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Texas Instruments SN74LS251NSR
- Part No.:
- SN74LS251NSR
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-SOIC (0.209", 5.30mm Width)
- Datasheet:
-
SN74LS251NSR.pdf
- Description:
- IC MULTIPLEXER 1 X 8:1 16SO
- Quantity:
- Payment:
- Shipping:
Inventory:3,950
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Product details
Overview
SN74LS251NSR from Texas Instruments is a dual 4-line-to-1-line data selector/multiplexer with three-state outputs, implemented in low-power Schottky TTL logic. It features 8 data inputs (I₀–I₇), 3 address inputs (A, B, C), an enable input (E̅), and a complementary output (Y̅) plus true output (Y). Operating from 4.75 V to 5.25 V at 0°C to 70°C, it supports high-speed digital routing in bus-oriented systems such as microprocessor data paths and memory address decoding.
For engineers reviewing the SN74LS251NSR datasheet, SN74LS251NSR pinout, SN74LS251NSR application, or SN74LS251NSR equivalent, key selection considerations include its 16-pin SOP (NS) package, three-state output capability for bus sharing, TTL-compatible input thresholds, propagation delay of 22 ns typical at VCC = 5 V, and guaranteed fan-out of 20 LS-TTL loads.
Technical Context
The SN74LS251NSR implements combinational logic selection using three binary address lines to route one of eight data inputs to the Y output, while Y̅ provides the inverted signal. Its three-state outputs are controlled by the active-high enable (E̅) input, allowing direct connection to shared data buses without external gating.
Designed for TTL system integration, it meets standard LS-TTL voltage thresholds (VIH = 2.0 V min, VIL = 0.8 V max), drives ±8 mA output current, and maintains noise margins of 0.4 V (high) and 0.4 V (low) under nominal conditions. The device does not include latching or clocked functionality - operation is purely level-sensitive and asynchronous.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Logic Family | LS-TTL - ensures compatibility with legacy 74LS-series systems and predictable interfacing with standard TTL inputs/outputs. |
| Supply Voltage | 4.75 V to 5.25 V - requires tightly regulated +5 V rail; not tolerant of wider ranges or undervoltage brownout. |
| Propagation Delay | 22 ns typical (A/B/C → Y), 25 ns max - enables reliable operation up to ~20 MHz toggle rate in critical path timing. |
| Output Drive | ±8 mA (IOH/IOL) - sufficient to drive 20 LS-TTL inputs or directly interface with 74ALS/AS devices without buffers. |
| Operating Temperature | 0°C to 70°C - commercial-grade rating; unsuitable for extended industrial or automotive ambient environments. |
| Three-State Enable | Active-low E̅ input - places Y and Y̅ in high-impedance state when asserted, enabling bus arbitration in multi-source systems. |
| Input Loading | 1.6 mA max IIL, 20 µA max IIH - presents light DC load to upstream drivers, minimizing fan-out constraints. |
Pinout & Package
SOP (NS) package - 16-pin small-outline plastic package with 1.27 mm lead pitch, 10.0 mm × 4.4 mm body, and maximum height of 2.00 mm. RoHS-compliant, NIPDAU lead finish, MSL Level-1.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 (E̅) | Output Enable (active low) | Asserting low enables Y/Y̅ outputs; high forces both into high-impedance state for bus isolation. |
| 2 (I₀) | Data Input 0 | One of eight independent data sources selected when A,B,C = 000; TTL-compatible voltage thresholds apply. |
| 3 (I₁) | Data Input 1 | Selected when A,B,C = 001; identical electrical specs to I₀–I₇; no internal termination or biasing. |
| 4 (I₂) | Data Input 2 | Selected when A,B,C = 010; shares same DC and AC characteristics across all eight data inputs. |
| 5 (I₃) | Data Input 3 | Selected when A,B,C = 011; electrically symmetrical with other data inputs; no priority encoding. |
| 6 (I₄) | Data Input 4 | Selected when A,B,C = 100; supports synchronous or asynchronous data routing depending on system control timing. |
| 7 (I₅) | Data Input 5 | Selected when A,B,C = 101; no internal latching - output follows input changes within propagation delay window. |
| 8 (I₆) | Data Input 6 | Selected when A,B,C = 110; valid only when E̅ is low; undefined behavior if E̅ is high during address change. |
| 9 (I₇) | Data Input 7 | Selected when A,B,C = 111; final data source in 3-bit decoded selection; no glitch suppression circuitry. |
| 10 (GND) | Ground Reference | Primary return path for all internal logic and I/O; must be low-impedance and decoupled near VCC pin. |
| 11 (Y) | True Output | Active-high selected data; three-state capable; driven only when E̅ = low and address stable. |
| 12 (Y̅) | Inverted Output | Complementary to Y; simultaneously three-stated with Y; no separate enable control. |
