Texas Instruments SN74HCT257N

Overview

SN74HCT257N 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 4.5–5.5 V, delivers ±6-mA output drive, exhibits typical propagation delay of 17 ns at 5.5 V, and draws ≤80 µA supply current. It routes four independent 2-input data channels to four outputs under address-select and active-low output-enable control.

For engineers reviewing the SN74HCT257N datasheet, SN74HCT257N pinout, SN74HCT257N application, or SN74HCT257N equivalent, key selection criteria include TTL-compatible input thresholds, high-current 3-state bus interface capability, guaranteed 4.5–5.5 V operation, and PDIP-16 packaging for through-hole prototyping and industrial control logic.

Technical Context

The SN74HCT257N implements four independent 2:1 multiplexers sharing a common active-low output-enable (OE) signal. Each channel selects between inputs A and B based on the A/B address line, driving its Y output only when OE is low; otherwise, all outputs enter high-impedance state.

Its HCT logic family ensures TTL-voltage compatibility (VIH = 2 V, VIL = 0.8 V) while maintaining CMOS power efficiency. Input leakage remains ≤1 µA, and output off-state current is ≤5 µA, enabling clean bus isolation in multi-source systems without contention.

Key Specifications

Pinout & Package

SN74HCT257N is housed in a 16-pin plastic dual in-line package (PDIP), measuring 19.31 mm × 6.35 mm, with through-hole mounting and industry-standard pin spacing (0.1 inch).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS257N, SN74HCT257N offers superior bus-driving capability and true 3-state isolation; compared with SN74HC257N, it guarantees TTL input compatibility and tighter timing control at 5 V - making it optimal for robust, drop-in replacement in legacy 5-V bus designs.

Availability

SN74HCT257N is available at Aetrix Electronics and suitable for industrial control systems, test instrumentation, and embedded diagnostics requiring stable component supply and long-term manufacturability.

Supply support for SN74HCT257N 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 delivering analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74HCT257N belongs to TI's 74HCT logic family, engineered specifically to bridge TTL and CMOS domains in 5-V systems - emphasizing bus interface reliability, noise immunity, and pin-for-pin compatibility with legacy 74LS devices.

FAQ

What is the recommended bypass capacitor for SN74HCT257N?

A 0.1-µF ceramic capacitor is recommended directly between VCC (pin 16) and GND (pin 8) to suppress high-frequency noise and stabilize the supply during output switching. For enhanced broadband decoupling, TI recommends paralleling this with a 1-µF capacitor placed close to the device. Proper placement minimizes inductance and ensures reliable operation in noisy industrial environments where SN74HCT257N is commonly deployed.

Does SN74HCT257N support hot insertion into a live bus?

No, SN74HCT257N does not support hot insertion. Its absolute maximum ratings specify that VCC must be applied before any input signals, and the device lacks bus-fault protection or power-up sequencing logic. Applying inputs before VCC risks latch-up or damage. For safe operation in systems requiring live bus access, SN74HCT257N must be powered and stabilized prior to enabling OE or applying address/data signals.

How does the A/B pin control channel selection in SN74HCT257N?

The A/B pin (pin 1) is a global select line that determines, for all four channels simultaneously, whether the A-input (e.g., 1A, 2A, 3A, 4A) or B-input (e.g., 1B, 2B, 3B, 4B) is routed to the corresponding Y-output (1Y–4Y). When A/B = low, all Y outputs reflect their A inputs; when A/B = high, all Y outputs reflect their B inputs - enabling synchronized data source switching across the 4-bit bus.

What happens to the outputs of SN74HCT257N when OE is high?

When OE (pin 15) is high, all four Y outputs (pins 4, 7, 9, 12) enter a high-impedance (Hi-Z) state, effectively disconnecting them from the bus. This is confirmed by electrical characteristics showing IOZ ≤ 5 µA at VCC or GND. The Hi-Z state prevents bus contention and allows multiple SN74HCT257N devices or other drivers to share the same data lines safely - a core function in bus-oriented systems where SN74HCT257N is deployed.

Can unused inputs on SN74HCT257N be left floating?

No, unused inputs on SN74HCT257N must never be left floating. TI explicitly requires all unused inputs to be tied to VCC or GND to prevent undefined logic states, increased power consumption, or oscillation. For example, if only two channels are used, the A/B, 3A/3B, 4A/4B, and associated Y pins must be terminated per the recommended operating conditions - ensuring stable operation and meeting the design intent of SN74HCT257N in industrial control and test applications.