Texas Instruments SN74LS240DW

Overview

SN74LS240DW 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 with independent active-low output-enable (1G, 2G/2G) control, 15 ns maximum propagation delay at 5 V, ±24 mA output drive capability, and hysteresis-enhanced noise immunity. It is used in server memory subsystems to isolate and strengthen address signals between CPU and DRAM modules.

For engineers reviewing the SN74LS240DW datasheet, SN74LS240DW pinout, SN74LS240DW application, or SN74LS240DW equivalent, key selection considerations include its 3-state bus-driving capability, LS-family logic compatibility, dual-channel enable control, DC loading reduction via PNP inputs, and thermal performance in SOIC-20 packaging.

Technical Context

The SN74LS240DW implements two independent 4-bit inverting buffers, each with separate active-low output-enable inputs (1G on Pin 1, 2G/2G on Pin 19). Its PNP input structure reduces DC loading on upstream drivers, while built-in input hysteresis provides 400-mV noise margin. Outputs are fully 3-state capable, enabling direct connection to shared buses without contention when disabled.

It operates within a 4.75–5.25 V supply range and supports TTL-level input thresholds (VIH = 2 V, VIL = 0.8 V). The device drives terminated lines down to 133 Ω and delivers VOL ≤ 0.5 V at IOL = 24 mA, VOH ≥ 2.4 V at IOH = –3 mA, and exhibits tPLH/tPHL ≤ 18 ns under standard load conditions (RL = 667 Ω, CL = 45 pF).

Key Specifications

Pinout & Package

SN74LS240DW is housed in a 20-pin SOIC (DW) package measuring 12.80 mm × 7.50 mm, optimized for surface-mount assembly and moderate-power bus applications.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS244DW and SN74ALS240N, the SN74LS240DW provides deterministic inversion for address latching while maintaining industry-standard LS drive and timing - making it optimal for legacy memory interface designs where polarity and compatibility are prioritized over marginal speed gains.

Availability

SN74LS240DW is available at Aetrix Electronics and suitable for memory address buffering, server backplane signal conditioning, and industrial LED display driving requiring stable component supply across extended production lifecycles.

Supply support for SN74LS240DW 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, with over 90 years of innovation in logic, power, and signal chain technologies.

The SN74LS240DW belongs to TI's legacy 74LS logic family, engineered specifically for high-reliability, medium-speed bus interfacing in industrial control, computing, and communications infrastructure where TTL compatibility and proven robustness are critical.

FAQ

What is the function of Pin 19 (2G/2G) on the SN74LS240DW?

Pin 19 is the active-low output-enable input for Channel 2. When driven LOW, it enables inversion and driving of inputs 2A1–2A4 to outputs 2Y1–2Y4. When HIGH, all Channel 2 outputs enter high-impedance state. This pin is labeled "2G/2G" in the datasheet to indicate its dedicated role for the second 4-bit channel, distinct from Pin 1 (1G) for Channel 1. The SN74LS240DW relies on this dual-enable architecture for independent bus segment control.

Can the SN74LS240DW interface directly with 3.3-V microcontrollers?

Yes, the SN74LS240DW accepts 3.3-V logic inputs reliably due to its input tolerance down to 2 V and VIH specification of 2 V at 5-V supply. Its PNP input structure ensures compatibility without external level shifters. However, its outputs swing to 0 V / ~3.4 V (not full 0–3.3 V), so downstream 3.3-V receivers must tolerate TTL-high levels. The SN74LS240DW remains functional in mixed-voltage systems where signal direction is unidirectional from 3.3-V controller to 5-V bus.

What is the maximum continuous output current per pin for the SN74LS240DW?

The SN74LS240DW supports a maximum low-level output current (IOL) of 24 mA per pin under recommended operating conditions (VCC = 4.75–5.25 V, TA = 0–70°C). Exceeding this value risks exceeding absolute maximum ratings and may cause VOL degradation or thermal overstress. The datasheet specifies that no more than one output should be shorted at a time, and short-circuit duration must not exceed one second. This 24-mA rating defines the SN74LS240DW's ability to drive multiple TTL loads or terminate moderate-length traces.

Does the SN74LS240DW require external pull-up resistors on its enable pins?

Yes - to ensure outputs remain in high-impedance state during power-up or power-down sequences, both 1G (Pin 1) and 2G/2G (Pin 19) must be tied to VCC through pull-up resistors. The minimum resistor value is determined by the current-sinking capability of the driving source; TI recommends values between 1 kΩ and 10 kΩ for robust noise immunity and fast enable release. Leaving these pins floating risks undefined output states and potential bus contention, a critical design consideration for the SN74LS240DW in live-insertion or hot-swap systems.

How does the hysteresis in the SN74LS240DW improve system reliability?

The SN74LS240DW incorporates built-in input hysteresis (VT+ − VT− = 0.2–0.4 V), which creates distinct switching thresholds for rising and falling edges. This prevents oscillation or chatter on slow-rising signals - common in long PCB traces or unterminated buses - by requiring a larger voltage swing to toggle state. As a result, the SN74LS240DW maintains clean logic transitions in electrically noisy environments like industrial control cabinets or densely routed server motherboards, directly improving timing margin and reducing intermittent faults.