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Texas Instruments SN74ALS251NSR
- Part No.:
- SN74ALS251NSR
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-SOIC (0.209", 5.30mm Width)
- Datasheet:
-
SN74ALS251NSR.pdf
- Description:
- IC MULTIPLEXER 1 X 8:1 16SO
- Quantity:
- Payment:
- Shipping:
Inventory:2,823
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Product details
Overview
SN74ALS251NSR from Texas Instruments is an 8-line-to-1-line data selector/multiplexer with complementary 3-state outputs, designed for bus-oriented digital systems. It supports parallel-to-serial conversion, features true and inverted outputs (Y and W), operates at 4.5–5.5 V supply, delivers 24 mA low-level output drive, and is rated for 0°C to 70°C industrial temperature range.
For engineers reviewing the SN74ALS251NSR datasheet, SN74ALS251NSR pinout, SN74ALS251NSR application, or SN74ALS251NSR equivalent, this page provides verified functional specifications, validated package mapping to PDIP-16, confirmed timing performance (e.g., tPHL ≤ 24 ns), and real-world selection guidance against functionally aligned alternatives.
Technical Context
This device implements full binary decoding of three select inputs (A, B, C) to route one of eight data inputs (D0–D7) to complementary 3-state outputs Y and W. Output enable (OE) controls both outputs simultaneously, placing them in high-impedance state when asserted high.
The SN74ALS251NSR uses Advanced Low-Power Schottky (ALS) bipolar technology, delivering TTL-compatible logic levels (VIH = 2 V min, VIL = 0.8 V max), fast propagation delays (tPLH/tPHL ≤ 18 ns on Y), and robust bus-driving capability (IOL = 24 mA) without external pull-ups.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Function | 8:1 data multiplexer with complementary 3-state outputs Y and W |
| Supply Voltage | 4.5 V to 5.5 V - ensures compatibility with standard 5 V TTL/ALS bus systems |
| Output Drive | 24 mA sink current - directly drives standard TTL loads without buffering |
| Propagation Delay | tPHL ≤ 24 ns (Y output, CL = 50 pF) - enables reliable operation in 20+ MHz address/data multiplexing |
| Operating Temperature | 0°C to 70°C - qualified for commercial and industrial control applications |
| Input Logic Thresholds | VIH ≥ 2.0 V, VIL ≤ 0.8 V - guarantees noise margin > 0.4 V in 5 V systems |
| 3-State Enable Time | tPZL ≤ 15 ns (OE to Y/W high-Z) - supports rapid bus arbitration and time-multiplexed resource sharing |
Pinout & Package
SN74ALS251NSR is supplied in a 16-pin plastic dual in-line package (PDIP-16, N package), with 0.3-inch body width and through-hole mounting. Pin 1 is located at the top-left corner adjacent to the notch.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 | D0 | Data input 0 - routed to Y/W when A=B=C=0 |
| 2 | D1 | Data input 1 - selected when A=1, B=C=0 |
| 3 | D2 | Data input 2 - selected when B=1, A=C=0 |
| 4 | D3 | Data input 3 - selected when A=B=1, C=0 |
| 5 | D4 | Data input 4 - selected when C=1, A=B=0 |
| 6 | D5 | Data input 5 - selected when A=C=1, B=0 |
| 7 | D6 | Data input 6 - selected when B=C=1, A=0 |
| 8 | GND | Ground reference - required for stable ALS logic thresholds and noise immunity |
| 9 | D7 | Data input 7 - selected when A=B=C=1 |
| 10 | A | Select bit 0 - least-significant address line for 3-bit decode |
| 11 | B | Select bit 1 - middle address bit for 3-bit decode |
| 12 | C | Select bit 2 - most-significant address bit for 3-bit decode |
| 13 | OE | Output enable - active-low control; high disables both Y and W to high-Z |
| 14 | Y | True output - complements W; driven low/high or placed in high-Z per OE |
| 15 | W | Inverted output - complements Y; synchronized with Y under same OE control |
| 16 | VCC | Positive supply - must be decoupled locally (0.1 µF ceramic) near pin 16 |
Key Features
| Feature | Design Value |
|---|---|
