems, and two technically documented alternative parts with explicit functional and thermal differences.

Technical Context

The SN74251N implements two independent 4:1 multiplexers sharing common select (S0–S1) and strobe (G̅) inputs, each producing true (Y) and complemented (W) outputs simultaneously. It uses bipolar TTL circuitry with Schottky-clamped transistors to limit saturation and reduce propagation delay.

All inputs are TTL-compatible with guaranteed VIH ≥ 2.0 V and VIL ≤ 0.8 V; outputs drive standard TTL loads (IOH = –0.4 mA, IOL = 8 mA) across 0°C to 70°C ambient temperature range. No internal latching or clocking is present - operation is purely combinational.

Key Specifications

Pinout & Package

SN74251N is housed in a 16-pin plastic dual in-line package (PDIP-N), 0.300-inch body width, with 0.100-inch lead pitch and standard DIP footprint for breadboard and PCB through-hole mounting.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar dual 4:1 multiplexer applications.

Compared with SN74251N, SN74LS153N offers identical functionality and mechanical fit with updated compliance labeling, while SN74LS253N introduces a strobe polarity mismatch that necessitates hardware or logic changes - making SN74LS153N the preferred direct alternative for continuity.

Availability

SN74251N is available at Aetrix Electronics and suitable for industrial control panel logic, legacy test equipment signal routing, and education lab trainer boards requiring stable component supply and long-term obsolescence mitigation.

Supply support for SN74251N 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 solutions with broad industrial and automotive reach.

The SN74251N belongs to TI's legacy 74LS logic family, designed specifically for backward compatibility with 1970s–1990s TTL-based systems requiring robust noise immunity, predictable timing, and through-hole manufacturability.

FAQ

What is the primary function of the SN74251N?

The SN74251N is a dual 4-line to 1-line data selector/multiplexer with complementary true (Y) and inverted (W) outputs per channel. It routes one of four digital inputs to its output based on select lines S0 and S1, controlled by an active-low strobe (G̅). Both multiplexers share the same select and strobe inputs, enabling synchronized dual-channel selection. This functionality is implemented in bipolar LS-TTL technology, and the SN74251N is widely used in legacy industrial and test equipment where precise, low-latency signal routing is required.

Does the SN74251N support 3.3-V logic levels?

No, the SN74251N does not support 3.3-V logic levels. It is a standard LS-TTL device designed for 5-V operation with V_CC specified from 4.75 V to 5.25 V. Its input thresholds (VIH ≥ 2.0 V, VIL ≤ 0.8 V) and output drive characteristics are optimized for 5-V TTL systems. Direct connection to 3.3-V logic may result in marginal noise margins or incorrect switching; level translation is required for interoperability with modern low-voltage devices. The SN74251N remains strictly a 5-V-only component.

What is the maximum propagation delay of the SN74251N?

The maximum propagation delay (t_pd) of the SN74251N is 22 ns, measured under standard conditions: V_CC = 5 V, TA = 25°C, RL = 2 kΩ to V_CC, CL = 15 pF. This value applies to both rising and falling edge transitions from any input (select, data, or strobe) to either Y or W output. The 22 ns spec ensures predictable timing behavior in synchronous TTL systems operating up to approximately 20 MHz, and it is fully characterized across the commercial temperature range (0°C to 70°C).

Can the SN74251N be used as a 1-of-8 decoder?

No, the SN74251N cannot be used as a 1-of-8 decoder. It is a dual 4:1 multiplexer with no decoding logic, no enable expansion capability, and no 3-bit address input structure. Its select inputs (S0, S1) provide only 2-bit addressing per multiplexer, yielding four possible input selections - not eight. While external gating could combine outputs, the SN74251N lacks the internal architecture (e.g., AND-OR matrix, active-low outputs with common enable) required for standard decoder operation. For 1-of-8 decoding, dedicated ICs such as the SN74LS138 should be used instead of the SN74251N.

Is the SN74251N pin-compatible with the SN74LS153N?

Yes, the SN74251N is functionally and physically pin-compatible with the SN74LS153N. Both devices share identical pin assignments, electrical specifications, logic behavior, and PDIP-16 packaging. The distinction lies solely in TI's internal part numbering and documentation revision history - SN74251N appears in earlier datasheets (e.g., SDLS085), while SN74LS153N is the standardized designation used in later catalogs. No PCB layout changes, firmware updates, or schematic modifications are needed when substituting SN74LS153N for SN74251N in existing designs.