Texas Instruments SN74AS646NT

Overview

SN74AS646NT from Texas Instruments is an octal bus transceiver and register IC with 3-state outputs, designed for bidirectional data flow between two 8-bit buses (A and B) in synchronous digital systems. It integrates independent D-type flip-flop registers per bus, real-time and stored-data multiplexing via SAB/SBA controls, true (non-inverting) logic path, and supports clock frequencies up to 90 MHz at 5 V. It is used in legacy industrial control backplanes and military-grade data acquisition subsystems requiring deterministic timing and bus isolation.

For engineers reviewing the SN74AS646NT datasheet, SN74AS646NT pinout, SN74AS646NT application, or SN74AS646NT equivalent, this device is selected when high-speed (≤90 MHz), low-skew bidirectional bus management with on-chip storage and glitch-free multiplexing is required - especially where compatibility with TTL/AS logic families, 5-V operation, and NT-package DIP form factor are mandatory.

Technical Context

The SN74AS646NT implements dual 8-bit bidirectional transceivers with integrated edge-triggered D-flip-flop registers on both A and B buses. Data is latched on the low-to-high transition of CLKAB (A→B) or CLKBA (B→A), enabling synchronized capture from either bus into dedicated storage. The DIR input selects direction of real-time transfer, while OE enables/disables 3-state outputs.

Select-control inputs SAB and SBA determine whether output data originates from real-time bus signals or from the internal registers - with internal circuitry eliminating decoding glitches during transitions between modes. All control inputs (DIR, OE, SAB, SBA, CLKAB, CLKBA) are TTL-compatible and fully buffered.

Key Specifications

Pinout & Package

SN74AS646NT is housed in a 24-pin standard plastic DIP (NT) package with 300-mil width, suitable for through-hole prototyping and industrial board repair. Pin numbering follows JEDEC MS-001, with pins 1–12 on top row (left to right) and 13–24 on bottom row (right to left).

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74AS648NT and SN74ALS646ANT, the SN74AS646NT delivers the highest speed and drive strength in the NT-package octal transceiver family while maintaining full pin compatibility - making it optimal for performance-critical upgrades without PCB changes.

Availability

SN74AS646NT is available at Aetrix Electronics and suitable for industrial backplane interfaces, military data acquisition systems, and legacy test equipment requiring stable component supply with long-term obsolescence mitigation.

Supply support for SN74AS646NT 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 for industrial, automotive, and aerospace markets.

The SN74AS646NT belongs to TI's legacy Advanced Schottky TTL logic family, engineered for high-speed, noise-immune bus interfacing in mission-critical systems where reliability and long-term availability are essential.

FAQ

What is the maximum operating frequency of the SN74AS646NT?

The SN74AS646NT supports a maximum clock frequency of 90 MHz under recommended operating conditions (VCC = 4.5 V to 5.5 V, TA = 0°C to 70°C). This rating applies to both CLKAB and CLKBA inputs and is validated per TI's characterization data for SN74AS646 devices in NT packaging. The SN74AS646NT achieves this speed due to its Advanced Schottky process and optimized internal gate design.

Does the SN74AS646NT support both real-time and stored data modes?

Yes, the SN74AS646NT supports both real-time (transparent) and stored (registered) data transfer via dedicated SAB and SBA control inputs. When SAB = L, real-time A-bus data transfers to B-bus; when SAB = H, previously latched data from the A-register drives B-bus. Identical behavior applies to SBA for B→A paths. This dual-mode capability is implemented with internal glitch-elimination logic, ensuring robust mode switching.

Is the SN74AS646NT pin-compatible with other members of the '646/'648 family?

Yes, the SN74AS646NT shares identical 24-pin NT-package pinout with SN74ALS646A, SN74ALS648A, SN74AS648, and SN74ALS646. All share common pin assignments for CLKAB, CLKBA, DIR, OE, SAB, SBA, A1–A8, B1–B8, VCC, and GND. However, logic behavior differs: SN74AS646NT provides true (non-inverting) data path, whereas SN74AS648NT provides inverting path.

What is the output drive capability of the SN74AS646NT?

The SN74AS646NT provides 48 mA low-level output current (IOL) and –15 mA high-level output current (IOH) at VCC = 4.5 V, enabling direct drive of multiple TTL loads or moderate capacitive bus stubs. This drive strength exceeds that of ALS-family equivalents (e.g., SN74ALS646A: 24 mA IOL) and supports reliable signal integrity in electrically noisy industrial environments without external buffers.

Can the SN74AS646NT operate with a 3.3-V supply?

No, the SN74AS646NT is specified only for 4.5 V to 5.5 V operation and is not 3.3-V compatible. Its input thresholds (VIH = 2.0 V min, VIL = 0.8 V max) and internal circuitry are designed for 5-V TTL logic levels. Applying 3.3 V to VCC may result in undefined operation, insufficient noise margin, or failure to meet timing specifications. For 3.3-V systems, consider modern LVCMOS or LVT logic alternatives.