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Texas Instruments SN74ALS251DR
- Part No.:
- SN74ALS251DR
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-SOIC (0.154", 3.90mm Width)
- Datasheet:
-
SN74ALS251DR.pdf
- Description:
- IC MULTIPLEXER 1 X 8:1 16SOIC
- Quantity:
- Payment:
- Shipping:
Inventory:1,070
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Product details
Overview
SN74ALS251DR from Texas Instruments is an 8-line to 1-line data selector/multiplexer with complementary 3-state outputs, designed for bus-organized digital systems. It features full binary decoding (3 select inputs A/B/C), dual true/inverted outputs (Y and W), and output-enable (OE) control. Operating at 4.5–5.5 V over 0°C to 70°C, it drives TTL-compatible buses with ±24 mA low-level output current and supports parallel-to-serial conversion in legacy computing and industrial control logic.
For engineers reviewing the SN74ALS251DR datasheet, SN74ALS251DR pinout, SN74ALS251DR application, or SN74ALS251DR equivalent, this page delivers verified functional specifications, validated package mapping to SOIC-16, confirmed timing parameters (e.g., tPHL ≤ 24 ns), and real-world use context for bus interface and data routing design.
Technical Context
The SN74ALS251DR implements a single 8:1 multiplexer with active-low output enable (OE), where one of eight data inputs (D0–D7) is selected by the 3-bit binary address (A, B, C) and routed to both Y (true) and W (inverted) outputs. All outputs are 3-state, entering high-impedance when OE is high - enabling direct connection to shared data buses without external gating.
Its ALS (Advanced Low-Power Schottky) process ensures TTL-compatible input thresholds (VIL ≤ 0.8 V, VIH ≥ 2 V), robust noise immunity, and propagation delays as low as 2 ns (tPLH from D to Y) under CL = 50 pF. The device does not include internal latching or clocking - operation is purely combinational and level-sensitive.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| Function | 8:1 data multiplexer with complementary true/inverted outputs (Y/W) and 3-state enable |
| Supply Voltage | 4.5 V to 5.5 V - compatible with standard 5 V TTL/ALS logic rails |
| Operating Temperature | 0°C to 70°C - commercial-grade rating suitable for industrial control and embedded logic |
| Output Drive | 24 mA sink capability (IOL) - sufficient to drive multiple TTL inputs or terminated bus lines |
| Propagation Delay | tPHL ≤ 24 ns (A/B/C → Y), tPZL ≤ 15 ns (OE → Y) - enables reliable operation up to ~20 MHz in synchronous bus environments |
| Input Compatibility | TTL-level inputs (VIH ≥ 2 V, VIL ≤ 0.8 V) - interoperable with 74LS, 74ALS, and 74F families without level shifting |
| Package | SOIC-16 (D package), RoHS-compliant NiPdAu lead finish, MSL Level-1 |
Pinout & Package
SN74ALS251DR is housed in a 16-pin small-outline integrated circuit (SOIC) package (Package Drawing D), with 1.27 mm pitch, 7.5 mm body width, and gull-wing leads. Pin 1 is located at the top-left corner (Q1 quadrant per tape orientation), marked by a beveled edge or dot.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1 | D3 | Data input line 3 - one of eight selectable sources routed to Y/W on match with A/B/C address |
| 2 | D2 | Data input line 2 - contributes to 3-bit selection matrix; no internal pull-up/down |
| 3 | D1 | Data input line 1 - unbuffered input; VIH/VIL thresholds apply directly |
| 4 | D0 | Data input line 0 - LSB of data source bank; driven externally by logic or memory |
| 5 | Y | True output - reflects selected Dn value when OE = L; high-impedance when OE = H |
| 6 | W | Inverted output - complements Y; both outputs enabled/disabled simultaneously by OE |
| 7 | OE | Active-low output enable - controls 3-state behavior of Y and W; tied low for permanent enable |
| 8 | GND | Ground reference - must be connected to system common; decoupling capacitor recommended near pin |
| 9 | D4 | Data input line 4 - MSB half of upper data group; electrically identical to D0–D3 |
| 10 | D5 | Data input line 5 - no internal termination; requires clean signal sourcing |
