Texas Instruments SN74LS257BNSR

Overview

SN74LS257BNSR from Texas Instruments is a quadruple 2-line to 1-line data selector/multiplexer in a 16-pin SOP (NS) package, operating over 0°C to 70°C, with TTL-compatible inputs and outputs, 16 mA output drive capability, and typical propagation delay of 15 ns at V_CC = 5 V. It enables digital signal routing in bus management and address/data path selection.

For engineers reviewing the SN74LS257BNSR datasheet, SN74LS257BNSR pinout, SN74LS257BNSR application, or SN74LS257BNSR equivalent, key selection criteria include its quad 2:1 multiplexing function, TTL logic family compatibility, SOP-16 RoHS-compliant packaging, and industrial-temperature-range availability for legacy system upgrades and repair.

Technical Context

The SN74LS257BNSR integrates four independent 2-input multiplexers sharing common select (S) and enable (G̅) control lines. Each channel routes one of two data inputs (A or B) to a single output (Y) based on the logic state of S, with G̅ asserting low to enable operation.

It uses bipolar TTL circuitry with Schottky-clamped transistors to achieve speed-power trade-offs characteristic of the LS family: 15 ns typical t_PD, 2 mW typical power dissipation per multiplexer, and guaranteed fan-out of 20 LS-TTL loads.

Key Specifications

Pinout & Package

SOP-16 (NS) package: 16-pin small-outline package with gull-wing leads, 1.27 mm pitch, 10.4 mm × 5.3 mm body size, and maximum height of 2.00 mm. Pin 1 marked by beveled corner or dot.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS157N and SN74HC157DR, the SN74LS257BNSR uniquely delivers RoHS-compliant SOP packaging with guaranteed LS-TTL electrical compatibility-making it the sole drop-in replacement for legacy LS-based SMT designs requiring zero layout change.

Availability

SN74LS257BNSR is available at Aetrix Electronics and suitable for bus arbitration, address/data path selection, test mode routing, and legacy system repair requiring stable component supply across long-lifecycle industrial programs.

Supply support for SN74LS257BNSR 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 aerospace design expertise.

The SN74LS257BNSR belongs to TI's legacy 74LS logic family, engineered for high-speed, low-power digital signal routing in cost-sensitive, reliability-critical industrial control and instrumentation systems.

FAQ

What is the function of the G̅ pin on the SN74LS257BNSR?

The G̅ (enable) pin on the SN74LS257BNSR is an active-low control that enables or disables all four multiplexer outputs. When G̅ is low, outputs Y1–Y4 reflect the selected inputs (A or B) based on the S signal; when G̅ is high, all Y outputs enter high-impedance state. This allows bus sharing without contention. The SN74LS257BNSR requires G̅ to be actively driven-not left floating-to ensure predictable operation.

Can the SN74LS257BNSR operate at 3.3 V?

No, the SN74LS257BNSR is specified only for 4.75 V to 5.25 V operation and is not guaranteed to function correctly at 3.3 V. Its TTL input thresholds and internal bipolar transistor biasing require nominal 5 V supply. Attempting 3.3 V operation may result in marginal switching, increased propagation delay, or failure to recognize logic-high inputs. For 3.3 V systems, consider CMOS alternatives like SN74LVC157A-but note they are not pin- or logic-compatible with SN74LS257BNSR.

Is the SN74LS257BNSR pin-compatible with the SN74LS157?

Yes, the SN74LS257BNSR is functionally and pin-compatible with the SN74LS157 across all 16 pins-including identical pin assignments for G̅, S, A/B inputs, Y outputs, V_CC, and GND. Both implement the same quadruple 2:1 multiplexer logic with identical truth tables and timing characteristics. The SN74LS257BNSR extends this compatibility into RoHS-compliant SOP packaging, whereas SN74LS157 is offered only in PDIP.

What is the maximum clock frequency supported by the SN74LS257BNSR?

The SN74LS257BNSR does not operate on a clock signal-it is combinational logic with no internal clock. Its usable switching rate is determined by propagation delay (15 ns typical) and system-level setup/hold timing. In practice, it reliably supports data rates up to approximately 33 MHz in well-designed synchronous interfaces where input signals meet minimum pulse width (≥10 ns) and transition time (<5 ns) requirements. The SN74LS257BNSR performance remains stable across its full 0°C–70°C operating range.

Does the SN74LS257BNSR support hot-swap or live-insertion?

No, the SN74LS257BNSR is not designed for hot-swap operation. Its bipolar TTL architecture lacks bus-hold, power-up 3-state, or current-limiting features required for safe insertion into a live backplane. Applying V_CC or signals before power stabilization may cause latch-up or excessive current draw. Always power-cycle the host system before replacing the SN74LS257BNSR. For hot-swap applications, consider purpose-built hot-swap controllers or configurable logic with I²C configuration.