onsemi SN74LS240N

Overview

SN74LS240N from Texas Instruments is an octal inverting buffer/line driver with 3-state outputs, designed for memory address and bus driving applications in TTL-based systems. It features dual 4-bit channels (1A–1Y and 2A–2Y), active-low output-enable inputs (1G, 2G/2G), 24 mA low-level output drive, 5 V supply operation, and 14 ns typical propagation delay. It is commonly used to isolate and strengthen address/data bus signals in industrial control and legacy computing systems.

For engineers reviewing the SN74LS240N datasheet, pinout, applications, or equivalent options, this page delivers verified electrical specs, functional pin mapping, real-world use cases in bus buffering and memory interfacing, and validated alternative parts - all grounded in TI's SDLS144D production data sheet (Rev. D, October 2016).

Technical Context

The SN74LS240N implements two independent 4-bit inverting buffer sections, each controlled by a dedicated active-low output-enable input (1G and 2G). Its PNP-input structure reduces DC loading on upstream logic, while built-in input hysteresis improves noise immunity on shared bus lines. All outputs are 3-state capable, enabling direct connection to bidirectional or multipoint buses without external isolation.

It operates strictly within 4.75 V to 5.25 V at 0 °C to 70 °C, with guaranteed VOL ≤ 0.5 V at IOL = 24 mA and VOH ≥ 2.4 V at IOH = –3 mA. The device drives terminated lines down to 133 Ω and supports mixed-voltage interfacing - inputs tolerate 2 V minimum VIH and are compatible with 3.3-V logic sources.

Key Specifications

Pinout & Package

SN74LS240N uses a 20-pin Plastic Dual In-line Package (PDIP-N), body size 24.33 mm × 6.35 mm, with through-hole mounting and industry-standard pin spacing (0.1 inch).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal inverting buffer applications.

Compared with SN74LS244N, SN74LS240N provides essential signal inversion for address decoding; compared with SN74HC240N, it delivers superior noise immunity and bus drive in legacy industrial hardware - making it irreplaceable where LS-TTL compatibility and robustness are mandatory.

Availability

SN74LS240N is available at Aetrix Electronics and suitable for industrial control systems, legacy computing upgrades, and LED display manufacturing requiring stable component supply over extended product lifecycles.

Supply support for SN74LS240N 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 global semiconductor leader founded in 1930, specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74LS240N belongs to TI's legacy LS-TTL logic family, engineered specifically for high-reliability bus interfacing, memory addressing, and signal restoration in 5-V systems where noise immunity and drive strength outweigh power efficiency.

FAQ

What is the function of the 2G/2G pin on the SN74LS240N?

The 2G/2G pin (Pin 19) is the active-low output-enable input for Channel 2 of the SN74LS240N. When driven LOW, it enables the four 2A-to-2Y inverting outputs (2Y1–2Y4); when HIGH, those outputs enter high-impedance state. This pin is labeled "2G/2G" in TI's datasheet to indicate its dual symbolic representation in logic diagrams - it is a single physical terminal, not a differential pair. Its behavior is identical to Pin 1 (1G) for Channel 1.

Can SN74LS240N interface directly with 3.3-V microcontrollers?

Yes, the SN74LS240N can accept 3.3-V logic inputs reliably. Its minimum VIH is specified at 2.0 V under recommended operating conditions, and its inputs tolerate down to 2 V while maintaining full TTL noise margin. This allows direct connection from 3.3-V MCU GPIOs without level shifters - though the SN74LS240N itself requires a 5-V supply (VCC = 4.75–5.25 V) to operate correctly and deliver its rated 24 mA output drive.

What is the maximum capacitive load the SN74LS240N can drive while maintaining timing specs?

The SN74LS240N's switching characteristics (tPLH/tPHL) are characterized at CL = 45 pF with RL = 667 Ω. While it can drive higher capacitance, timing degrades linearly: at CL = 100 pF, propagation delay increases to ~25 ns. For reliable operation within datasheet limits, keep total load capacitance ≤45 pF per output - achieved via short traces (<5 cm), minimal stubs, and proper termination. Exceeding this requires derating timing margins in system-level validation.

Does SN74LS240N have built-in ESD protection, and what are its ratings?

Yes, the SN74LS240N meets JEDEC standards for ESD robustness: it is rated at 500 V HBM (Human Body Model) across all packages, and 500 V CDM (Charged Device Model) specifically for the N-package variant. These ratings reflect manufacturing-level protection - end-equipment design must still include board-level safeguards (e.g., series resistors, TVS diodes) for interfaces exposed to user handling or cable connections.

How does the PNP input structure of SN74LS240N reduce DC loading?

The PNP input stage draws negligible DC current when driven HIGH - typically ≤20 µA IIH - and sinks only –0.2 mA IIL when driven LOW. This contrasts with standard TTL inputs that sink ~1.6 mA. As a result, the SN74LS240N places far less burden on upstream drivers (e.g., microprocessor address latches), preventing voltage droop and enabling higher fan-out without bus buffers. This is explicitly leveraged in memory address bus applications where multiple SN74LS240N inputs connect to one source.