Texas Instruments SN74AS257NSR

Overview

SN74AS257NSR 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 SOP (NS) package, operates from 0°C to 70°C, supports 4.5–5.5 V supply, delivers ±48 mA output drive, and achieves propagation delays as low as 1 ns (A/B → Y). It enables selective routing of two 4-bit data sources onto shared bus lines in industrial control backplanes.

For engineers reviewing the SN74AS257NSR datasheet, SN74AS257NSR pinout, SN74AS257NSR application, or SN74AS257NSR equivalent, this page provides verified functional specifications, validated pin mapping for NS package, confirmed timing behavior under 50 pF load, and real-world substitution guidance aligned with TI's AS-family characterization.

Technical Context

The SN74AS257NSR implements four independent 2:1 multiplexers sharing a common A/B select line and output-enable (OE) control. Each channel selects between corresponding A and B inputs (1A/1B through 4A/4B) to drive its respective 3-state output (1Y–4Y), enabling simultaneous 4-bit data path selection without internal bus contention.

All outputs enter high-impedance state when OE is high, decoupling the device from the bus. Input thresholds are TTL-compatible (VIL = 0.8 V, VIH = 2 V), and output drive strength (IOL = 48 mA, IOH = −15 mA) supports direct connection to standard TTL loads without external buffering.

Key Specifications

Pinout & Package

SOP (NS) package: 16-pin small-outline plastic, 1.27 mm pitch, 8.1 mm × 10.4 mm body, 2.00 mm max height, lead-free NiPdAu plating, MSL Level-1 (unlimited floor life).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74ALS257ANSR and SN74F257N, the SN74AS257NSR delivers the highest speed (1–11 ns delays) and strongest drive (48 mA), making it optimal for high-throughput bus interfaces where timing margin is critical-though at higher static power consumption.

Availability

SN74AS257NSR is available at Aetrix Electronics and suitable for industrial backplane routing, legacy system bus arbitration, microprocessor address demultiplexing, and real-time signal path switching requiring stable component supply and long-term manufacturability.

Supply support for SN74AS257NSR 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, automotive, and communications reach.

The SN74AS257NSR belongs to TI's Advanced Schottky TTL logic family, engineered for high-speed digital systems requiring robust noise margins, fast propagation, and direct TTL bus interfacing without external buffers.

FAQ

What is the maximum clock frequency supported by SN74AS257NSR in a data-select application?

The SN74AS257NSR does not operate on a clock signal-it is combinatorial logic. Its usable data rate depends on propagation delay and system timing margins. With tPHL/tPLH ≤ 6 ns and tPZL/tPLZ ≤ 7 ns, it supports reliable operation in 4-bit bus systems running up to ~80 MHz (assuming 12 ns minimum cycle time), provided setup/hold times of downstream devices are met. The SN74AS257NSR itself imposes no internal clock limit.

Can SN74AS257NSR interface directly with CMOS logic operating at 5 V?

Yes, the SN74AS257NSR can drive 5 V CMOS inputs directly: its VOH ≥ 2.4 V (at IOL = 48 mA) exceeds the minimum VIH (2.0 V) for standard 5 V CMOS, and its VOL ≤ 0.5 V satisfies typical CMOS VIL ≤ 0.8 V. However, CMOS outputs driving SN74AS257NSR inputs must meet TTL input thresholds (VIH ≥ 2.0 V, VIL ≤ 0.8 V); many 5 V CMOS families do so, but verify with specific device datasheets.

What is the recommended PCB layout practice for SN74AS257NSR to ensure signal integrity?

Place a 0.1 µF ceramic decoupling capacitor between VCC (pin 16) and GND (pin 8) within 3 mm of the SN74AS257NSR. Route all input/output traces as controlled-impedance microstrips (if possible) with length matching across the 4-bit group. Avoid routing high-speed signals adjacent to sensitive analog traces. Use solid ground plane beneath the NS package and minimize via stubs on critical nets like A/B and OE.

Does SN74AS257NSR support hot-swap or live-insertion into a powered bus?

No, SN74AS257NSR is not hot-swap rated. Its absolute maximum ratings specify VI ≤ 7 V and disabled output voltage ≤ 5.5 V, but it lacks built-in current limiting, slew-rate control, or undervoltage lockout. Inserting SN74AS257NSR into a live 5 V bus risks transient overcurrent, bus contention, or latch-up. Always power-cycle the system before replacing the SN74AS257NSR.

How does the A/B select pin function when used with asynchronous control signals?

The A/B pin is purely combinational: output Y reflects the selected input (A if A/B = LOW, B if A/B = HIGH) with propagation delay (2–11 ns), regardless of OE state. When OE is HIGH, outputs float regardless of A/B state. For glitch-free operation with asynchronous A/B changes, synchronize A/B transitions to OE assertion or use external edge-triggered latching-since the SN74AS257NSR has no internal registers or metastability hardening.