| 13 (C) | Address Bit 2 (MSB) | Most significant select line; determines upper/lower half of 8-input set; level-sensitive only. |
| 14 (B) | Address Bit 1 | Middle select line; contributes to full 3-bit binary decode; no hysteresis or filtering. |
| 15 (A) | Address Bit 0 (LSB) | Least significant select line; completes 3-bit address; transitions cause immediate output update if E̅ active. |
| 16 (VCC) | Positive Supply | +5 V power supply; requires local 0.1 µF ceramic decoupling; sensitive to ripple > 100 mV peak-to-peak. |
Key Features
| Feature | Design Value |
|---|---|
| Dual complementary outputs (Y and Y̅) | Eliminates need for external inverters in applications requiring both polarities - reduces component count and board area. |
| Three-state outputs with common enable | Allows direct connection to multidrop data buses without contention; simplifies bus arbitration logic in CPU/peripheral interfaces. |
| Standard LS-TTL input/output compatibility | Interoperates seamlessly with existing 74LS, 74ALS, and 74F families without level-shifting or buffering. |
| No internal latches or clocks | Enables transparent, real-time data routing - ideal for address selection, test pattern generation, and combinatorial signal switching. |
| Guaranteed fan-out of 20 LS-TTL loads | Supports direct driving of multiple downstream gates without buffer amplification - lowers system power and complexity. |
Applications
| Microprocessor Address Decoding | Memory Bank Selection |
|---|---|
|
Use Scenario: Selecting between multiple ROM/RAM chips based on upper address bits in an 8-bit system. IC Role / Device Role / Timing Role: Data selector routing chip-select signals under control of A13–A15, synchronized with CPU read/write strobes. Use Value: Reduces discrete gate count by replacing 3× 74LS138 + OR-gate logic with single-package solution, cutting propagation delay by 15 ns. |
Use Scenario: Dynamically assigning 64 KB address space among four 16 KB SRAM banks in embedded controller designs. IC Role / Device Role / Timing Role: Address multiplexer generating bank-enable signals derived from A14/A15 and mode register bits. Use Value: Enables software-configurable memory mapping without PCB redesign; three-state outputs prevent bus conflicts during reconfiguration. |
| Test Pattern Multiplexing | Legacy Bus Arbitration |
|
Use Scenario: Feeding programmable test vectors from multiple shift registers into a DUT's parallel input port during production testing. IC Role / Device Role / Timing Role: Static data selector choosing between four 8-bit test sources under JTAG-controlled address lines. Use Value: Provides deterministic, glitch-free vector switching with sub-25 ns latency - critical for boundary-scan timing compliance. |
Use Scenario: Managing shared control bus access among UART, timer, and ADC peripherals in an 8051-based industrial module. IC Role / Device Role / Timing Role: Three-state bus switch enabling one peripheral to drive control lines while others tri-state their outputs. Use Value: Replaces discrete MOSFET switches or complex PLD logic with proven, radiation-tolerant TTL silicon - improves long-term reliability. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar data selector/multiplexer applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74LS151N | Identical logic function, pinout, and DC specs; differs only in PDIP (N) package vs. SOP (NS) for SN74LS251NSR. | Same use cases but requires through-hole assembly; lacks tape-and-reel packaging for automated SMT placement. | Select SN74LS151N only for legacy through-hole designs or prototyping where SOP reflow is unavailable. |
| SN74LS251DR | Same die, SOIC (D) package with 1.27 mm pitch; slightly larger body (10.3 mm × 5.3 mm) and different tape dimensions (A₀/B₀). | Compatible in function and timing, but footprint differs - not drop-in replaceable without PCB revision. | Choose SN74LS251DR when SOIC land patterns are already committed; SN74LS251NSR requires NS-specific layout. |
Compared with SN74LS151N and SN74LS251DR, the SN74LS251NSR offers optimized SMT manufacturability via its SOP-16 (NS) tape-and-reel format, tighter height constraint (2.00 mm max), and industry-standard MSL-1 handling - making it preferred for high-volume automated assembly where board space and thermal profile are constrained.
Availability
SN74LS251NSR is available at Aetrix Electronics and suitable for microprocessor address decoding, memory bank selection, test pattern multiplexing, and legacy bus arbitration requiring stable component supply and long-term obsolescence management.
Supply support for SN74LS251NSR 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 communications reach.
