Texas Instruments SN74LS42NSR

Overview

SN74LS42NSR from Texas Instruments is a TTL BCD-to-decimal decoder IC that converts 4-bit binary-coded decimal input into active-low 10-line decimal outputs, with typical propagation delay of 45 ns at VCC = 5 V and IOH = –0.4 mA, fan-out of 20 LS-TTL loads, and operating temperature range of 0°C to 70°C. It is used in digital display drivers, address decoders, and numeric selection logic in legacy industrial control panels.

For engineers reviewing the SN74LS42NSR datasheet, SN74LS42NSR pinout, SN74LS42NSR application, or SN74LS42NSR equivalent, key selection considerations include its active-low decoded outputs, TTL-compatible input thresholds, absence of internal pull-ups, fixed BCD decoding logic (0–9 only), and compatibility with standard 16-pin PDIP packaging for through-hole prototyping and repair.

Technical Context

This decoder implements combinational logic mapping BCD inputs A–D (D = MSB) to ten mutually exclusive active-low outputs Y0–Y9, where each output asserts low only when its corresponding decimal value (0–9) is present; inputs 10–15 (1010–1111) produce all-high outputs (no decode). No enable or latch functionality is integrated.

Designed for direct interface with TTL logic families, SN74LS42NSR features standard LS-TTL input thresholds (VIH = 2.0 V min, VIL = 0.8 V max), output drive capability of –0.4 mA (IOH) and 8 mA (IOL), and operates strictly from a single 5-V supply with no biasing or external components required.

Key Specifications

Pinout & Package

SN74LS42NSR is supplied in a 16-pin plastic dual in-line package (PDIP), with 0.3-inch body width and standard through-hole footprint. Pin spacing is 0.1 inch, and the package includes notch-marked orientation for correct PCB insertion.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar BCD-to-decimal decoding applications.

Compared with SN74LS42NSR, SN74LS145N offers marginally faster decoding with identical form/fit/function, while 74HC42N enables lower-power operation but necessitates board-level changes due to push-pull outputs and different voltage thresholds.

Availability

SN74LS42NSR is available at Aetrix Electronics and suitable for legacy industrial control upgrades, repair of vintage test equipment, and educational digital logic labs requiring stable component supply and long-term obsolescence support.

Supply support for SN74LS42NSR 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 consumer design support.

SN74LS42NSR belongs to TI's legacy 74LS logic family, engineered for reliable, low-cost digital decoding in commercial-grade systems where deterministic timing and TTL interoperability are essential.

FAQ

What is the function of SN74LS42NSR in a digital system?

SN74LS42NSR performs fixed BCD-to-decimal decoding: it accepts a 4-bit BCD input (A–D) and asserts exactly one of ten active-low outputs (Y0–Y9) corresponding to decimal digits 0–9. Inputs 10–15 produce no active output. The SN74LS42NSR is not programmable and contains no enable or latch features - its behavior is purely combinational and hardwired.

Does SN74LS42NSR have built-in pull-up resistors on its outputs?

No, SN74LS42NSR does not include internal pull-up resistors. Its outputs are open-collector, meaning external pull-up resistors (typically 1–10 kΩ to +5 V) are required to establish defined HIGH logic levels. This architecture allows wired-OR connections and direct driving of LEDs or relay drivers without additional buffering.

Can SN74LS42NSR operate from a 3.3 V supply?

No, SN74LS42NSR is specified only for 4.75 V to 5.25 V operation. Applying 3.3 V will result in incorrect logic thresholds, degraded noise margins, and unreliable output states. For 3.3 V systems, consider CMOS alternatives like 74HC42, but note that SN74LS42NSR itself requires strict +5 V compliance.

What is the maximum fan-out of SN74LS42NSR?

SN74LS42NSR supports a fan-out of 20 LS-TTL loads per output. This rating assumes standard loading conditions (IOH = –0.4 mA, IOL = 8 mA) and reflects its ability to drive multiple downstream TTL inputs without signal degradation. Exceeding this limit may cause timing violations or logic errors.

Is SN74LS42NSR pin-compatible with SN74LS145N?

Yes, SN74LS42NSR and SN74LS145N share identical pin assignments, electrical specifications, and functional behavior. Both are 16-pin PDIP BCD-to-decimal decoders with active-low open-collector outputs and identical truth tables. SN74LS145N is a documented drop-in replacement per TI documentation and has been validated in field-replaceable contexts.