Texas Instruments SN74AS646DWR

Overview

SN74AS646DWR from Texas Instruments is an octal bus transceiver and register with 3-state outputs, designed for bidirectional data flow between two 8-bit buses (A and B) in TTL-compatible 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 up to 90 MHz clock frequency with 1 ns propagation delay on A→B paths.

For engineers reviewing the SN74AS646DWR datasheet, SN74AS646DWR pinout, SN74AS646DWR application, or SN74AS646DWR equivalent, this device is critical for high-speed bus isolation, registered data forwarding, and glitch-free bus arbitration in legacy industrial control backplanes, test equipment data paths, and military-grade digital subsystems requiring deterministic timing and dual-bus storage.

Technical Context

The SN74AS646DWR implements dual 8-bit D-register banks synchronized to separate clocks (CLKAB for A→register, CLKBA for B→register), enabling independent capture of data from either bus. Its DIR input selects direction of real-time transfer (A→B or B→A), while OE enables/disables all outputs without affecting internal register operation.

Select-control inputs SAB and SBA determine whether output data originates from real-time bus signals or stored register contents-using dedicated circuitry that eliminates decoding glitches during transitions between modes. The device operates exclusively in true (non-inverting) logic mode and does not support open-collector outputs.

Key Specifications

Pinout & Package

SN74AS646DWR uses a 24-pin SOIC (DW) package with 300-mil width, surface-mount footprint, and standard pin numbering per TI DW package outline.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74ALS646ADWR and SN74AS648DW, the SN74AS646DWR delivers highest clock rate and sink current for true-path applications but consumes more power; SN74ALS646ADWR trades speed for efficiency, while SN74AS648DW maintains performance with inverted logic polarity.

Availability

SN74AS646DWR is available at Aetrix Electronics and suitable for industrial backplane interfaces, military data acquisition systems, automated test equipment fixtures, and legacy computer memory expansion requiring stable component supply and long-term obsolescence management.

Supply support for SN74AS646DWR 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 for industrial, automotive, and aerospace markets.

The SN74AS646DWR belongs to TI's legacy Advanced Schottky TTL logic product line, engineered for high-speed, registered bus interfacing in mission-critical systems where reliability and deterministic timing supersede CMOS power efficiency.

FAQ

What is the maximum clock frequency supported by SN74AS646DWR?

The SN74AS646DWR 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 characterization data in SDAS039F. The SN74AS646DWR achieves this performance using Advanced Schottky TTL architecture, making it the fastest variant in the SN74AS64x family.

Does SN74AS646DWR support open-collector outputs?

No, SN74AS646DWR provides only 3-state (high-impedance) outputs - not open-collector. The device family documentation explicitly states "3 state" output logic for SN74AS646 variants, and no configuration or external circuitry can convert its outputs to open-collector mode. For open-collector alternatives, engineers should consider SN74ALS648A or other members of the SN74ALS64x series.

How does the SN74AS646DWR handle simultaneous data capture from both A and B buses?

The SN74AS646DWR supports independent, simultaneous data capture: A-bus data is latched into the A-register on the low-to-high transition of CLKAB, while B-bus data is latched into the B-register on the low-to-high transition of CLKBA. These clocks operate asynchronously, allowing concurrent registration without conflict. The SN74AS646DWR retains both datasets until explicitly routed via SAB/SBA controls or transferred in real-time mode.

What is the purpose of the NC pins on SN74AS646DWR?

SN74AS646DWR has six NC (No internal connection) pins - located at positions 14, 15, 16, 17, 18, and 19 - which are physically present but electrically unconnected to die circuitry. Per TI datasheet SDAS039F, these pins must remain unconnected in PCB layout; routing traces or applying bias to them may cause mechanical stress or unintended coupling. Their presence accommodates package compatibility across the broader SN74x64x family.

Can SN74AS646DWR operate with a 3.3 V supply?

No, SN74AS646DWR requires a 5 V nominal supply (4.5 V to 5.5 V range) and is not 3.3 V compatible. Its Advanced Schottky TTL input thresholds (VIH = 2.0 V min, VIL = 0.8 V max) and output voltage levels (VOH ≥ 2.4 V, VOL ≤ 0.5 V) are specified only for 5 V operation. Attempting 3.3 V supply will result in non-compliant logic levels, marginal timing, and potential functional failure. For 3.3 V systems, level translators or LVCMOS-compatible alternatives are required.