Texas Instruments CD74HCT251E

Overview

CD74HCT251E from Texas Instruments is an 8-input, three-state digital multiplexer with true (Y) and complement (Y̅) outputs, designed for bus-oriented systems requiring logic-level translation and output isolation. It operates at 4.5 V to 5.5 V, delivers 12 ns typical data-to-output propagation delay at 5 V/15 pF, supports –55 °C to 125 °C operation, and drives up to 10 LSTTL loads.

For engineers reviewing the CD74HCT251E datasheet, CD74HCT251E pinout, CD74HCT251E application, or CD74HCT251E equivalent, this device serves as a direct functional and pin-compatible replacement for the 74LS251 in mixed-logic bus interfaces where CMOS power efficiency and TTL input compatibility are required.

Technical Context

The CD74HCT251E implements an 8:1 multiplexing function using high-speed silicon-gate CMOS technology, with address decoding across three select inputs (S2–S0) and active-low output enable (OE) controlling high-impedance state entry/exit. Its HCT logic family ensures LSTTL-compatible input thresholds (VIL ≤ 0.8 V, VIH ≥ 2.0 V) while maintaining CMOS output drive strength and noise immunity.

True and complement outputs operate simultaneously under enable, enabling differential signaling or logic redundancy without external inversion. The three-state architecture allows bidirectional bus sharing, and propagation delays are balanced across all eight data paths (I0–I7), ensuring deterministic timing in synchronous data routing applications.

Key Specifications

Pinout & Package

CD74HCT251E is housed in a 16-pin plastic dual in-line package (PDIP-N), with nominal body dimensions of 19.31 mm × 6.35 mm and through-hole mounting compatibility.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 8-input multiplexer applications.

Compared with CD74HCT251E, SN74HCT251N provides identical functionality with branding alignment, while MC74HCT251NG adds AEC-Q100 qualification-making it suitable for automotive applications where process traceability and stress-test reporting are mandatory.

Availability

CD74HCT251E is available at Aetrix Electronics and suitable for industrial control systems, automotive sensor networks, and test equipment requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for CD74HCT251E 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 specializing in analog, embedded processing, and logic solutions, with decades of heritage in high-reliability logic families.

The CD74HCT251E belongs to TI's 74HCT high-speed CMOS logic series, engineered specifically for seamless migration from bipolar TTL systems while delivering CMOS power efficiency and noise immunity.

FAQ

What is the operating voltage range for the CD74HCT251E?

The CD74HCT251E operates from 4.5 V to 5.5 V DC. This narrow range ensures strict compatibility with standard 5 V LSTTL logic families while preventing overvoltage stress on internal CMOS structures. Operation outside this window may cause undefined behavior or permanent damage, per absolute maximum ratings.

Does the CD74HCT251E support true and complement outputs simultaneously?

Yes, the CD74HCT251E provides concurrent true (Y) and complement (Y̅) outputs. When enabled (OE = L), both outputs reflect the selected input in non-inverted and inverted form respectively-enabling single-chip generation of differential logic pairs or parity bits without external gates.

How does the three-state enable (OE) function on the CD74HCT251E?

The OE pin on the CD74HCT251E is active-low: when held low, Y and Y̅ drive their respective logic states; when pulled high, both outputs enter high-impedance (Z) state. This allows multiple CD74HCT251E devices to share a common bus without contention, controlled entirely by OE logic level.

Is the CD74HCT251E pin-compatible with the 74LS251?

Yes, the CD74HCT251E is functionally and pin-compatible with the 74LS251. It retains identical pin assignments, truth table behavior, and timing relationships-while upgrading input compatibility to LSTTL levels and reducing supply current by >90% compared to the original bipolar device.

What is the maximum propagation delay for the CD74HCT251E across its full temperature range?

The maximum data-to-output propagation delay for the CD74HCT251E is 53 ns over the full –55 °C to 125 °C range (at VCC = 4.5 V, CL = 50 pF). This worst-case value ensures timing margin compliance in safety-critical or high-reliability systems where ambient extremes affect gate switching speed.