Texas Instruments SN74LS646NT3

Overview

SN74LS646NT3 from Texas Instruments is an octal bus transceiver with 3-state outputs and internal 3-line-to-8-line decoder, designed for bidirectional data flow control in TTL-level address/data buses. It features 8-bit parallel I/O, active-low output enable (OE\), and dual-function control inputs (DIR and G\) enabling direction selection and output gating. It operates at VCC = 5 V, supports 0°C to 70°C ambient, and is used in legacy industrial control backplanes and microprocessor system expansion interfaces.

For engineers reviewing the SN74LS646NT3 datasheet, SN74LS646NT3 pinout, SN74LS646NT3 application, or SN74LS646NT3 equivalent, key selection considerations include its bidirectional 3-state bus interface capability, integrated direction decoding logic, TTL-compatible input thresholds, 5-V single-supply operation, and SOIC-24 package thermal and layout constraints.

Technical Context

The SN74LS646NT3 integrates a 3-line-to-8-line decoder with an octal transceiver in a single 24-pin SOIC package, eliminating external decode logic for bus arbitration. Its DIR input selects data flow direction (A→B or B→A), while G\ enables/disables all outputs simultaneously, and OE\ provides independent 3-state control per byte group.

It uses standard LS-TTL circuitry with Schottky-clamped bipolar transistors, delivering typical propagation delay of 15 ns (max 25 ns) at VCC = 5 V, fan-out of 20 LS-TTL loads, and guaranteed noise margin of 0.4 V under worst-case conditions. Input clamping diodes and output current limiting support robust operation in noisy industrial environments.

Key Specifications

Pinout & Package

SN74LS646NT3 is housed in a 24-pin small-outline integrated circuit (SOIC) package with 300-mil body width, gull-wing leads, and JEDEC MS-013 compliant footprint. Thermal resistance θJA is 90°C/W, supporting continuous operation at full load within specified ambient range.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74LS245N and SN74ALS646N, the SN74LS646NT3 uniquely combines decode and transceive functions in one package, offering lowest component count for multi-slot backplanes, while maintaining proven LS-TTL noise margins and compatibility with decades-old system schematics.

Availability

SN74LS646NT3 is available at Aetrix Electronics and suitable for industrial backplane interfaces, microprocessor system expansion, and legacy test equipment bus control requiring stable component supply and long-term obsolescence mitigation.

Supply support for SN74LS646NT3 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 solutions with broad industrial, automotive, and communications portfolio coverage.

The SN74LS646NT3 belongs to TI's legacy 74LS logic family, engineered specifically for reliability and interoperability in TTL-based computing and control systems deployed from the 1980s through early 2000s.

FAQ

What is the function of the DIR pin on the SN74LS646NT3?

The DIR (Direction) pin on the SN74LS646NT3 controls bidirectional data flow: when high, data passes from Port A to Port B; when low, data flows from Port B to Port A. This pin works in conjunction with G\ and OE\ to provide precise bus arbitration. The SN74LS646NT3 uses DIR to determine real-time path polarity without requiring external logic, making it ideal for dynamic bus-sharing applications where direction changes frequently.

Does the SN74LS646NT3 support 3.3-V logic levels?

No, the SN74LS646NT3 is strictly a 5-V TTL device with VIL = 0.8 V and VIH = 2.0 V thresholds, and is not 3.3-V tolerant. Driving its inputs from 3.3-V sources may result in marginal or incorrect logic recognition, and applying 3.3-V signals to outputs during 3-state disable does not guarantee safe interfacing. For mixed-voltage systems, level-shifting circuitry or a 3.3-V-compatible alternative such as SN74LVC245A must be used alongside the SN74LS646NT3.

What is the difference between G\ and OE\ on the SN74LS646NT3?

G\ (Group Enable) is an active-low input that globally disables all outputs on Port B, independent of DIR state, while OE\ (Output Enable) is an active-low input that controls 3-state operation for both Port A and Port B outputs. G\ provides coarse-grained bus isolation-e.g., disabling an entire peripheral slot-whereas OE\ allows fine-grained timing control, such as synchronizing output release with clock edges. Both must be asserted for functional operation of the SN74LS646NT3.

Is the SN74LS646NT3 pin-compatible with SN74LS645N?

No, the SN74LS646NT3 is not pin-compatible with SN74LS645N. While both are octal transceivers, SN74LS645N has 20 pins and lacks the integrated 3-to-8 decoder, resulting in different pin assignments for control signals and I/O. The SN74LS646NT3 uses a 24-pin SOIC package with dedicated DIR, G\, and OE\ inputs arranged to support its decoder-transceiver integration-making direct PCB replacement impossible without layout revision.

What package type is used for the SN74LS646NT3?

The SN74LS646NT3 is supplied in a 24-pin small-outline integrated circuit (SOIC) package, designated DW, with 300-mil body width and gull-wing leads. It complies with JEDEC MS-013, has a moisture sensitivity level (MSL) rating of Level-1, and supports peak reflow temperatures up to 260°C. This package is compatible with standard surface-mount assembly processes and offers improved thermal performance over older PDIP variants of the same logic family.