The SN74LS251NSR belongs to TI's legacy 74LS logic family, designed specifically for backward-compatible upgrades in TTL-based digital systems where speed, power efficiency, and interoperability with existing 74xx circuits are essential.
FAQ
What is the primary function of the SN74LS251NSR?
The SN74LS251NSR is an 8-input, 3-address-line data selector/multiplexer with complementary true (Y) and inverted (Y̅) three-state outputs. It routes one of eight digital inputs to the outputs based on the binary value of address lines A, B, and C. Its enable input (E̅) controls output state - when high, both Y and Y̅ enter high-impedance mode. This makes the SN74LS251NSR ideal for bus-oriented applications like address decoding and signal routing in TTL-based systems.
Does the SN74LS251NSR support 3.3 V logic levels?
No, the SN74LS251NSR is strictly a 5 V-only device with LS-TTL input thresholds (VIL ≤ 0.8 V, VIH ≥ 2.0 V) and output swing referenced to 5 V. It cannot reliably interface with 3.3 V CMOS or LVTTL systems without level translation. Driving its inputs from 3.3 V sources may result in marginal or invalid logic recognition, and connecting its 5 V outputs to 3.3 V–tolerant inputs risks overvoltage damage. For mixed-voltage designs, the SN74LS251NSR must be used within a dedicated 5 V domain or paired with bidirectional translators.
Can the SN74LS251NSR be used as a demultiplexer?
While the SN74LS251NSR is fundamentally a multiplexer, it can be configured as a 1-to-8 demultiplexer by applying the data signal to the enable input (E̅) and using the address lines (A, B, C) to select the output channel - but only indirectly. Since it lacks dedicated demux outputs, this requires external inversion and gating of Y/Y̅ to derive eight distinct active-low enables. Unlike purpose-built demuxes (e.g., 74LS138), the SN74LS251NSR does not provide decoded, mutually exclusive outputs - so true 1:8 demux functionality is not natively supported and introduces timing and loading complications.
What is the maximum clock/data toggle frequency supported by the SN74LS251NSR?
The SN74LS251NSR has no internal clock; its operation is purely combinational and asynchronous. The practical maximum toggle rate depends on propagation delay (22 ns typical) and system setup/hold timing. Assuming clean address transitions and stable data, it supports reliable switching up to approximately 20 MHz in worst-case path analysis. However, metastability or glitches may occur if address lines change concurrently with data transitions - proper synchronization of control signals is required for robust high-speed use of the SN74LS251NSR.
Is the SN74LS251NSR pin-compatible with the SN74LS151?
Yes, the SN74LS251NSR is functionally and pin-compatible with the SN74LS151 across all 16 pins - same pin assignments for I₀–I₇, A–C, E̅, Y, Y̅, VCC, and GND. Both share identical logic behavior, DC specs, and AC timing. The distinction lies solely in part numbering convention and packaging variants (e.g., SN74LS151 is commonly offered in PDIP, while SN74LS251NSR specifies SOP-16), but the core silicon and pinout are interchangeable for design reuse and second-sourcing purposes.
SN74LS251NSR Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74LS
- Package/Case:
- 16-SOIC (0.209", 5.30mm Width)
- Packaging:
- Tape & Reel (TR)
- Product Status:
- Active
- Type:
- Multiplexer
- Circuit:
- 1 x 8:1
- Independent Circuits:
- 1
- Current - Output High, Low:
- 2.6mA, 8mA
- Voltage Supply Source:
- Single Supply
- Voltage - Supply:
- 4.75V ~ 5.25V
- Operating Temperature:
- 0°C ~ 70°C
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Surface Mount
- Supplier Device Package:
- 16-SO
SN74LS251NSR FAQ
1.How can I place an order for SN74LS251NSR through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74LS251NSR 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 SN74LS251NSR reliable?
The price and inventory of SN74LS251NSR are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74LS251NSR is usually 5 days.
3.What payment methods are accepted for SN74LS251NSR?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74LS251NSR transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74LS251NSR?
SN74LS251NSR orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74LS251NSR 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 SN74LS251NSR?
For technical support, including SN74LS251NSR datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74LS251NSR requirements.
6.How does Aetrix verify that SN74LS251NSR is sourced from the original manufacturer or authorized distributors?
All SN74LS251NSR 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 SN74LS251NSR meets industry standards.
7.What is the process for return or replacement of SN74LS251NSR?
All SN74LS251NSR units undergo pre-shipment inspection (PSI). If there is an issue with SN74LS251NSR, 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 SN74LS251NSR part is unused and in its original packaging.
Return procedure for SN74LS251NSR:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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