Texas Instruments SN74AS257D

Overview

SN74AS257D from Texas Instruments is a 4-bit dual 2-line-to-1 data selector/multiplexer with 3-state outputs, designed for bus interface in high-performance TTL systems. It features 16-pin SOIC (D) package, operates from 0°C to 70°C, supports 5 V supply, and delivers ±15 mA high-level and 48 mA low-level output drive. It enables bidirectional data routing from two 4-bit sources to four shared bus lines under A/B select control.

For engineers reviewing the SN74AS257D datasheet, SN74AS257D pinout, SN74AS257D application, or SN74AS257D equivalent, this page provides verified functional specifications, validated pin assignments, confirmed bus-interface timing behavior, and real-world substitution guidance for legacy TTL system upgrades and industrial control backplane designs.

Technical Context

The SN74AS257D implements two independent 2:1 multiplexers per 4-bit channel, each selecting between corresponding A and B inputs based on the common A/B select line. Its 3-state outputs are disabled when OE is high, isolating the Y outputs from the bus without loading.

It uses advanced Schottky (AS) TTL logic with optimized propagation delays: tPLH/tPHL ≤ 6 ns for data paths (A/B → Y) and ≤ 7.5 ns for enable path (OE → Y), ensuring tight timing margins in synchronous bus arbitration and address/data multiplexing applications.

Key Specifications

Pinout & Package

SN74AS257D is housed in a 16-pin SOIC (D) package per JEDEC MS-012, 3.9 mm body width, 1.75 mm height max, with 1.27 mm lead pitch and gull-wing leads.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar data multiplexer applications.

Compared with SN74ALS257AD, SN74AS257D delivers 40% faster propagation and 6× stronger sink current, making it preferable for high-speed bus arbitration; versus SN74AS258N, it serves complementary functions-selection vs. distribution-requiring careful functional validation before substitution.

Availability

SN74AS257D is available at Aetrix Electronics and suitable for industrial control backplanes, legacy system upgrades, programmable logic interfaces, and test equipment signal routing requiring stable component supply and long-term obsolescence management.

Supply support for SN74AS257D 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 founded in 1930, specializing in analog, embedded processing, and logic solutions with broad industrial and automotive qualification.

The SN74AS257D belongs to TI's legacy 74AS logic family, engineered for high-speed TTL-compatible systems where propagation delay, output drive, and bus interfacing are critical-targeting instrumentation, control panels, and retro-computing infrastructure.

FAQ

What is the maximum clock rate supported by SN74AS257D in a synchronous bus application?

The SN74AS257D does not operate on a clock signal-it is combinational logic. Its usable data rate depends on propagation delay and system setup/hold timing. With tPHL/tPLH ≤ 6 ns and tPZL/tPLZ ≤ 9.5 ns, it supports reliable bus switching up to ~80 MHz in well-terminated, low-capacitance (<50 pF) environments. For SN74AS257D-based designs, ensure OE and A/B transitions meet minimum pulse width requirements (≥2 ns) specified in the datasheet.

Can SN74AS257D replace SN74ALS257A in an existing PCB layout?

Yes, SN74AS257D is pin-compatible with SN74ALS257A in the same SOIC (D) or PDIP (N) package and shares identical pinout, voltage thresholds, and 3-state behavior. However, SN74AS257D draws higher supply current (ICC ≈ 30 mA vs. 14 mA) and switches faster, which may require verifying decoupling capacitance and signal integrity margins. The SN74AS257D marking on the device is "AS257".

What is the recommended decoupling strategy for SN74AS257D in high-speed operation?

Place a 0.1 µF X7R ceramic capacitor between VCC (pin 16) and GND (pin 15) as close as possible to the SN74AS257D package-lead length ≤ 2 mm. For boards with multiple SN74AS257D devices or dense routing, add a 4.7 µF tantalum or polymer capacitor per 4–6 ICs on the same 5 V rail. Avoid shared vias between VCC and GND; use dedicated thermal pads and short traces to minimize inductance affecting SN74AS257D transient response.

Does SN74AS257D support mixed-voltage interfacing, such as 3.3 V controllers driving its inputs?

No-SN74AS257D is a 5 V-only TTL device. Its VIH minimum is 2.0 V, but 3.3 V logic outputs may not reliably exceed this threshold under capacitive load or temperature variation. Direct connection risks marginal switching. Use a TTL-input buffer (e.g., SN74LVC1G17) or level translator (e.g., TXS0102) between 3.3 V controllers and SN74AS257D inputs. The SN74AS257D itself must be powered at 4.5–5.5 V.

How does the 3-state output behavior of SN74AS257D interact with bus termination?

When OE is high, SN74AS257D outputs enter high-impedance (≥100 kΩ) state, effectively disconnecting from the bus. This allows safe sharing with other drivers-provided bus termination (e.g., 220 Ω pull-up to 5 V or Thevenin) is placed externally and sized for the total stub capacitance and longest trace. Do not rely on SN74AS257D internal leakage (IOZL/IOZH ≤ ±50 µA) for bus biasing; external termination defines idle voltage. This behavior is consistent across all operating conditions of SN74AS257D.