Texas Instruments SN74LS42D

Overview

SN74LS42D from Texas Instruments is a TTL BCD-to-decimal decoder IC that converts 4-bit binary-coded decimal input into active-low 1-of-10 decoded outputs, with typical propagation delay of 45 ns at VCC = 5 V and operating current of 28 mA, used in address decoding and numeric display driver circuits for legacy industrial control panels.

For engineers reviewing the SN74LS42D datasheet, SN74LS42D pinout, SN74LS42D application, or SN74LS42D equivalent, key selection considerations include its active-low open-collector outputs, 0°C to 70°C commercial temperature range, SOIC-16 package, TTL logic compatibility, and absence of internal pull-up resistors requiring external termination.

Technical Context

The SN74LS42D implements combinational logic using bipolar transistor-transistor logic (TTL) architecture, with four buffered BCD inputs (A0–A3) and ten individually driven active-low outputs (Y0–Y9). It decodes only valid BCD codes (0000–1001), leaving outputs Y10–Y15 unimplemented and undefined for invalid inputs (1010–1111).

Input thresholds are defined per TTL standards: VIH ≥ 2.0 V, VIL ≤ 0.8 V; output drive capability is rated at IOH = –0.4 mA (high-state leakage) and IOL = 8 mA (low-state sink), supporting direct interface with LED displays via series current-limiting resistors.

Key Specifications

Pinout & Package

SN74LS42D is housed in a 16-pin SOIC (Small Outline Integrated Circuit) package with 1.27 mm pitch, 7.5 mm body width, and gull-wing leads. The package is RoHS-compliant, moisture sensitivity level 1, and rated for peak reflow at 260°C.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS42D, SN74LS145N offers greater output sink strength at the cost of speed, while 74HC42D enables ultra-low power and wider supply range but breaks TTL interoperability - neither is pin-compatible without circuit modification.

Availability

SN74LS42D is available at Aetrix Electronics and suitable for industrial panel displays, legacy computer address decoding, and electromechanical control logic requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS42D 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 solutions with broad industrial and automotive reach.

The SN74LS42D belongs to TI's legacy TTL logic family, designed specifically for reliable, deterministic decoding in fixed-function digital systems where predictability and interoperability with 1970s–1990s hardware ecosystems are essential.

FAQ

What is the function of SN74LS42D in a digital system?

The SN74LS42D functions as a 4-line BCD-to-10-line decimal decoder, translating binary-coded decimal inputs (0000–1001) into one of ten active-low output lines (Y0–Y9). It is used in address selection, display digit enabling, and discrete control signal generation. Its open-collector outputs require external pull-up resistors, and it operates exclusively within the TTL logic voltage domain. SN74LS42D does not decode invalid BCD codes (1010–1111), ensuring safe operation in manual-input systems.

Does SN74LS42D support 5 V-only operation?

Yes, SN74LS42D is specified for operation only within 4.75 V to 5.25 V - it is not tolerant of 3.3 V or wider supply ranges. Operation outside this window may cause incorrect decoding, increased power dissipation, or permanent damage. The device draws approximately 28 mA typical supply current at 5 V, and requires a local 0.1 µF ceramic bypass capacitor at the VCC pin. SN74LS42D has no internal regulation or multi-supply capability.

Can SN74LS42D drive LEDs directly?

Yes, SN74LS42D can drive LEDs directly via its active-low open-collector outputs, provided appropriate series current-limiting resistors are used. Each output sinks up to 8 mA continuously, sufficient for standard indicator LEDs at ~2 mA. Since outputs are active-low, LEDs must be connected anode-to-VCC and cathode-to-output. SN74LS42D does not provide internal pull-ups or current regulation - external resistor calculation is mandatory based on forward voltage and desired current.

Is SN74LS42D pin-compatible with SN74LS42N?

Yes, SN74LS42D (SOIC-16) and SN74LS42N (PDIP-16) share identical pin numbering, signal assignment, and electrical functionality - they differ only in package type and thermal/mounting characteristics. Both use the same 16-pin layout with VCC at pin 16 and GND at pin 8. SN74LS42D is not pin-compatible with non-16-pin variants or other decoder families such as SN74LS138. Always verify PCB footprint before substitution.

What happens when invalid BCD codes are applied to SN74LS42D?

When invalid BCD inputs (1010 through 1111) are applied to SN74LS42D, all ten outputs (Y0–Y9) remain inactive (high-impedance/open-circuit), producing no asserted low signal. This behavior prevents erroneous activation of downstream devices during switch bounce, encoder misalignment, or data corruption. SN74LS42D does not assert any output for these codes - it is not a 4-to-16 line decoder. This feature is intrinsic to the SN74LS42D logic design and requires no external logic.