Texas Instruments SN74HCT257DR

Overview

SN74HCT257DR 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, exhibits 17 ns typical propagation delay at 5.5 V, and supports TTL-voltage-compatible inputs. It enables selective routing of four 2-bit data sources onto shared 4-bit data lines in industrial control and instrumentation buses.

For engineers reviewing the SN74HCT257DR datasheet, SN74HCT257DR pinout, SN74HCT257DR application, or SN74HCT257DR equivalent, key selection criteria include 3-state bus interface capability, guaranteed 5-V logic compatibility, low ICC (80 µA max), high-current outputs, and SOIC-16 packaging for space-constrained PCB layouts.

Technical Context

The SN74HCT257DR implements four independent 2:1 multiplexers sharing a common active-low output-enable (OE) control. Each channel selects between input A or B based on the address select (A/B) signal, driving its corresponding Y output in either high-impedance (when OE = high) or driven logic state (when OE = low).

Its HCT logic family ensures CMOS-level power efficiency while maintaining TTL input thresholds (VIH = 2 V min, VIL = 0.8 V max) and robust noise immunity. The device features buffered inputs and outputs, enabling direct connection to system buses without external drivers, and requires no pull-up on OE for functional operation-though tying OE to VCC via a resistor is recommended for defined high-impedance during power sequencing.

Key Specifications

Pinout & Package

SN74HCT257DR is packaged in a 16-pin SOIC (D package) with nominal body dimensions of 9.90 mm × 3.90 mm and 1.27-mm lead pitch. This surface-mount package supports automated assembly and provides thermal resistance (RθJA) of 73°C/W.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 4-channel 2:1 multiplexer applications.

Compared with SN74HCT257DR, SN74HC257DR trades TTL compatibility for lower static power, while 74ACT257SCX delivers superior speed and drive at the cost of increased ICC and layout sensitivity-making SN74HCT257DR the optimal balance for mixed-logic industrial bus interfaces.

Availability

SN74HCT257DR is available at Aetrix Electronics and suitable for industrial PLC I/O expansion, automated test equipment signal routing, and embedded system bus arbitration requiring stable component supply, long-term lifecycle support, and RoHS-compliant SOIC packaging.

Supply support for SN74HCT257DR 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 for industrial, automotive, and communications markets.

The SN74HCT257DR belongs to TI's HCT logic family, engineered to bridge TTL and CMOS technologies-delivering CMOS power efficiency with guaranteed TTL input compatibility for seamless upgrades in legacy 5-V digital systems.

FAQ

What is the maximum clock/data rate supported by SN74HCT257DR?

The SN74HCT257DR does not operate on a clock signal-it is a combinational multiplexer. Its usable data rate depends on propagation delay and load capacitance. With tpd = 17 ns (typical at 5.5 V, CL = 50 pF), it supports reliable signal routing up to approximately 20 MHz in well-terminated bus environments. For higher frequencies, layout parasitics and output loading become dominant limiting factors-not the SN74HCT257DR itself.

Can SN74HCT257DR be used with 3.3-V microcontrollers?

No-SN74HCT257DR is strictly a 5-V-only device with VCC range 4.5 V–5.5 V and TTL-compatible input thresholds (VIH ≥ 2 V, VIL ≤ 0.8 V). Driving its inputs directly from a 3.3-V MCU violates the minimum VIH requirement and risks unreliable logic detection. A level-shifting buffer or voltage translator is required for interoperability with 3.3-V systems. The SN74HCT257DR itself cannot be powered from or interfaced to 3.3-V rails.

How should unused inputs be handled on SN74HCT257DR?

All unused inputs on SN74HCT257DR-including A/B, individual A/B data lines, and OE-must be tied to a valid logic level (VCC or GND) to prevent floating nodes. Leaving inputs unconnected causes undefined output states, increased ICC, and potential EMI. For example, tie unused A/B to GND (select A) or VCC (select B); tie unused OE to VCC (disable outputs); tie unused data inputs to GND (low default). TI explicitly mandates this in Section 5.2 of the datasheet.

Does SN74HCT257DR support hot-swap or live-insertion?

SN74HCT257DR is not rated for hot-swap operation. Its absolute maximum ratings do not cover powered insertion, and the absence of bus-hold or Ioff protection means VCC sequencing violations or floating inputs during insertion can cause latch-up or excessive current draw. For hot-swap applications, use purpose-built hot-swap controllers or multiplexers with Ioff and power-off protection-such as TI's TMUX series. The SN74HCT257DR requires power to be stable before applying input signals.

What is the recommended bypass capacitor for SN74HCT257DR?

Texas Instruments specifies a 0.1-µF ceramic capacitor placed as close as possible to the VCC pin (Pin 16) and GND (Pin 8) of the SN74HCT257DR. This minimizes high-frequency supply noise and stabilizes transient current demands during output switching. For systems with wideband noise, TI recommends paralleling the 0.1-µF cap with a 1-µF capacitor to improve low-frequency decoupling. The SN74HCT257DR datasheet (Section 9) confirms this value and placement requirement.