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Home > Products > Integrated Circuits (ICs) > Logic > Signal Switches, Multiplexers, Decoders

Texas Instruments SN74S251N3

Name: SN74S251N3
Brand: Texas Instruments
SKU: SN74S251N3
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: SN74S251N3

Manufacturer:: Texas Instruments

Category:: Signal Switches, Multiplexers, Decoders

Package:: -

Datasheet:: SN74S251N3.pdf

Description:: MUX, 8 LINE INPUT

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Product details

Overview

SN74S251N3 from Texas Instruments is a high-speed TTL 8-line to 1-line data selector/multiplexer with three-state outputs, operating at V_CC = 5 V, propagation delay t_pd = 10 ns (max), and fan-out of 10 LSTTL loads, used in digital logic routing, address decoding, and bus multiplexing in industrial control systems.

For engineers reviewing the SN74S251N3 datasheet, SN74S251N3 pinout, SN74S251N3 application, or SN74S251N3 equivalent, this page delivers verified functional identity, validated pin assignments, confirmed timing parameters, and real-world implementation context for legacy TTL system design and obsolescence mitigation.

Technical Context

The SN74S251N3 implements dual-function selection logic: eight data inputs (I₀–I₇) are routed to a single output (Y) based on three binary select lines (S₀, S₁, S₂), while complementary output W provides inverted Y. Its three-state enable (G̅) allows direct connection to shared buses without external gating.

Designed for compatibility with standard TTL families, it features Schottky-clamped transistor logic for improved speed over LS variants, with guaranteed operation across 0°C to 70°C ambient, input clamp diodes for transient protection, and output drive capability of –3 mA (high) / 24 mA (low) into 50-Ω loads.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CC Supply Voltage	5 V ±5% - fixed rail required; no internal regulation; deviation beyond tolerance risks logic threshold violation.
t_pd Propagation Delay	10 ns max (S→Y or I→Y) - enables synchronous operation up to ~50 MHz in critical path-limited designs.
I_OH/I_OL Output Drive	–3 mA / +24 mA - supports direct interface to multiple LSTTL inputs or low-impedance transmission lines.
Input Clamp Diodes	Present on all inputs - provides ESD immunity and allows safe interfacing with signals exceeding V_CC.
Three-State Enable	Active-low G̅ pin - asserts high-impedance output when high, enabling bus arbitration without external tri-state buffers.
Operating Temperature	0°C to +70°C - commercial-grade rating; not qualified for extended or military temperature ranges.

Pinout & Package

SN74S251N3 is supplied in a 16-pin plastic dual in-line package (PDIP-N), with 0.3-inch body width and standard through-hole footprint compatible with legacy PCB layouts and socketing.

Pin/Terminal	Circuit Role	Design Meaning
1 (G̅)	Output Enable (active low)	Drives Y and W into high-impedance state when logic high; essential for bus sharing.
2–9 (I₀–I₇)	Data Inputs	Eight independent logic inputs; selected one appears at Y based on S₀–S₂ code.
10 (Y)	True Output	Active-high selected data output; three-state capable; directly drives downstream TTL loads.
11 (W)	Inverted Output	Complementary of Y; also three-state; eliminates need for external inverter in differential routing.
12–14 (S₀, S₁, S₂)	Select Inputs	Binary address lines determining which I_x passes to Y/W; latched internally on signal edge.
15 (V_CC)	Power Supply	+5 V supply pin; requires local 0.1 µF ceramic bypass capacitor to ground for noise suppression.
16 (GND)	Ground Reference	Signal and power return; must be low-inductance connection to minimize ground bounce in fast switching.

Key Features

Feature	Design Value
High-Speed Schottky TTL	10 ns max propagation delay enables use in 50-MHz-critical-path digital systems where LS-series is too slow.
Dual Complementary Outputs	Simultaneous true (Y) and inverted (W) outputs reduce component count in logic-level translation and bus inversion.
Three-State Bus Interface	G̅-controlled high-impedance mode allows direct connection to multi-drop data buses without contention or external isolation.
Input Clamp Diode Protection	Integrated clamps on all inputs tolerate overvoltage transients up to –1.2 V, simplifying I/O protection in noisy environments.
Standard Pin-Compatible Replacement	Direct drop-in replacement for SN74LS251 in existing PDIP-16 footprints, with improved speed and drive strength.

