Texas Instruments SN74F251BDR

Overview

SN74F251BDR from Texas Instruments is an 8-line-to-1-line data selector/multiplexer with 3-state complementary outputs (Y and W), binary address decoding (A, B, C), strobe-controlled enable (G), and bus-compatible output drive. It operates from 4.5 V to 5.5 V at 0°C to 70°C and supports parallel-to-serial conversion in TTL-level digital systems.

For engineers reviewing the SN74F251BDR datasheet, SN74F251BDR pinout, SN74F251BDR application, or SN74F251BDR equivalent, this device is selected for bus-interfaced data routing, address decoding in legacy microprocessor systems, and logic-level signal selection where true/inverted outputs and high-impedance control are required.

Technical Context

The SN74F251BDR implements full binary decoding of three select inputs (A, B, C) to route one of eight data inputs (D0–D7) to active-low and active-high complementary outputs (W and Y). Its strobe input (G) enables synchronous 3-state control: when G = HIGH, both outputs enter high-impedance mode, decoupling the device from the bus.

Designed for F-series TTL logic, it features guaranteed switching performance (tPLH/tPHL ≤ 12.5 ns at VCC = 5 V, CL = 50 pF), rail-to-rail input voltage tolerance (−1.2 V to 7 V), and robust output drive (IOL = 24 mA, IOH = −3 mA), enabling direct interface with standard TTL and CMOS loads without external buffering.

Key Specifications

Pinout & Package

SN74F251BDR is packaged in a 16-pin SOIC (D package), 7.5 mm × 10.3 mm body, with 1.27 mm pitch and RoHS-compliant NiPdAu lead finish. Pin 1 orientation follows Q1 quadrant per tape-and-reel specification.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar 8:1 multiplexer applications.

Compared with SN74LS251N and 74F257N, the SN74F251BDR delivers superior bus drive, faster propagation, and true/inverted outputs in a single 8:1 structure-making it optimal for time-critical, high-fanout multiplexing where signal polarity diversity matters.

Availability

SN74F251BDR is available at Aetrix Electronics and suitable for industrial control panels, legacy microprocessor upgrades, and automated test equipment requiring stable component supply and long-term obsolescence management.

Supply support for SN74F251BDR 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 SN74F251BDR belongs to TI's 74F logic family-designed specifically for high-speed, low-power TTL-compatible digital systems requiring reliable 5-V operation and bus-interface capability.

FAQ

What is the maximum clock/data rate supported by the SN74F251BDR?

The SN74F251BDR does not operate on a clock signal-it is combinatorial logic. Its maximum usable data rate depends on propagation delay and system timing margins. With tPHL/tPLH ≤ 12.5 ns and setup/hold times inherent to TTL, it reliably supports asynchronous data selection up to ~40 MHz in well-designed PCB layouts. The SN74F251BDR is not intended for synchronous clock-domain crossing but excels in address decoding and static bus routing where timing is controlled externally.

Does the SN74F251BDR require pull-up or pull-down resistors on its inputs?

No, the SN74F251BDR has TTL-compatible input thresholds (VIH = 2.0 V, VIL = 0.8 V) and internal input clamping diodes. Inputs driven by standard 5-V logic sources do not require external biasing. However, floating inputs must be avoided: unused select (A/B/C) or data (D0–D7) pins should be tied to VCC or GND to prevent metastability and excess current draw. The SN74F251BDR datasheet explicitly warns against unterminated inputs due to potential shoot-through current in the input stage.

Can the SN74F251BDR drive a 50-pF capacitive load at full speed?

Yes-the SN74F251BDR's switching characteristics are specified with CL = 50 pF, R1 = R2 = 500 Ω, and VCC = 5 V. All published tPLH/tPHL and tPZL/tPHZ values assume this exact load condition. Driving heavier capacitance (>50 pF) will increase propagation delay and reduce edge rates; for loads exceeding 50 pF, buffer stages or layout optimization (shorter traces, ground planes) are recommended to maintain timing integrity in the SN74F251BDR design.

Is the SN74F251BDR pin-compatible with the SN74F151B?

No-the SN74F251BDR is not pin-compatible with the SN74F151B. While both are 8:1 multiplexers, the SN74F151B has active-low outputs (Y̅ and W̅) and different pin assignments (e.g., strobe on pin 7 vs pin 7 on SN74F251BDR, but inverted logic sense). The SN74F251BDR uses active-high Y and active-low W with true/inverted polarity, whereas SN74F151B outputs are both active-low. Swapping them requires PCB redesign and logic inversion-SN74F251BDR is not a drop-in replacement.

What is the meaning of "3-state" in the SN74F251BDR context?

In the SN74F251BDR, "3-state" refers to the ability of outputs Y and W to assume high-impedance (Hi-Z) mode-neither sourcing nor sinking significant current-when the strobe input G is HIGH. This electrically disconnects the outputs from the bus, allowing multiple devices to share the same signal lines without contention. The SN74F251BDR achieves this via totem-pole output transistors that fully turn off, resulting in IOZL/IOZH ≤ ±50 µA, making it safe for use in wired-OR or multi-drop bus architectures.