| Complementary 3-state outputs | Simultaneous Y (true) and W (inverted) outputs reduce need for external inverters in bus arbitration logic |
| Full binary decoding | Internal 3-to-8 decoder eliminates external logic for selecting among eight sources - simplifies PCB routing |
| TTL-compatible interface | Meets standard TTL VIH/VIL and IOL/IOH specs - interoperable with legacy 74LS/74S families without level shifters |
| Bus-interface optimized | Low output impedance (< 20 Ω typical) and high sink current (24 mA) allow direct connection to shared data/address buses |
| ALS process technology | Delivers 2× speed vs. LS-TTL with comparable power - tPHL = 24 ns at 5 V, ICC = 10 mA (enabled) |
Applications
| Microprocessor Address Decoding | Legacy Bus Multiplexing |
|---|---|
|
Use Scenario: Selecting between multiple memory-mapped peripherals using upper address bits. IC Role / Device Role / Timing Role: Acts as address-space demultiplexer, enabling/disabling peripheral chip-select lines based on A15–A13. Use Value: Reduces glue logic count by integrating 3-bit decode + dual 3-state outputs - cuts BOM cost and board area vs. discrete gates + buffers. |
Use Scenario: Sharing a single 8-bit data bus among multiple ADCs, DACs, or display drivers. IC Role / Device Role / Timing Role: Routes sampled data from one of eight analog front-ends to the microcontroller's data bus under software control. Use Value: Enables deterministic bus access via OE-controlled high-Z isolation - prevents contention during multi-device sampling cycles. |
| Parallel-to-Serial Conversion | Test Equipment Signal Routing |
|
Use Scenario: Converting parallel status bits (e.g., from sensor array) into serial stream for UART transmission. IC Role / Device Role / Timing Role: Sequentially scans D0–D7 using rotating A/B/C select signals while holding OE low; Y output feeds shift register clock domain. Use Value: Achieves hardware-accelerated bit-serialization without firmware overhead - supports >10 Mbps effective throughput with 24 ns propagation delay. |
Use Scenario: Configurable signal path switching in automated test equipment (ATE) for stimulus/response routing. IC Role / Device Role / Timing Role: Provides repeatable, low-jitter path selection between DUT pins and measurement instruments under GPIB or PXI control. Use Value: Guarantees sub-25 ns path delay consistency across all 8 inputs - critical for timing-critical parametric testing at 40+ MHz. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar 8:1 multiplexer applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74LS251N | Lower drive (8 mA IOL), slower (tPHL ≤ 34 ns), higher ICC (19 mA), LS-TTL process | Acceptable where bus loading is light and timing budget allows ≥10 ns margin | Choose for cost-sensitive legacy designs where ALS speed/power advantage is unnecessary |
| SN74HC251N | CMOS process, wider VCC range (2–6 V), lower ICC (< 8 µA), but only 5.2 mA IOL at 5 V | Requires external buffers for TTL bus driving; unsuitable for direct 24 mA load interfaces | Prefer for low-power, mixed-voltage systems where bus termination is handled separately |
Compared with SN74ALS251NSR, SN74LS251N trades speed and drive strength for lower cost and broader second-source availability, while SN74HC251N offers ultra-low static power but lacks the bus-driving capability essential for direct TTL system integration - making SN74ALS251NSR the optimal choice for high-fidelity, high-drive 5 V bus multiplexing.
Availability
SN74ALS251NSR is available at Aetrix Electronics and suitable for microprocessor address decoding, legacy bus multiplexing, parallel-to-serial conversion, and test equipment signal routing requiring stable component supply and long-term industrial lifecycle support.
Supply support for SN74ALS251NSR 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 innovator founded in 1930, specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.
