Texas Instruments SN74LS240NE4

Overview

SN74LS240NE4 from Texas Instruments is an octal inverting buffer/line driver with 3-state outputs, designed for memory address and bus driving applications. It features dual 4-bit channels, active-low output-enable (1G and 2G), 5 V operation, 14 ns typical propagation delay, and PNP input structure reducing DC loading on buses.

For engineers reviewing the SN74LS240NE4 datasheet, SN74LS240NE4 pinout, SN74LS240NE4 application, or SN74LS240NE4 equivalent, key selection criteria include 3-state bus isolation capability, TTL-compatible input thresholds, hysteresis-enhanced noise immunity, and compatibility with terminated lines down to 133 Ω in high-density PCB layouts.

Technical Context

The SN74LS240NE4 implements two independent 4-bit inverting buffer sections, each controlled by a dedicated active-low output-enable input (1G and 2G). Its PNP input stage minimizes current draw from upstream logic, while built-in input hysteresis provides 400 mV noise margin-critical for reliable operation in electrically noisy server and telecom backplanes.

Each channel drives one set of four inverted outputs (1Y1–1Y4 and 2Y1–2Y4) from corresponding A-side inputs (1A1–1A4 and 2A1–2A4). When either G input is high, its associated Y outputs enter high-impedance state, enabling bidirectional bus sharing without contention.

Key Specifications

Pinout & Package

SN74LS240NE4 is supplied in a 20-pin plastic dual in-line package (PDIP-N), with 2.54 mm lead pitch and body size 24.33 mm × 6.35 mm.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS244N, SN74LS240NE4 provides inverted outputs essential for address decoding; compared with SN74ALS240N, it offers proven legacy reliability at slightly higher power and latency-ideal for cost-sensitive industrial retrofits.

Availability

SN74LS240NE4 is available at Aetrix Electronics and suitable for memory address buffering, LED display multiplexing, telecom backplane interface, and industrial I/O expansion requiring stable component supply across extended product lifecycles.

Supply support for SN74LS240NE4 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 delivering analog and embedded processing solutions for industrial, automotive, and communications markets.

The SN74LS240NE4 belongs to TI's legacy LS-TTL logic family, engineered specifically for high-reliability bus interfacing in memory systems, instrumentation, and industrial control where deterministic timing and noise immunity are critical.

FAQ

What is the function of the 1G and 2G pins on the SN74LS240NE4?

The 1G and 2G pins are active-low output-enable inputs controlling Channels 1 and 2 independently. When 1G is low, inputs 1A1–1A4 pass through as inverted outputs 1Y4–1Y1; when high, those outputs go high-impedance. The same applies to 2G and Channel 2. This enables selective bus arbitration without external logic. SN74LS240NE4 requires both enables to be asserted for full 8-bit operation.

Can SN74LS240NE4 interface with 3.3-V logic inputs?

Yes. SN74LS240NE4 inputs tolerate down to 2 V and meet VIH ≥ 2 V and VIL ≤ 0.8 V specifications, making them compatible with 3.3-V CMOS outputs. No level-shifting circuitry is required-this feature simplifies mixed-voltage designs such as upgrading legacy 5-V systems with modern 3.3-V peripherals. SN74LS240NE4 maintains full TTL output drive regardless of input voltage level.

What is the maximum capacitive load SN74LS240NE4 can drive reliably?

SN74LS240NE4 is characterized for CL = 45 pF in switching tests (tPLH/tPHL = 14 ns typical), but real-world performance depends on trace impedance and termination. With proper 133 Ω series termination, it reliably drives >100 pF distributed loads on 15 cm FR-4 traces. Exceeding 60 pF un-terminated may cause ringing due to fast edges; always use local 0.1 µF VCC decoupling and minimize stub lengths. SN74LS240NE4 datasheet specifies CL = 45 pF as the test condition-not an absolute limit.

Does SN74LS240NE4 have built-in ESD protection?

Yes. SN74LS240NE4 meets JEDEC JS-001 Human-Body Model (HBM) specification with 500 V ESD rating, verified per industry-standard testing. This level of protection supports safe handling in standard manufacturing environments without requiring additional external TVS devices for board-level ESD immunity. However, SN74LS240NE4 does not specify Charged-Device Model (CDM) rating in its official datasheet, so CDM-sensitive assembly processes should follow TI's recommended handling guidelines.

How does the PNP input structure benefit SN74LS240NE4 in bus applications?

The PNP input stage reduces input current (IIL ≤ –0.2 mA) compared to standard TTL, lowering DC loading on upstream drivers and increasing fan-out capability-up to 20 LS-TTL loads per input. This minimizes voltage droop on shared buses and improves signal integrity in daisy-chained configurations. SN74LS240NE4 leverages this to maintain stable logic thresholds even when driving multiple receivers, a key advantage in memory address distribution networks.