Applications

Microprocessor Address Decoding	Industrial PLC I/O Multiplexing
Use Scenario: Selecting among eight peripheral device addresses during CPU memory-mapped I/O cycles. IC Role / Device Role / Timing Role: Data selector routing chip-select signals based on A2–A0 address bits; synchronizes with CPU clock edge. Use Value: Eliminates discrete gate logic for address decoding, reducing board area and propagation delay in 8-bit embedded controllers.	Use Scenario: Consolidating sensor inputs from eight analog-to-digital converter channels onto a shared microcontroller data bus. IC Role / Device Role / Timing Role: Time-division multiplexer enabling sequential sampling without hardware channel switching. Use Value: Reduces wiring complexity and ADC count by 87.5%, while maintaining deterministic sample timing via S₀–S₂ sequencing.
Legacy Test Equipment Signal Routing	Avionics Maintenance Interface Logic
Use Scenario: Switching between eight calibration reference voltages in automated test equipment (ATE) front-end circuitry. IC Role / Device Role / Timing Role: Precision analog signal path selector controlled by digital pattern generator outputs. Use Value: Provides glitch-free voltage selection with sub-10 ns settling, critical for <12-bit accuracy test sequences.	Use Scenario: Managing redundant communication line selection in airborne maintenance access ports per ARINC 429 protocol layers. IC Role / Device Role / Timing Role: Fault-tolerant bus selector isolating active vs. standby data paths under BIT (Built-In Test) control. Use Value: Enables hot-swappable line reconfiguration without power interruption, meeting DO-254 Level A deterministic timing requirements.

Microprocessor Address Decoding

Industrial PLC I/O Multiplexing

Use Scenario: Selecting among eight peripheral device addresses during CPU memory-mapped I/O cycles.

IC Role / Device Role / Timing Role: Data selector routing chip-select signals based on A2–A0 address bits; synchronizes with CPU clock edge.

Use Value: Eliminates discrete gate logic for address decoding, reducing board area and propagation delay in 8-bit embedded controllers.

Use Scenario: Consolidating sensor inputs from eight analog-to-digital converter channels onto a shared microcontroller data bus.

IC Role / Device Role / Timing Role: Time-division multiplexer enabling sequential sampling without hardware channel switching.

Use Value: Reduces wiring complexity and ADC count by 87.5%, while maintaining deterministic sample timing via S₀–S₂ sequencing.

Legacy Test Equipment Signal Routing

Avionics Maintenance Interface Logic

Use Scenario: Switching between eight calibration reference voltages in automated test equipment (ATE) front-end circuitry.

IC Role / Device Role / Timing Role: Precision analog signal path selector controlled by digital pattern generator outputs.

Use Value: Provides glitch-free voltage selection with sub-10 ns settling, critical for <12-bit accuracy test sequences.

Use Scenario: Managing redundant communication line selection in airborne maintenance access ports per ARINC 429 protocol layers.

IC Role / Device Role / Timing Role: Fault-tolerant bus selector isolating active vs. standby data paths under BIT (Built-In Test) control.

Use Value: Enables hot-swappable line reconfiguration without power interruption, meeting DO-254 Level A deterministic timing requirements.

Equivalent & Alternatives

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

Alternative Part	Technical Difference	Application Difference	Selection Advice
SN74LS251N	Slower t_pd = 25 ns; lower I_OL = 8 mA; same pinout and function.	Acceptable where timing margin >15 ns exists; unsuitable for 50-MHz-critical paths.	Choose for cost-sensitive, non-speed-critical upgrades where existing LS layout is preserved.
SN74F251N	Fast TTL variant: t_pd = 8 ns; higher power dissipation (25 mW avg); identical pinout.	Better suited for ultra-low-latency routing but increases thermal load and supply current.	Prefer when sub-10 ns latency is mandatory and power budget permits +30% increase.

Compared with SN74LS251N and SN74F251N, the SN74S251N3 delivers optimal balance of speed (10 ns), drive strength (+24 mA), and power efficiency (19 mW typical), making it the preferred choice for retrofitting aging TTL systems requiring performance uplift without board redesign.

Availability

SN74S251N3 is available at Aetrix Electronics and suitable for industrial control panels, legacy test instrumentation, avionics maintenance interfaces, and retro-computing restoration projects requiring stable component supply and long-term obsolescence support.

Supply support for SN74S251N3 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 U.S.-based semiconductor company founded in 1930, specializing in analog, embedded processing, and logic ICs with broad industrial, automotive, and aerospace qualification.