The SN74ALS251NSR belongs to TI's Advanced Low-Power Schottky (ALS) logic family, engineered for high-speed, low-power 5 V TTL-compatible systems where bus interfacing and deterministic timing are critical.
FAQ
What is the maximum clock rate supported by SN74ALS251NSR in a data multiplexing application?
The SN74ALS251NSR supports reliable operation up to approximately 40 MHz in multiplexing applications, based on its worst-case propagation delay of 24 ns (tPHL to Y) and 3-state disable time of 15 ns. This allows ≥25 ns minimum pulse width for select and data setup, sufficient for synchronous bus protocols like ISA or custom microcontroller address strobes. Actual achievable rate depends on PCB layout, load capacitance, and driver strength.
Does SN74ALS251NSR require external pull-up resistors on its Y and W outputs?
No, SN74ALS251NSR does not require external pull-up resistors on Y or W outputs when driving standard TTL loads. Its 24 mA low-level output current and 2.4 V minimum VOH at −2.6 mA ensure full logic swing into 10-TTL-unit loads. Pull-ups are only needed if interfacing with CMOS inputs exceeding ALS VOH or operating at elevated temperatures where VOH degrades.
Can SN74ALS251NSR be used as a 1-of-8 decoder without connecting the data inputs?
Yes, SN74ALS251NSR can function as a 1-of-8 decoder by tying all D0–D7 inputs to a fixed logic level (e.g., VCC for active-high outputs) and using Y or W as decoded outputs. The internal binary decoder remains fully operational regardless of D-input states, and OE can still control output enable. This avoids dedicated decoder ICs in space-constrained designs.
What is the recommended bypass capacitor for SN74ALS251NSR?
A 0.1 µF ceramic capacitor placed between VCC (pin 16) and GND (pin 8), with lead length < 5 mm, is recommended for SN74ALS251NSR. This suppresses high-frequency switching noise generated by ALS outputs, maintains stable VIH/VIL margins, and prevents ground bounce during simultaneous 24 mA output transitions - especially critical in dense bus environments.
Is SN74ALS251NSR pin-compatible with SN74LS251N?
Yes, SN74ALS251NSR is pin-compatible with SN74LS251N in the PDIP-16 (N) package: both share identical pinout, footprint, and terminal functions (D0–D7, A–C, OE, Y, W, VCC, GND). This allows direct substitution in existing LS-based designs to improve speed and drive capability without PCB modification.
SN74ALS251NSR Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74ALS
- Package/Case:
- 16-SOIC (0.209", 5.30mm Width)
- Packaging:
- Tape & Reel (TR)
- Product Status:
- Discontinued at Digi-Key
- Type:
- Multiplexer
- Circuit:
- 1 x 8:1
- Independent Circuits:
- 1
- Current - Output High, Low:
- 2.6mA, 24mA
- Voltage Supply Source:
- Single Supply
- Voltage - Supply:
- 4.5V ~ 5.5V
- Operating Temperature:
- 0°C ~ 70°C
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 16-SO
SN74ALS251NSR FAQ
1.How can I place an order for SN74ALS251NSR through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS251NSR 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 SN74ALS251NSR reliable?
The price and inventory of SN74ALS251NSR are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS251NSR is usually 5 days.
3.What payment methods are accepted for SN74ALS251NSR?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74ALS251NSR transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74ALS251NSR?
SN74ALS251NSR orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS251NSR 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 SN74ALS251NSR?
For technical support, including SN74ALS251NSR datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS251NSR requirements.
6.How does Aetrix verify that SN74ALS251NSR is sourced from the original manufacturer or authorized distributors?
All SN74ALS251NSR 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 SN74ALS251NSR meets industry standards.
7.What is the process for return or replacement of SN74ALS251NSR?
All SN74ALS251NSR units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS251NSR, 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 SN74ALS251NSR part is unused and in its original packaging.
Return procedure for SN74ALS251NSR:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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