Texas Instruments SN74HC251N

Overview

SN74HC251N from Texas Instruments is an 8-to-1 data selector/multiplexer with complementary 3-state outputs, designed for bus-oriented digital systems. It features full binary decoding of three select inputs (A, B, C), strobe-controlled output enable (OE), and operates across 2 V to 6 V supply range. With typical propagation delay of 9 ns at 5 V and ±6-mA drive capability, it supports parallel-to-serial conversion in industrial control and test equipment.

For engineers reviewing the SN74HC251N datasheet, SN74HC251N pinout, SN74HC251N application, or SN74HC251N equivalent, key selection criteria include 3-state output compatibility with LSTTL loads, wide VCC tolerance, low ICC (≤80 μA), complementary Y/W outputs, and PDIP-16 packaging for through-hole prototyping and legacy board replacement.

Technical Context

The SN74HC251N implements a full 3-bit binary decoder driving eight input channels (D0–D7) to dual complementary outputs (Y and W), both controlled by a single active-low output-enable (OE) signal. Its 3-state architecture allows direct connection to shared data buses without external isolation logic.

Operation requires all unused inputs tied to VCC or GND per TI SCBA004 guidance; timing behavior is specified for CL = 50 pF and CL = 150 pF, with tpd ranging from 15 ns (6 V, 25°C, 50 pF) to 77 ns (6 V, 25°C, 150 pF). Input transition rates are constrained to ≤400 ns at 6 V to ensure reliable switching.

Key Specifications

Pinout & Package

SN74HC251N uses a 16-pin Plastic Dual In-line Package (PDIP) with 19.31 mm × 6.35 mm body size and through-hole mounting. Pin numbering follows standard DIP orientation with Pin 1 marked by notch or dot.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HCT251N, SN74HC251N offers lower static power and wider VCC flexibility but requires CMOS-level input drive; versus CD74HC251E, it is functionally identical with no design impact-selection depends solely on procurement channel preference and lifecycle status.

Availability

SN74HC251N is available at Aetrix Electronics and suitable for industrial control, test equipment, and legacy system upgrades requiring stable component supply and long-term obsolescence management.

Supply support for SN74HC251N 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 with over 50 years of innovation in industrial and automotive electronics.

The SN74HC251N belongs to TI's 74HC logic family, engineered for high-speed, low-power CMOS operation in noise-sensitive industrial environments and cost-sensitive consumer applications.

FAQ

What is the maximum clock frequency supported by SN74HC251N for reliable data selection?

The SN74HC251N does not operate on a clock signal-it is combinatorial logic. Its maximum usable switching rate is determined by propagation delay: at VCC = 5 V and CL = 50 pF, tpd(max) = 60 ns, supporting input select changes up to ~16 MHz for clean setup/hold timing. For SN74HC251N, reliable operation requires stable A/B/C inputs for ≥60 ns before output stabilization.

Can SN74HC251N drive a 50-Ω transmission line directly?

No. SN74HC251N is rated for ±6 mA drive at 5 V and is intended for bus loading (e.g., 15 LSTTL units), not RF or high-speed transmission line termination. Driving a 50-Ω line would require >100 mA at 5 V, far exceeding SN74HC251N's absolute maximum output current (±35 mA). Use dedicated line drivers or buffers for such applications.

Is SN74HC251N compatible with 3.3-V microcontroller GPIOs?

Yes, with constraints. At VCC = 3.3 V, SN74HC251N accepts VIH ≥ 2.0 V and VIL ≤ 0.99 V per spec, matching most 3.3-V MCU outputs. However, its outputs swing rail-to-rail (0–3.3 V), making it fully compatible as a level-translating multiplexer between 3.3-V controllers and 5-V peripherals when VCC = 5 V is used.

What happens to Y and W outputs when OE is left floating?

Leaving OE floating violates TI's recommended operating conditions and risks undefined output states. Per SCBA004, all unused inputs-including OE-must be tied to VCC or GND. If OE floats near mid-supply, both Y and W may enter partial conduction, causing excessive ICC, bus contention, or logic glitches. Always pull OE to VCC (disabled) or GND (enabled) via resistor or direct connection.

Does SN74HC251N support hot insertion or live bus swapping?

No. SN74HC251N lacks hot-swap protection features such as power-up sequencing control, Ioff protection, or back-drive immunity. Applying VCC while signals are present on Dn, A/B/C, or OE may exceed absolute maximum ratings (e.g., VI < 0 or VI > VCC), risking latch-up or permanent damage. Power sequencing must follow VCC-first, signal-later order for safe operation.