Texas Instruments SN74HC646DW

Overview

SN74HC646DW from Texas Instruments is an octal bus transceiver and register IC with 3-state outputs, designed for bidirectional data flow control between two 8-bit buses (A and B) in TTL- and CMOS-compatible systems. It features independent A/B registers, real-time and stored-data multiplexing via SAB/SBA controls, ±6-mA output drive at 5 V, 11 ns typical propagation delay, and operates across 2 V to 6 V supply range.

For engineers reviewing the SN74HC646DW datasheet, SN74HC646DW pinout, SN74HC646DW application, or SN74HC646DW equivalent, key selection criteria include dual-register storage capability, 3-state bus isolation timing (ten/tdis ≤ 61 ns at 4.5 V), CLKAB/CLKBA edge-triggered latching, DIR/OE-controlled direction and enable logic, and SOIC-24 package compatibility with industrial temperature range (–40°C to 85°C).

Technical Context

The SN74HC646DW integrates eight identical bidirectional channels, each with D-type flip-flops clocked on low-to-high transitions of CLKAB (A→B) or CLKBA (B→A). Its control architecture supports four distinct bus-management functions: real-time transfer, stored-data transfer, simultaneous A/B storage, and isolation mode - all governed by OE, DIR, SAB, and SBA inputs.

It implements true data paths without inversion, maintains input functionality during output disable (enabling concurrent storage), and uses internal 3-state drivers capable of sinking/sourcing ±6 mA at 5 V while consuming ≤80 µA ICC. The device requires no external pull-ups on OE for power-up high-impedance safety when tied to VCC via resistor.

Key Specifications

Pinout & Package

SN74HC646DW is housed in a 24-pin SOIC (DW) package with standard 0.300-inch body width and 1.27 mm pitch. Pin 1 is located at top-left corner (Q1 quadrant per tape specification), with GND at pin 12 and VCC at pin 24.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC646DW, SN74HCT646DW offers superior noise immunity in legacy 5 V systems but consumes more quiescent power, while SN74LV646APW delivers higher speed and lower voltage operation at the cost of package incompatibility and revised pin assignments - neither is pin-compatible nor drop-in replaceable.

Availability

SN74HC646DW is available at Aetrix Electronics and suitable for industrial PLC backplane interfaces, legacy system bus bridges, test equipment data acquisition hubs, and automated test fixture control requiring stable component supply and long-term lifecycle support.

Supply support for SN74HC646DW 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 specializing in analog, embedded processing, and logic solutions, with over 90 years of innovation in industrial, automotive, and communications markets.

The SN74HC646DW belongs to TI's 74HC logic family, engineered for high-noise-immunity, low-power, and wide-voltage-operation digital interfacing in industrial control and instrumentation systems.

FAQ

What is the maximum clock frequency supported by SN74HC646DW?

The SN74HC646DW supports a maximum clock frequency of 31 MHz at VCC = 4.5 V and CL = 50 pF, as specified in its timing characteristics table. This value assumes proper signal integrity, controlled impedance routing, and adherence to recommended operating conditions including VIH/VIL thresholds and input transition times. Exceeding this frequency may result in setup/hold violations or metastability in register capture.

Does SN74HC646DW require external pull-up resistors on its control pins?

Yes - SN74HC646DW requires a pull-up resistor on the OE pin to ensure high-impedance state during power-up or power-down; TI specifies minimum resistance based on driver sink capability. DIR, SAB, SBA, CLKAB, and CLKBA should also be actively driven or pulled to valid logic levels (VCC or GND) per SCBA004 guidelines, as floating CMOS inputs risk increased ICC and erratic behavior.

Can SN74HC646DW operate reliably at 3.3 V supply voltage?

Yes - SN74HC646DW is fully specified for operation from 2 V to 6 V, including 3.3 V nominal systems. At VCC = 3.3 V, it maintains guaranteed VOH ≥ 2.5 V and VOL ≤ 0.55 V under load, supports tpd ≈ 14 ns (typ), and retains full register functionality and 3-state control. Input thresholds scale proportionally, ensuring compatibility with standard 3.3 V CMOS logic families.

How does the SN74HC646DW handle simultaneous access to both A and B buses?

The SN74HC646DW prevents bus contention by enforcing strict directional control: only one bus (A or B) may drive the other at any time, determined by DIR and OE states. When OE is high, both buses enter high-impedance isolation regardless of DIR. During active transfer, the selected direction path is enabled while the opposite port remains input-only - no internal short-circuit paths exist between A and B terminals.

Is SN74HC646DW pin-compatible with SN74LS646 or other 74LS-series transceivers?

No - SN74HC646DW is not pin-compatible with SN74LS646. While both are octal transceivers with similar pin count and function labels, SN74LS646 uses a 24-pin PDIP package with different pin assignments (e.g., CLKAB/CLKBA positions differ), lacks SAB/SBA select inputs, and has incompatible electrical characteristics including higher ICC and TTL-level thresholds. Direct replacement requires PCB redesign and logic-level validation.