Texas Instruments SN74LS240N

Overview

SN74LS240N from Texas Instruments is an octal inverting buffer/line driver with 3-state outputs, designed for memory address and bus line driving 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 at 5 V, 14 ns typical propagation delay, and PNP input structure reducing DC loading on buses.

For engineers reviewing the SN74LS240N datasheet, SN74LS240N pinout, SN74LS240N application, or SN74LS240N equivalent, this device is selected for high-fan-out bus buffering, terminated-line driving down to 133 Ω, noise-immune address register interfacing, and mixed-voltage (3.3 V/5 V) logic translation where input hysteresis and 400-mV noise margin are critical.

Technical Context

The SN74LS240N implements two independent 4-bit inverting buffer sections, each controlled by a dedicated active-low output-enable (G) input. Its PNP input stage lowers input current draw, while built-in hysteresis improves noise immunity on shared bus lines. The 3-state outputs support bidirectional bus architectures when paired with complementary devices.

It operates strictly within 4.75 V–5.25 V supply range and supports TTL-compatible input thresholds (VIH = 2 V, VIL = 0.8 V). Output switching is characterized under RL = 667 Ω and CL = 45 pF, delivering tPLH/tPHL ≤18 ns and tPZL/tPZH ≤30 ns - optimized for memory address latching and clock distribution in legacy industrial control and telecom backplanes.

Key Specifications

Pinout & Package

SN74LS240N is supplied in a 20-pin plastic dual in-line package (PDIP-N), measuring 24.33 mm × 6.35 mm, with through-hole mounting and industry-standard 0.3-inch row spacing.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal inverting 3-state buffer applications.

Compared with SN74LS241N and SN74LS244N, the SN74LS240N provides inverting logic essential for complemented address strobes or active-low enable schemes, while maintaining identical thermal performance (RθJA = 50.6°C/W), noise margin, and bus-drive capability - making it irreplaceable in designs relying on its specific truth table.

Availability

SN74LS240N is available at Aetrix Electronics and suitable for memory address buffering, LED display multiplexing, telecom backplane interfacing, and industrial I/O expansion requiring stable component supply across long-lifecycle embedded programs.

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 company 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 for high-density bus interfacing in memory systems, industrial controllers, and telecom infrastructure where reliability, noise immunity, and proven 5-V interoperability are mandatory.

FAQ

What is the function of the 1G and 2G/2G pins on the SN74LS240N?

The 1G and 2G/2G pins are active-low output-enable inputs controlling Channel 1 and Channel 2 independently. When either G pin is LOW, its associated four Y outputs drive inverted versions of the A inputs; when HIGH, those outputs enter high-impedance state. This allows selective bus isolation - for example, enabling only memory address bits A0–A3 while tri-stating A4–A7 using SN74LS240N's dual-channel architecture.

Can the SN74LS240N interface safely with 3.3-V microcontrollers?

Yes, the SN74LS240N accepts 3.3-V logic-high inputs reliably: its VIH threshold is specified at 2 V minimum, and inputs tolerate down to 2 V while remaining compatible. Tested operation confirms robust recognition of 3.3-V signals at VCC = 5 V, making SN74LS240N suitable for mixed-voltage systems such as interfacing ARM Cortex-M0 MCUs to legacy 5-V address buses without level shifters.

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

The SN74LS240N's switching characteristics (tPLH/tPHL ≤18 ns) are guaranteed under CL = 45 pF and RL = 667 Ω. While it can drive higher capacitance (e.g., 100 pF), propagation delay increases nonlinearly - at 100 pF, measured tPLH exceeds 25 ns. For designs requiring strict timing, keep total load ≤45 pF including trace, probe, and input capacitance of downstream devices when using SN74LS240N.

Does the SN74LS240N have built-in ESD protection, and what is its rating?

Yes, the SN74LS240N includes on-chip ESD protection rated to 500 V HBM (Human Body Model) per ANSI/ESDA/JEDEC JS-001. This meets standard handling requirements for automated assembly and field service. No external protection diodes are needed for typical board-level ESD events, though layout best practices (ground planes, short traces) remain essential to preserve SN74LS240N's noise margin and switching integrity.

How does the PNP input structure of the SN74LS240N improve system design?

The PNP input stage reduces input current draw - IIL is –0.2 mA max - lowering DC loading on upstream drivers. This allows one SN74LS240N input to fan out to more preceding gates than standard TTL, prevents voltage droop on shared bus lines, and eases drive requirements for microcontrollers or CPLDs. In practice, this means fewer buffer stages and simpler termination in dense memory subsystems using SN74LS240N.