Texas Instruments SN74LS640N3

Overview

SN74LS640N from Texas Instruments is an octal bus transceiver with 3-state outputs, designed for bidirectional data flow control in TTL-level parallel bus systems. It features 8-bit non-inverting data paths, independent output-enable (OE) and direction (DIR) controls, and operates at 0°C to 70°C with typical propagation delay of 15 ns and maximum supply current of 48 mA.

For engineers reviewing the SN74LS640N datasheet, SN74LS640N pinout, SN74LS640N application, or SN74LS640N equivalent, key selection considerations include its 20-pin PDIP package, TTL-compatible input thresholds, 3-state bus isolation capability, and compatibility with legacy LS logic families in industrial control backplanes and retro-computing interfaces.

Technical Context

The SN74LS640N implements dual 8-bit bidirectional buffers using standard TTL circuitry, where DIR selects data flow direction (A-to-B or B-to-A) and OE independently disables both A- and B-side outputs into high-impedance state. Its internal structure avoids bus contention by ensuring only one side drives at any time.

It conforms to the SN74LS640 family specification SDLS189 (Rev. March 1988), supports DC-coupled TTL signaling, and requires no external biasing or termination for standard 5 V bus operation. Input clamp diodes and output short-circuit protection are integrated per TI's LS process design rules.

Key Specifications

Pinout & Package

SN74LS640N uses a 20-pin plastic dual in-line package (PDIP-N) with 0.3-inch body width and standard 0.1-inch pin pitch. Pin 1 is marked by a notch or dot; pin numbering proceeds counter-clockwise from top-left corner.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS245N and SN74ALS640N, the SN74LS640N offers the most direct implementation of DIR-controlled bidirectional flow in legacy TTL systems, balancing proven reliability, wide temperature margin, and minimal control signal overhead - making it optimal for maintenance of installed base equipment and backward-compatible upgrades.

Availability

SN74LS640N is available at Aetrix Electronics and suitable for industrial backplane interfaces, retrocomputing bus bridges, and legacy test equipment data links requiring stable component supply and long-term obsolescence management.

Supply support for SN74LS640N 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, automotive, and aerospace qualification coverage.

The SN74LS640N belongs to TI's legacy 74LS logic family, engineered for robustness in noisy environments and designed specifically for TTL-compatible bus interfacing in control systems and computing infrastructure from the 1970s–1990s era.

FAQ

What is the maximum clock rate supported by SN74LS640N in a synchronous bus application?

The SN74LS640N does not include an internal clock; it is an asynchronous transceiver. Its 15 ns maximum propagation delay supports reliable operation in systems with bus cycle times ≥30 ns - corresponding to effective data rates up to ~33 MHz in tightly timed control protocols. For SN74LS640N-based designs, timing margins must account for worst-case trace delays and loading, and the device itself imposes no inherent frequency ceiling beyond its specified tPD and setup/hold requirements.

Can SN74LS640N be used with 3.3 V logic systems?

No - SN74LS640N is strictly a 5 V TTL device. Its input thresholds (VIH = 2.0 V min, VIL = 0.8 V max) and output levels (VOH = 2.7 V min at 400 µA, VOL = 0.5 V max at 8 mA) are defined for 5 V operation. Direct connection to 3.3 V logic risks undriven inputs, marginal noise margins, and potential damage from output voltage overshoot. Use level translators like SN74LVC4245 or discrete resistor dividers when interfacing SN74LS640N with 3.3 V systems.

Does SN74LS640N support hot-swap or live-insertion into an active bus?

SN74LS640N is not hot-swap rated. While its 3-state outputs prevent contention during normal OE-controlled disable, the device lacks bus-fault protection, slew-rate control, or power-up sequencing features required for safe live insertion. Applying power to SN74LS640N while its I/O pins are biased by an active bus may cause latch-up or excessive current draw. Always power-cycle the subsystem before inserting or removing SN74LS640N in operational environments.

How does the DIR pin behavior differ between SN74LS640N and SN74LS245N?

SN74LS640N uses a single DIR pin to select direction: DIR = HIGH enables A→B flow; DIR = LOW enables B→A flow. In contrast, SN74LS245N has two independent enables - pin 1 (A→B EN) and pin 19 (B→A EN) - requiring external logic to ensure only one is active. This makes SN74LS640N simpler to control with microcontroller GPIO but incompatible with SN74LS245N's pinout and control architecture without redesign.

Is there a RoHS-compliant version of SN74LS640N available?

Yes - SN74LS640N.A and SN74LS640N are both RoHS-compliant (lead-free NiPdAu finish), as confirmed in TI's Packaging Addendum. They replace legacy SNPB (tin-lead) versions and meet JEDEC J-STD-020 moisture sensitivity Level-1 requirements. No reflow profile changes are needed versus standard SnPb processes, and the devices maintain identical electrical specifications and 0°C to +70°C operating range.