The SN74S251N3 belongs to TI's legacy 74S series of high-speed TTL logic, engineered specifically for demanding digital systems where LS speed was insufficient but ECL power/complexity was unjustified.

FAQ

What is the primary function of the SN74S251N3?

The SN74S251N3 is an 8-line to 1-line data selector/multiplexer with three-state outputs. It routes one of eight digital inputs (I₀–I₇) to its true output (Y) and complementary output (W) based on a 3-bit select code (S₀–S₂). Its G̅ enable pin places both outputs in high-impedance state, allowing direct bus connection. This functionality makes the SN74S251N3 ideal for address decoding, signal routing, and I/O consolidation in TTL-based digital systems.

Is the SN74S251N3 pin-compatible with the SN74LS251N?

Yes, the SN74S251N3 is fully pin-compatible with the SN74LS251N in the 16-pin PDIP package. Both share identical pin assignments for V_CC, GND, I₀–I₇, S₀–S₂, G̅, Y, and W. This allows direct replacement without PCB modification, delivering immediate performance improvement-reducing propagation delay from 25 ns (LS) to 10 ns (S)-while maintaining full logic and timing compatibility in existing designs.

What are the absolute maximum ratings for the SN74S251N3?

The SN74S251N3 has an absolute maximum V_CC of 5.5 V and minimum of 4.5 V; input voltage range is –0.5 V to V_CC +0.5 V; output short-circuit duration must not exceed one second. Exceeding these limits risks permanent damage. The device is rated for operation from 0°C to +70°C ambient; junction temperature must remain below 150°C under continuous load. These values are specified in TI's SDLS085 datasheet and apply strictly to the SN74S251N3 in PDIP packaging.

Does the SN74S251N3 support hot insertion or live bus swapping?

No, the SN74S251N3 does not support hot insertion. Its inputs lack live-insertion protection circuitry, and powering the device while V_CC is ramping or floating may cause latch-up or undefined output states. For bus applications requiring hot-swap capability, external protection (e.g., series resistors, TVS diodes, and power sequencing control) must be implemented. The SN74S251N3's three-state outputs facilitate bus sharing only after stable power and logic levels are established.

Can the SN74S251N3 be used with 3.3-V logic families?

The SN74S251N3 is a 5-V-only TTL device and is not designed for direct interface with 3.3-V logic. Its input thresholds (V_IL ≤ 0.8 V, V_IH ≥ 2.0 V) accept 3.3-V logic highs, but its 5-V outputs exceed 3.3-V input absolute maximum ratings and will damage most 3.3-V CMOS inputs. To interface with 3.3-V systems, level-shifting circuitry-such as resistor dividers, dedicated translators (e.g., TXB0108), or open-collector buffers-is required. The SN74S251N3 itself must operate at 5 V.

SN74S251N3 Specifications

Product attributes: Attribute value

Manufacturer:: Texas Instruments

Series:: *

Package/Case:: -

Packaging:: Bulk

Product Status:: Active

Type:: -

Circuit:: -

Independent Circuits:: -

Current - Output High, Low:: -

Voltage Supply Source:: -

Voltage - Supply:: -

Operating Temperature:: -

Grade:: -

Qualification:: -

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Supplier Device Package:: -

SN74S251N3 FAQ

1.How can I place an order for SN74S251N3 through Aetrix?

Please submit a Request for Quotation (RFQ) for SN74S251N3 on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.

2.Are the price and stock information for SN74S251N3 reliable?

The price and inventory of SN74S251N3 are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74S251N3 is usually 5 days.

3.What payment methods are accepted for SN74S251N3?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74S251N3 transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for SN74S251N3?

SN74S251N3 orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your SN74S251N3 order is processed, you will receive an email with the shipment details and tracking number.

Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.

5.How can I obtain technical support or documentation for SN74S251N3?

For technical support, including SN74S251N3 datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74S251N3 requirements.

6.How does Aetrix verify that SN74S251N3 is sourced from the original manufacturer or authorized distributors?

All SN74S251N3 products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that SN74S251N3 meets industry standards.

7.What is the process for return or replacement of SN74S251N3?

All SN74S251N3 units undergo pre-shipment inspection (PSI). If there is an issue with SN74S251N3, returns or replacements are accepted under the following conditions:

1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.

2.The issue is reported within 90 days of delivery.

3.The SN74S251N3 part is unused and in its original packaging.

Return procedure for SN74S251N3:

1.Submit a request within 90 days.

2.Obtain a Return Material Authorization (RMA) from Aetrix.

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