Texas Instruments SN74LS240NG4

Overview

SN74LS240NG4 from Texas Instruments is an octal inverting buffer/line driver with dual 4-bit independent 3-state outputs, designed for memory address and bus driving applications. It features 15 ns maximum propagation delay at 5 V, ±24 mA output drive capability, PNP input structure reducing DC loading, and hysteresis-enhanced noise margins. It operates in 0°C to 70°C ambient temperature and drives terminated lines down to 133 Ω.

For engineers reviewing the SN74LS240NG4 datasheet, SN74LS240NG4 pinout, SN74LS240NG4 application, or SN74LS240NG4 equivalent, this device is selected for high-density bus buffering where low-power TTL compatibility, fast enable/disable timing (tPZH/tPZL ≤23 ns), and robust noise immunity in legacy industrial control and memory subsystems are required.

Technical Context

The SN74LS240NG4 implements two independent 4-bit inverting buffer sections, each controlled by a dedicated active-low output-enable (G) input (Pin 1 for Channel 1, Pin 19 for Channel 2). Its PNP input stage reduces input current draw, enabling higher fan-in on shared buses. Hysteresis (0.2–0.4 V) at all inputs improves noise rejection in electrically noisy environments such as motor drive backplanes or telecom line cards.

It operates strictly at 5 V nominal supply (4.75–5.25 V), supports 3.3-V logic-level inputs due to 2 V VIH tolerance, and delivers true 3-state outputs capable of driving high-capacitance bus loads up to 45 pF with RL = 667 Ω. Output disable timing (tPZH/tPZL) and enable timing (tPLZ/tPHZ) are fully characterized per JEDEC standards.

Key Specifications

Pinout & Package

SN74LS240NG4 is packaged in a 20-pin plastic DIP (PDIP-N) with 24.33 mm × 6.35 mm body size and 2.54 mm lead pitch.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS244N, SN74LS240NG4 provides inverted outputs essential for address decoding pipelines; compared with SN74HC240N, it delivers superior noise margin and legacy TTL compatibility at the cost of higher static current and fixed 5 V operation.

Availability

SN74LS240NG4 is available at Aetrix Electronics and suitable for memory subsystems, industrial I/O expansion, LED display drivers, and telecom line card interfaces requiring stable component supply over extended production lifecycles.

Supply support for SN74LS240NG4 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 headquartered in Dallas, Texas, specializing in analog, embedded processing, and logic solutions for industrial, automotive, and communications markets.

The SN74LS240NG4 belongs to TI's legacy 74LS logic family, engineered for high-reliability bus interface applications in systems where compatibility with decades-old TTL designs and robust noise performance are mandatory.

FAQ

What is the function of Pin 19 on the SN74LS240NG4?

Pin 19 is the active-low output-enable input (2G) for Channel 2. When driven LOW, it enables the four inverted outputs (2Y1–2Y4) on Pins 9, 7, 5, and 3. When HIGH, those outputs enter high-impedance state. This pin must be pulled HIGH via a resistor during power-up to prevent bus contention, as specified in TI's SDLS144D datasheet for SN74LS240NG4.

Can SN74LS240NG4 interface directly with 3.3-V microcontrollers?

Yes, SN74LS240NG4 accepts 3.3-V logic inputs reliably: its VIH minimum is 2.0 V, well below 3.3 V logic HIGH levels. However, its outputs swing to ~3.4 V (VOH) at 5 V supply, so interfacing with 3.3-V receivers requires verification of input tolerance - SN74LS240NG4 itself does not perform level translation in the reverse direction.

What is the maximum capacitive load SN74LS240NG4 can drive while maintaining 15 ns propagation delay?

Per TI's SDLS144D datasheet, SN74LS240NG4 maintains ≤15 ns tPLH/tPHL with CL = 45 pF and RL = 667 Ω at VCC = 5 V and TA = 25°C. Driving >45 pF increases delay nonlinearly; for 100 pF loads, typical tpd exceeds 22 ns. Layout best practice is to limit trace capacitance and use local 0.1 µF decoupling.

Does SN74LS240NG4 have built-in ESD protection?

Yes, SN74LS240NG4 is rated for 500 V HBM (Human Body Model) per ANSI/ESDA/JEDEC JS-001. This meets standard handling requirements for automated assembly and bench testing. No external protection diodes are needed for typical PCB handling, though system-level surge protection remains the designer's responsibility.

How does the PNP input structure benefit SN74LS240NG4 in bus applications?

The PNP input structure in SN74LS240NG4 reduces input current draw (IIL ≤ –0.2 mA) compared to standard TTL, lowering DC loading on shared buses. This allows more devices to be connected without violating fan-out limits and reduces voltage droop on weak drivers - a key advantage in dense memory address or control bus implementations using SN74LS240NG4.