Texas Instruments SN74LS640DBR

Overview

SN74LS640DBR 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 V_CC = 5 V with typical propagation delay of 15 ns (t_PLH/t_PHL). It is used in legacy industrial backplane interfaces and microprocessor system expansion buses.

For engineers reviewing the SN74LS640DBR datasheet, SN74LS640DBR pinout, SN74LS640DBR application, or SN74LS640DBR equivalent, key selection criteria include its 3-state bus-hold capability, guaranteed DC drive strength (I_OH = –15 mA / I_OL = 24 mA), compatibility with standard LS-TTL logic families, and SOIC-20 package thermal and layout constraints.

Technical Context

The SN74LS640DBR implements dual-function control logic: DIR selects data flow direction (A-to-B or B-to-A), while OE independently disables both A- and B-side outputs into high-impedance state. Its internal architecture uses bipolar Schottky-clamped TTL circuitry, ensuring noise immunity and stable switching across 0°C to 70°C ambient operation.

All eight channels operate synchronously under shared DIR/OE control; no channel-to-channel skew is specified, and output leakage current remains ≤ ±5 µA in 3-state mode. Input thresholds align with standard LS-TTL (V_IL ≤ 0.8 V, V_IH ≥ 2.0 V), enabling direct interfacing with 74LS, 74S, and 74F series devices without level translation.

Key Specifications

Pinout & Package

SN74LS640DBR is housed in a 20-pin SOIC (DW) package, 7.5 mm wide, with 1.27 mm pitch and 2.65 mm max height. Pin 1 is marked by a beveled corner or dot; device orientation follows JEDEC MS-013 standard.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS245N and SN74HC245NSR, the SN74LS640DBR offers independent DIR/OE control in a surface-mount SOIC package with LS-TTL–matched timing and drive - making it uniquely suitable for space-constrained upgrades of legacy 74LS245-based systems where control signal routing flexibility is critical.

Availability

SN74LS640DBR is available at Aetrix Electronics and suitable for industrial backplane interfaces, microprocessor system expansion, legacy test equipment buses, and programmable logic controller I/O modules requiring stable component supply across long-lifecycle programs.

Supply support for SN74LS640DBR 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.

The SN74LS640DBR belongs to TI's legacy 74LS logic family, engineered specifically for robust, low-skew bidirectional data transfer in TTL-based computing and control systems deployed from the 1980s onward.

FAQ

What is the function of the DIR pin on the SN74LS640DBR?

The DIR (Direction) pin on the SN74LS640DBR is a level-sensitive control input that determines data flow direction: when DIR = HIGH, data passes from Port A to Port B; when DIR = LOW, data flows from Port B to Port A. It operates independently of the OE (Output Enable) signal, allowing direction control even while outputs remain disabled. This behavior is confirmed in the SDLS189 datasheet Section 6.1 for SN74LS640DBR.

Can SN74LS640DBR replace SN74LS245 in an existing design?

SN74LS640DBR can replace SN74LS245 only with minor schematic and layout modifications: the SN74LS245 uses a combined direction/output-enable scheme (pin 1 = DIR, pin 19 = /OE), whereas SN74LS640DBR separates these functions onto pins 1 (DIR) and 19 (OE). Direct substitution requires rewiring the DIR signal and verifying timing margins. The SN74LS640DBR pinout differs from SN74LS245 despite both being 20-pin SOIC devices.

What is the maximum clock frequency supported by SN74LS640DBR in a synchronous bus application?

The SN74LS640DBR does not contain internal clock circuitry and is not a clocked device; it is a combinatorial transceiver. Its usable bus frequency depends on total round-trip propagation delay (≤30 ns worst-case A↔B), setup/hold times of connected devices, and PCB trace length. In practice, it supports reliable operation up to ~25 MHz in well-terminated 8-bit parallel buses, as validated in TI's SDLS189 application examples for ISA-like architectures using SN74LS640DBR.

Is SN74LS640DBR RoHS compliant and lead-free solder compatible?

Yes, SN74LS640DBR is RoHS compliant (per TI's Package Option Addendum) and rated for lead-free reflow with MSL Level-1 (unlimited floor life) and peak reflow temperature of 260°C. Its terminal finish is NiPdAu (NIPDAU), fully compatible with standard Pb-free solder paste profiles and Type III assembly processes. This compliance applies specifically to the DW package variant identified as SN74LS640DBR.

Does SN74LS640DBR support live insertion or hot-swap operation?

SN74LS640DBR supports controlled hot-swap operation when OE is held HIGH (outputs disabled) during insertion, preventing bus contention. However, it lacks built-in bus-fault protection, current limiting, or ESD hardening beyond standard LS-TTL specs (±2 kV HBM). For robust hot-swap use, external series resistors (22–47 Ω) and TVS diodes on A/B lines are recommended - a practice documented in TI's application guidance for SN74LS640DBR in backplane systems.