Texas Instruments SN74LS645DW

Overview

SN74LS645DW from Texas Instruments is an octal bus transceiver with 3-state outputs, designed for bidirectional data flow control between two 8-bit buses in TTL-level systems. It features independent output-enable (OE) and direction (DIR) controls, operates at 0°C to 70°C, supports 5 V supply, and delivers typical propagation delay of 15 ns (t_PLH/t_PHL) under standard loading.

For engineers reviewing the SN74LS645DW datasheet, SN74LS645DW pinout, SN74LS645DW application, or SN74LS645DW equivalent, this page provides verified functional identity, confirmed SOIC-20 package mapping, validated 3-state bidirectional bus interface behavior, and real-world substitution guidance aligned with TI's SDLS189 specification.

Technical Context

The SN74LS645DW implements dual-function control logic: DIR selects data flow direction (A→B or B→A), while OE independently disables all outputs into high-impedance state. Its TTL-compatible inputs accept standard LS thresholds (V_IL ≤ 0.8 V, V_IH ≥ 2.0 V), and outputs drive ±24 mA loads with guaranteed VOH ≥ 2.7 V and VOL ≤ 0.5 V at VCC = 5 V.

It belongs to the legacy 74LS family and shares identical AC/DC electrical specifications with SN74LS645N and SN74LS645NSR variants - including t_PD = 15 ns max, I_CC = 34 mA max, and fan-out of 20 LS-TTL loads - confirming consistent timing and loading behavior across SOIC, PDIP, and SOP packages.

Key Specifications

Pinout & Package

SN74LS645DW uses a 20-pin SOIC (Small Outline Integrated Circuit) package with 1.27 mm pitch, 7.5 mm body width, and 2.65 mm max height per TI DW0020A outline. Pin 1 is marked by a beveled corner or notch orientation.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal 3-state bus transceiver applications.

Compared with SN74LS645N and SN74LS645NSR, the SN74LS645DW offers identical electrical performance in a SOIC-20 package optimized for fine-pitch surface-mount reliability and thermal dissipation - making it ideal for compact, high-density PCBs where board space and reflow compatibility are prioritized over through-hole serviceability or reel logistics.

Availability

SN74LS645DW is available at Aetrix Electronics and suitable for industrial PLC backplane interfaces, data acquisition system bus isolation, and legacy computer memory expansion requiring stable component supply and long-term obsolescence management support.

Supply support for SN74LS645DW 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 founded in 1930, specializing in analog, embedded processing, and logic solutions with broad industrial, automotive, and communications applications.

The SN74LS645DW belongs to TI's legacy 74LS logic family, engineered specifically for robust, low-speed bidirectional bus interfacing in commercial-grade systems where TTL compatibility, predictable timing, and field-proven reliability are essential.

FAQ

What is the primary function of the SN74LS645DW?

The SN74LS645DW is an octal 3-state bidirectional bus transceiver that routes 8-bit data between two buses under independent direction (DIR) and output-enable (OE) control. It maintains full TTL compatibility, supports 5 V operation, and delivers 15 ns max propagation delay - making it suitable for legacy industrial control and instrumentation bus architectures where deterministic timing and bus contention avoidance are critical. The SN74LS645DW remains functionally identical to its PDIP and SOP variants despite its obsolete status.

Is the SN74LS645DW pin-compatible with other 74LS645 variants?

Yes, the SN74LS645DW is pin-compatible with SN74LS645N (PDIP-20) and SN74LS645NSR (SOP-20): all share identical pin numbering, signal assignments, and electrical behavior per TI's SDLS189 datasheet. The SN74LS645DW uses SOIC-20 (DW) packaging with 1.27 mm pitch, matching the land pattern requirements of the SOP variant and differing only in body width and thermal profile - not pinout or logic function.

What are the key timing parameters of the SN74LS645DW?

The SN74LS645DW specifies a maximum propagation delay (t_PD) of 15 ns under standard load conditions (VCC = 5 V, TA = 25°C, CL = 15 pF), with typical values of 10 ns for both t_PLH and t_PHL. Setup and hold times are not applicable since it lacks internal latching; instead, it operates synchronously with external control signals. These timing characteristics are fully validated across the 0°C to 70°C operating range and align precisely with those of SN74LS645N and SN74LS645NSR.

Can the SN74LS645DW be used in new designs despite its obsolete status?

Yes - while TI lists SN74LS645DW as obsolete, Aetrix Electronics maintains active inventory and lifecycle coordination for this part to support legacy system maintenance, industrial retrofitting, and backward-compatible upgrades. Its identical functionality to active variants like SN74LS645NSR and SN74LS645-1DWR ensures seamless integration, and its SOIC-20 footprint remains widely supported in modern SMT assembly lines. The SN74LS645DW continues to meet original TI specifications without derating.

What is the recommended power supply decoupling for the SN74LS645DW?

TI recommends placing a 0.1 µF ceramic capacitor between VCC (pin 20) and GND (pin 10) as close as possible to the SN74LS645DW package - ideally within 5 mm - to suppress high-frequency switching noise and stabilize local supply voltage during output transitions. For systems with multiple SN74LS645DW devices or heavy bus loading, adding a bulk 4.7 µF tantalum or aluminum electrolytic capacitor per 5–10 ICs on the same 5 V rail further improves transient response. This decoupling strategy applies identically to all SN74LS645 variants, including SN74LS645DW.