Texas Instruments SN74LS440DW

Overview

SN74LS440 from Texas Instruments is a quadruple tridirectional bus transceiver in a 24-pin SOIC package, operating at 4.75–5.25 V supply, with typical propagation delay of 18 ns and output drive capability of ±8 mA, used for bidirectional data routing between TTL-compatible buses in legacy industrial control backplanes.

For engineers reviewing the SN74LS440 datasheet, SN74LS440 pinout, SN74LS440 application, or SN74LS440 equivalent, key selection considerations include tridirectional control logic (1A/1B/DIR), TTL-level compatibility, 24-pin SOIC mechanical fit, and guaranteed 18 ns max propagation delay across commercial temperature range.

Technical Context

The SN74LS440 implements four independent tridirectional data paths, each controlled by a dedicated direction input (DIR) and enabled via separate active-low output-enable (OE) signals per port pair. It uses standard TTL bipolar technology with Schottky-clamped inputs and outputs, supporting true three-state operation on both A- and B-side buses.

Each transceiver channel supports simultaneous bidirectional flow under DIR control: when DIR = LOW, data flows A→B; when DIR = HIGH, data flows B→A. All outputs are fully compatible with LS-TTL input thresholds and drive requirements, and feature internal clamping diodes for transient protection.

Key Specifications

Pinout & Package

SN74LS440 is housed in a 24-pin small-outline integrated circuit (SOIC) package with 300-mil body width and standard 1.27-mm pitch. Pin numbering follows JEDEC MS-013, with Pin 1 marked by notch or dot.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar tridirectional bus transceiver applications.

Compared with SN74LS245N and 74ACT125, the SN74LS440 uniquely integrates tridirectional control per channel in a single 24-pin package, eliminating external logic and reducing interconnect complexity - essential for space-constrained legacy backplane upgrades where timing and pin count are tightly coupled.

Availability

SN74LS440 is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy test equipment refurbishment, and embedded instrumentation subsystem bridging requiring stable component supply and long-term obsolescence mitigation.

Supply support for SN74LS440 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 U.S.-based semiconductor company founded in 1930, specializing in analog, embedded processing, and logic ICs with broad industrial and aerospace heritage.

The SN74LS440 belongs to TI's classic 74LS logic family, designed specifically for reliable, low-power, TTL-compatible bus interfacing in mission-critical industrial and military electronic systems of the 1980s–1990s era.

FAQ

What is the function of the DIR pin on the SN74LS440?

The DIR (Direction) pin on the SN74LS440 controls data flow direction for all four tridirectional channels simultaneously: when DIR is LOW, data transfers from A-side to B-side; when DIR is HIGH, data flows from B-side to A-side. This eliminates the need for external direction logic and simplifies bus arbitration in systems like industrial backplanes where dynamic direction switching is required. The SN74LS440 relies on this single pin to coordinate bidirectional behavior across its entire 24-pin interface.

Does the SN74LS440 support hot-swap or live-insertion?

No, the SN74LS440 does not support hot-swap or live-insertion. Its TTL bipolar design lacks bus-hold, power-on reset, or I_OFF protection features required for safe insertion into powered systems. Applying V_CC before establishing proper GND connection may cause latch-up or parametric shift. For SN74LS440 integration, power sequencing must follow GND-first, then V_CC, and signal lines must be inactive until supply stabilization - consistent with 1980s-era backplane design practices.

What is the maximum clock rate supported by the SN74LS440 in a synchronous bus system?

The SN74LS440 supports reliable operation in synchronous bus systems up to 20 MHz, derived from its 18 ns maximum propagation delay and 5 ns minimum pulse width specification. At 20 MHz (50 ns period), it provides sufficient margin for setup/hold timing with standard LS-TTL controllers. While the SN74LS440 itself has no internal clock, its timing parameters directly constrain achievable bus throughput - making it suitable for legacy 16-bit microprocessor buses such as those used with the Z80 or 8086 in systems where the SN74LS440 serves as the primary data path manager.

Can the SN74LS440 replace the SN74LS245 in an existing design?

No, the SN74LS440 cannot directly replace the SN74LS245 because they differ fundamentally in architecture: the SN74LS245 is octal unidirectional with separate A→B and B→A paths, while the SN74LS440 is quadruple tridirectional with shared A/B pins and a global DIR input. Replacing SN74LS245 with SN74LS440 would require redesigning PCB traces, updating control logic to drive DIR, and verifying timing under bidirectional switching - meaning the SN74LS440 is not a drop-in substitute but a functional alternative requiring system-level validation.

Is the SN74LS440 RoHS compliant?

The original SN74LS440 was manufactured prior to RoHS regulation and contains leaded solder and leaded die attach materials. Texas Instruments does not offer a RoHS-compliant version of the SN74LS440. For new designs requiring RoHS compliance, consider modern alternatives such as the SN74LVC245A with appropriate level-shifting and timing analysis. Legacy SN74LS440 units supplied by Aetrix Electronics are sourced from verified obsolete-component channels and retain original specifications - the SN74LS440 remains unchanged in form, fit, and function from its 1988 production release.