Texas Instruments SN74HCT257DT

Overview

SN74HCT257DT from Texas Instruments is a quadruple 2-line to 1-line data selector/multiplexer with 3-state outputs, designed for bus-organized digital systems. It operates at 4.5 V–5.5 V, delivers ±6 mA output drive at 5 V, exhibits typical propagation delay of 17 ns (at 5.5 V, CL = 50 pF), and features TTL-voltage-compatible inputs for seamless integration with legacy logic families in industrial control and instrumentation interfaces.

For engineers reviewing the SN74HCT257DT datasheet, SN74HCT257DT pinout, SN74HCT257DT application, or SN74HCT257DT equivalent, this page provides verified functional modes, switching characteristics, bus-interface design meaning, and validated alternative options for high-reliability 4-bit multiplexing in temperature-stable embedded systems.

Technical Context

The SN74HCT257DT implements four independent 2:1 multiplexers sharing a common active-low output-enable (OE) control, enabling simultaneous selection of one of two 4-bit data sources onto a single 4-bit bus. Each channel's output enters high-impedance state when OE is high, isolating the bus during contention or power sequencing.

Its HCT logic family ensures CMOS-level power efficiency (80 µA max ICC) while maintaining TTL input thresholds (VIH = 2 V min, VIL = 0.8 V max) and robust output drive (±6 mA at 5 V). Propagation delay is load-dependent: 17 ns typical at 5.5 V/50 pF, increasing to 19 ns at 5.5 V/150 pF.

Key Specifications

Pinout & Package

SN74HCT257DT is supplied in a 16-pin SOIC (D) package measuring 9.90 mm × 3.90 mm, optimized for surface-mount assembly and thermal dissipation (RθJA = 73°C/W).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar quad 2:1 multiplexer applications.

Compared with SN74HCT257DT, SN74HC257DR trades speed and 5-V optimization for wider voltage flexibility, while 74ACT257SCX sacrifices quiescent current and noise immunity for raw speed-making SN74HCT257DT the optimal balance for industrial 5-V bus systems demanding reliability, compatibility, and predictable timing.

Availability

SN74HCT257DT is available at Aetrix Electronics and suitable for industrial PLC I/O expansion, legacy system bus multiplexing, and embedded controller data path switching requiring stable component supply, long-term lifecycle assurance, and traceable sourcing for safety-rated equipment.

Supply support for SN74HCT257DT 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 over 50 years of innovation in industrial, automotive, and communications markets.

The SN74HCT257DT belongs to TI's HCT logic family, engineered specifically for 5-V TTL-compatible bus interfacing in industrial control, instrumentation, and legacy system upgrades where reliability, noise immunity, and precise timing are critical.

FAQ

What is the recommended power-up sequence for SN74HCT257DT to avoid bus contention?

TI recommends tying the OE (pin 15) to VCC through a pullup resistor during power-up to ensure outputs remain in high-impedance state until system initialization completes. The minimum resistor value depends on the driver's sink capability; a 10-kΩ resistor is commonly used. This prevents unintended data leakage onto shared buses before firmware configures the SN74HCT257DT's address and enable states.

Does SN74HCT257DT support mixed-voltage operation between 3.3 V and 5 V logic domains?

No, SN74HCT257DT is specified only for 4.5 V–5.5 V operation and features TTL-compatible input thresholds (VIH ≥ 2 V, VIL ≤ 0.8 V), making it unsuitable for direct 3.3 V logic interfacing without level translation. Its outputs swing rail-to-rail within the 5-V supply, so connecting to 3.3 V inputs risks overvoltage damage unless current-limiting or clamping is implemented externally.

How does the SN74HCT257DT handle unused inputs, and what is the risk of leaving them floating?

All unused inputs on SN74HCT257DT-including A/B (pin 1), individual data lines (pins 2–4, 5–7, 10–11, 13–14), and OE (pin 15)-must be tied to VCC or GND. Floating inputs cause undefined logic states, increased ICC, and potential oscillation due to internal input stage metastability, which can corrupt bus data or trigger false transitions in downstream logic. TI explicitly mandates this in Section 5.2 of the datasheet.

What is the maximum capacitive load SN74HCT257DT can drive while maintaining its 17 ns propagation delay specification?

The 17 ns typical propagation delay for SN74HCT257DT is guaranteed only at CL = 50 pF, as tested per Figure 6-1 in the datasheet. At CL = 150 pF, tpd increases to 19 ns (5.5 V). Driving loads beyond 150 pF will further degrade timing and may exceed setup/hold margins in synchronous systems; for heavier loads, buffer stages or reduced clock rates are required to maintain timing compliance.

Can SN74HCT257DT be used in automotive applications, and what temperature rating applies?

SN74HCT257DT is rated for –40°C to +85°C operation (per SN74HCT257DR/N ordering variants), meeting industrial temperature requirements but not AEC-Q100 automotive qualification. It lacks automotive-specific screening, failure-in-time (FIT) data, or enhanced ESD protection required for under-hood or safety-critical vehicle subsystems. Use in automotive contexts requires full system-level validation and derating per TI's application report SCBA004.