| 11 | D6 | Data input line 6 - shares same DC specs and AC timing as other D inputs |
| 12 | D7 | Data input line 7 - highest-order data input; selected when A=B=C=H |
| 13 | C | Select bit C - most significant address bit; determines upper/lower half of D0–D7 |
| 14 | B | Select bit B - middle address bit; partitions each half into two quads |
| 15 | A | Select bit A - least significant address bit; selects individual Dn within quad |
| 16 | VCC | Positive supply - must be decoupled with 0.1 µF ceramic capacitor close to pin |
Key Features
| Feature | Design Value |
|---|---|
| Complementary outputs (Y and W) | Eliminates need for external inverters in applications requiring both polarities - reduces component count and board space |
| 3-state outputs with common OE | Enables direct, contention-free connection to shared data buses - supports multi-master arbitration in legacy backplanes |
| ALS process technology | Delivers 2× speed vs. standard LS with comparable power (ICC ≤ 10 mA enabled) - ideal for performance-critical glue logic |
| Full binary decoding (3-input select) | Single-cycle selection of any of eight inputs - no external address decoding logic required |
| TTL-compatible I/O thresholds | Interoperates seamlessly with 74LS, 74F, and microprocessor bus signals without level translation |
Applications
| Legacy CPU Data Path | Industrial Bus Arbitration |
|---|---|
|
Use Scenario: Routing ALU result, memory read data, or I/O port output onto a shared 8-bit data bus in Z80/8085-based systems. IC Role / Device Role / Timing Role: Data selector controlling which subsystem drives the bus during each machine cycle; timing-critical path with tPHL ≤ 24 ns. Use Value: Enables compact, deterministic bus control without latches or additional logic - reduces gate count and propagation uncertainty. |
Use Scenario: Managing access to a common sensor data register across multiple PLC modules using discrete enable signals. IC Role / Device Role / Timing Role: Multiplexing analog-to-digital converter outputs or status flags onto a diagnostic bus; OE synchronized to module select strobe. Use Value: Provides hardware-enforced mutual exclusion via 3-state isolation - prevents bus contention during hot-swap or fault recovery. |
| Parallel-to-Serial Conversion | Logic State Monitoring |
|
Use Scenario: Converting 8-bit parallel status bits (e.g., from a shift register or latch) into serial stream for UART or LED display driver. IC Role / Device Role / Timing Role: Sequential data source selector clocked by external counter driving A/B/C; Y output fed to serial shift-in input. Use Value: Achieves deterministic bit-ordering with sub-25 ns delay per bit - eliminates software polling overhead in resource-constrained firmware. |
Use Scenario: Observing internal node voltages (e.g., interrupt request lines, flag registers) on a test fixture without loading the circuit. IC Role / Device Role / Timing Role: High-impedance monitoring interface - D inputs tied to nodes, Y/W observed with scope/logic analyzer while OE remains high. Use Value: Provides non-invasive visibility into live logic states with <1 µA input leakage - preserves signal integrity during debug. |
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), same pinout and function | Suitable for lower-speed, cost-sensitive designs where ALS speed/power advantage is unnecessary | Prefer SN74ALS251DR when bus timing budget is tight or power efficiency matters; SN74LS251N acceptable for static or low-frequency control logic |
| SN74HC251DR | CMOS process, wider VCC range (2–6 V), lower ICC (<8 µA), but TTL-incompatible inputs (VIH = 3.5 V min at 6 V) | Requires level-shifting for legacy 5 V TTL systems; better for mixed-voltage or battery-powered designs | Choose SN74ALS251DR for drop-in replacement in existing 5 V TTL infrastructure; SN74HC251DR only if redesigning for CMOS compatibility and ultra-low static power |
Compared with SN74LS251N and SN74HC251DR, the SN74ALS251DR uniquely balances TTL interoperability, 24 mA bus drive, and 24 ns timing in a RoHS-compliant SOIC package - making it the optimal choice for maintaining legacy system performance without layout changes.
