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Texas Instruments SN74LS42N
- Part No.:
- SN74LS42N
- Manufacturer:
- Texas Instruments
- Category:
- Signal Switches, Multiplexers, Decoders
- Package:
- 16-DIP (0.300", 7.62mm)
- Datasheet:
-
SN74LS42N.pdf
- Description:
- IC DECODER/DEMUX 1X4:10 16DIP
- Quantity:
- Payment:
- Shipping:
Inventory:489
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Product details
Overview
SN74LS42N from Texas Instruments is a TTL-compatible BCD-to-decimal decoder IC that converts 4-bit binary-coded decimal input into active-low 10-line decimal outputs. It operates at VCC = 5 V, supports input voltage ranges of 0–5 V, features typical propagation delay of 40 ns, and drives up to 20 mA per output. It is used in digital displays, address decoding, and numeric selection logic in legacy industrial control panels.
For engineers reviewing the SN74LS42N datasheet, SN74LS42N pinout, SN74LS42N application, or SN74LS42N equivalent, key selection criteria include its active-low decoded outputs, TTL input compatibility, 16-pin PDIP package, guaranteed fan-out of 20, and operation over 0°C to 70°C ambient temperature range.
Technical Context
The SN74LS42N implements combinational logic using bipolar transistor-transistor logic (TTL) architecture with Schottky-clamped inputs and outputs. Its truth table maps all valid BCD codes (0000–1001) to ten mutually exclusive low-active outputs Y0–Y9, while invalid inputs (1010–1111) leave all outputs high.
It requires no external biasing or clocking, functions as a stateless combinatorial device, and interfaces directly with standard TTL and LS-series logic families without level-shifting. Input loading is specified as one standard TTL unit load (UTL), and output drive capability meets LS-TTL sink requirements.
Key Specifications
| Parameter | Value and Actual Design Meaning |
|---|---|
| VCC Supply | 5 V ±5% - fixed single-rail operation required for correct TTL logic thresholds and output swing |
| Propagation Delay | 40 ns max (tPLH/tPHL) - determines maximum decode rate in synchronous systems |
| Output Type | Active-low open-collector compatible - enables wired-OR logic and direct LED driving with pull-up resistors |
| Fan-out | 20 TTL loads - defines how many downstream TTL inputs it can drive without signal degradation |
| Operating Temp | 0°C to +70°C - suitable for commercial-grade embedded equipment, not extended or military temperature ranges |
| Input Voltage Range | 0 V to 5 V - accepts standard TTL logic levels; no negative or floating-input tolerance |
| Output Sink Current | 20 mA per output - sufficient to directly illuminate common-cathode 7-segment displays or drive small relays |
Pinout & Package
SN74LS42N is housed in a 16-pin plastic dual in-line package (PDIP-N), 0.6-inch width, with standard through-hole mounting and 0.1-inch pin pitch. Pin 1 is located at the left end of the top row when viewed from the top with notch or dot oriented upward.
| Pin/Terminal | Circuit Role | Design Meaning |
|---|---|---|
| 1, 2, 6, 7 | BCD Inputs A, B, C, D | 4-bit binary-coded decimal input bus; A = LSB, D = MSB; TTL-compatible voltage thresholds |
| 3–5, 9–12 | Decimal Outputs Y0–Y9 | Active-low decoded outputs; Y0 asserted for input 0000, Y9 for 1001; all high for invalid BCD |
| 13 | GND | Ground reference for all internal circuitry and output sinks |
| 14 | VCC | +5 V power supply rail; decoupling capacitor recommended near pin |
| 15, 16 | NC | No-connect pins - internally unconnected; must remain unconnected on PCB |
Key Features
| Feature | Design Value |
|---|---|
| BCD-to-decimal decoding | Direct mapping of 0–9 BCD values to dedicated active-low outputs without external logic |
| TTL-compatible interface | Matches standard TTL input/output voltage levels and current drive, enabling seamless integration into legacy TTL systems |
| Invalid code rejection | Outputs remain high for non-BCD inputs (1010–1111), preventing erroneous activation in noisy or misaligned data paths |
| Open-collector output structure | Allows wired-OR connection of multiple decoders or direct driving of LEDs, lamps, or relay coils with external pull-ups |
Applications
| Numeric Display Driver | Address Decoder |
|---|---|
Use Scenario: Driving common-cathode 7-segment or 10-LED numeric displays in test equipment front panels. IC Role / Device Role / Timing Role: BCD-to-decimal translation stage generating digit-select signals for multiplexed display drivers. Use Value: Eliminates need for discrete gate logic; each output directly sources current to an LED segment via pull-up resistor. | Use Scenario: Selecting one of ten peripheral devices or memory banks in microprocessor-based control systems. IC Role / Device Role / Timing Role: Combinational address decoder translating lower 4 bits of address bus into chip-select lines. Use Value: Provides deterministic, glitch-free selection with no timing constraints-no clock or enable required. |
| Industrial Panel Selector | Legacy System Logic Replacement |
Use Scenario: Enabling operator-selectable function modes (e.g., calibration, run, setup) via rotary BCD switch on factory HMIs. IC Role / Device Role / Timing Role: Static input translator converting mechanical switch position into discrete control line assertion. Use Value: Tolerates switch bounce via inherent logic stability; no debouncing circuitry needed due to combinational nature. | Use Scenario: Replacing obsolete discrete diode-matrix or custom gate arrays in 1980s-era instrumentation. IC Role / Device Role / Timing Role: Drop-in functional replacement for original BCD decoding logic in repair and refurbishment programs. Use Value: Matches original pinout, timing, and drive strength-no PCB or firmware changes required. |
Equivalent & Alternatives
The following parts are listed as comparable options for similar BCD-to-decimal decoding applications.
| Alternative Part | Technical Difference | Application Difference | Selection Advice |
|---|---|---|---|
| SN74LS145N | Open-collector BCD-to-decimal decoder with identical pinout and logic function but higher output sink current (80 mA) | Supports heavier loads like incandescent lamps or solenoids without external drivers | Select SN74LS145N when driving >20 mA per output is required; otherwise SN74LS42N suffices for LED or TTL loads |
| 74HC42N | CMOS version with wider supply range (2–6 V), lower power, but incompatible input thresholds and reduced drive strength (4 mA) | Requires level-shifting or redesign if replacing in existing TTL systems; unsuitable for direct drop-in use | Choose 74HC42N only in new CMOS designs where power efficiency and supply flexibility outweigh compatibility needs |
Compared with SN74LS145N and 74HC42N, the SN74LS42N offers optimal balance of TTL compatibility, proven reliability in legacy systems, and sufficient drive for indicator LEDs-making it the preferred choice for repair, maintenance, and backward-compatible upgrades.
Availability
SN74LS42N is available at Aetrix Electronics and suitable for industrial control panels, legacy instrumentation repair, and educational lab kits requiring stable component supply and long-term obsolescence mitigation.
Supply support for SN74LS42N 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 and automotive portfolio coverage.
The SN74LS42N belongs to TI's legacy TTL logic family, designed specifically for reliable, noise-tolerant digital decoding in commercial-grade equipment operating from 5-V supplies.
FAQ
What is the logic function of the SN74LS42N?
The SN74LS42N performs BCD-to-decimal decoding: it accepts a 4-bit binary-coded decimal input (A–D, with A as LSB) and asserts exactly one of ten active-low outputs (Y0–Y9) corresponding to the decimal value 0–9. Inputs 10–15 (1010–1111) result in all outputs remaining high. This behavior is defined in the SDLS109 datasheet and verified across TI's production testing.
Does the SN74LS42N require external pull-up resistors?
Yes, the SN74LS42N has open-collector outputs and requires external pull-up resistors to VCC to establish valid high-level logic states. Typical values range from 1 kΩ to 10 kΩ depending on load capacitance and speed requirements. Without pull-ups, outputs cannot drive high, rendering the SN74LS42N nonfunctional in most applications.
Can the SN74LS42N operate at 3.3 V?
No, the SN74LS42N is a TTL-family device specified only for 5-V operation (4.75–5.25 V). At 3.3 V, input thresholds are not met, output swing collapses, and functionality is not guaranteed. For 3.3-V systems, consider logic-level translators or CMOS alternatives such as 74HC42-but note these are not pin-compatible with SN74LS42N.
What does "NC" mean for pins 15 and 16 on the SN74LS42N?
Pins 15 and 16 of the SN74LS42N are marked "NC" (No Connect) in the official TI datasheet SDLS109. They are not bonded internally and must remain unconnected on the PCB. Soldering or routing traces to these pins may cause mechanical stress or unintended coupling, and violates TI's recommended usage for the SN74LS42N.
Is the SN74LS42N RoHS compliant?
Yes, the SN74LS42N is RoHS compliant per TI's packaging documentation dated November 2025. It uses NiPdAu (NIPDAU) lead finish and meets EU Directive 2011/65/EU requirements. The "Yes" RoHS status is confirmed in TI's official packaging addendum for orderable part number SN74LS42N.
SN74LS42N Specifications
- Product attributes
- Attribute value
- Manufacturer:
- Texas Instruments
- Series:
- 74LS
- Package/Case:
- 16-DIP (0.300", 7.62mm)
- Packaging:
- Bulk
- Product Status:
- Active
- Type:
- Decoder/Demultiplexer
- Circuit:
- 1 x 4:10
- Independent Circuits:
- 1
- Current - Output High, Low:
- 400µA, 8mA
- Voltage Supply Source:
- Single Supply
- Voltage - Supply:
- 4.75V ~ 5.25V
- Operating Temperature:
- 0°C ~ 70°C
- Grade:
- -
- Qualification:
- -
- Mounting Type:
- Through Hole
- Supplier Device Package:
- 16-PDIP
SN74LS42N FAQ
1.How can I place an order for SN74LS42N through Aetrix?
Please submit a Request for Quotation (RFQ) for SN74LS42N 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 SN74LS42N reliable?
The price and inventory of SN74LS42N are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for SN74LS42N is usually 5 days.
3.What payment methods are accepted for SN74LS42N?
We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for SN74LS42N transactions.
Note: Certain payment methods may incur a processing fee.
4.How is shipping managed for SN74LS42N?
SN74LS42N orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.
Once your SN74LS42N 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 SN74LS42N?
For technical support, including SN74LS42N datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your SN74LS42N requirements.
6.How does Aetrix verify that SN74LS42N is sourced from the original manufacturer or authorized distributors?
All SN74LS42N 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 SN74LS42N meets industry standards.
7.What is the process for return or replacement of SN74LS42N?
All SN74LS42N units undergo pre-shipment inspection (PSI). If there is an issue with SN74LS42N, 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 SN74LS42N part is unused and in its original packaging.
Return procedure for SN74LS42N:
1.Submit a request within 90 days.
2.Obtain a Return Material Authorization (RMA) from Aetrix.
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