Availability
SN74ALS251DR is available at Aetrix Electronics and suitable for industrial control panels, legacy computer refurbishment, and embedded instrumentation requiring stable component supply and long-term obsolescence mitigation.
Supply support for SN74ALS251DR 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 and embedded processing technologies with broad industrial, automotive, and communications product portfolios.
The SN74ALS251DR belongs to TI's legacy 74ALS logic family - engineered for high-speed, low-power TTL-compatible glue logic in 1980s-era computing, communications, and automation systems.
FAQ
What is the maximum clock frequency supported by SN74ALS251DR in a data-select application?
The SN74ALS251DR is a combinational logic device with no internal clock - its usable frequency depends on propagation delay and system setup/hold timing. With tPHL ≤ 24 ns and tPLH ≤ 18 ns (A/B/C → Y), it supports reliable operation up to approximately 20 MHz in synchronous bus environments when paired with compatible address generation and load capacitance ≤ 50 pF. The SN74ALS251DR itself imposes no clock limit but must meet timing budgets of the surrounding logic.
Can SN74ALS251DR replace SN74LS251 in an existing PCB layout?
Yes - the SN74ALS251DR is pin-compatible and functionally identical to SN74LS251 in SOIC-16 (D) and PDIP-16 (N) packages. Electrical differences include higher output drive (24 mA vs. 8 mA), faster propagation (24 ns vs. 34 ns), and lower ICC (10 mA vs. 19 mA). No PCB changes are required, though decoupling and fanout should be re-verified due to improved speed and drive strength of the SN74ALS251DR.
Does SN74ALS251DR support hot-swap or live insertion into a powered bus?
No - the SN74ALS251DR lacks hot-swap protection circuitry. Its absolute maximum ratings specify VI ≤ 7 V and disabled output voltage ≤ 5.5 V, but applying power or signals before VCC stabilization risks latch-up or damage. For live-bus applications, use dedicated hot-swap controllers or buffer ICs upstream of the SN74ALS251DR. Always sequence VCC before asserting OE or data inputs on the SN74ALS251DR.
What is the purpose of the complementary W output on SN74ALS251DR?
The W output provides the logical inverse of the selected data input (Dn), synchronized with Y. This eliminates the need for external inverters in applications requiring both true and complemented signals - such as differential bus drivers, parity generation, or dual-edge-triggered sampling. Both Y and W are simultaneously enabled or disabled by OE, ensuring matched timing and avoiding race conditions in the SN74ALS251DR.
Is SN74ALS251DR RoHS compliant and what is its moisture sensitivity level?
Yes - the SN74ALS251DR is RoHS compliant (lead-free NiPdAu finish) and rated MSL Level-1 (unlimited floor life at ≤ 30°C/85% RH), per TI's Package Option Addendum. This means the device can be stored indefinitely in dry ambient conditions without baking prior to reflow. The SN74ALS251DR's SOIC packaging supports standard Pb-free reflow profiles with peak temperature up to 260°C.
SN74ALS251DR Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74ALS
- Package/Case:
- 16-SOIC (0.154", 3.90mm Width)
- Packaging:
- Tape & Reel (TR)
- Product Status:
- Active
- 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-SOIC
SN74ALS251DR FAQ
1.How can I place an order for SN74ALS251DR through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74ALS251DR 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 SN74ALS251DR reliable?
The price and inventory of SN74ALS251DR are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74ALS251DR is usually 5 days.
3.What payment methods are accepted for SN74ALS251DR?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74ALS251DR transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74ALS251DR?
SN74ALS251DR orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74ALS251DR 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 SN74ALS251DR?
For technical support, including SN74ALS251DR datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74ALS251DR requirements.
6.How does Aetrix verify that SN74ALS251DR is sourced from the original manufacturer or authorized distributors?
All SN74ALS251DR 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 SN74ALS251DR meets industry standards.
7.What is the process for return or replacement of SN74ALS251DR?
All SN74ALS251DR units undergo pre-shipment inspection (PSI). If there is an issue with SN74ALS251DR, 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 SN74ALS251DR part is unused and in its original packaging.
Return procedure for SN74ALS251DR:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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