Texas Instruments SN74HC646NT

Overview

SN74HC646NT 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/CMOS systems. It features independent clocked D-type registers per bus, dual select controls (SAB/SBA), direction control (DIR), and output enable (OE), operating across 2 V to 6 V with typical propagation delay of 11 ns at 5 V and ±6-mA drive capability.

For engineers reviewing the SN74HC646NT datasheet, SN74HC646NT pinout, SN74HC646NT application, or SN74HC646NT equivalent, key selection considerations include real-time vs. registered data multiplexing, isolated bus storage during OE high, 3-state output timing (ten/tdis ≈ 61 ns at 4.5 V), and compatibility with LSTTL loads (up to 15 units).

Technical Context

The SN74HC646NT implements dual 8-bit D-type flip-flop registers clocked on low-to-high transitions of CLKAB (A→B) or CLKBA (B→A), enabling synchronous capture of data from either bus. Its transceiver logic supports four distinct bus-management modes: real-time transfer, stored-data transfer, simultaneous A/B storage, and isolation mode - all controlled via OE, DIR, SAB, and SBA inputs.

Each channel includes independent 3-state output drivers with ±6-mA sink/source capability at 5 V, low input current (≤1 µA), and CMOS-level input thresholds (VIH = 3.15 V, VIL = 1.35 V at VCC = 4.5 V). The device maintains functional integrity across –40°C to 85°C and supports load capacitances up to 150 pF.

Key Specifications

Pinout & Package

SN74HC646NT is supplied in a 24-pin plastic dual in-line package (PDIP-NT) with standard through-hole mounting and 0.3-inch body width. Pin numbering follows JEDEC MS-001, with Pin 1 located at bottom-left corner when notch faces upward.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HC646NT, SN74HC645N lacks register functionality and multiplexing controls, limiting it to basic bus steering; SN74ACT646N delivers superior speed and drive but sacrifices the wide 2–6 V operating range essential for mixed-voltage designs.

Availability

SN74HC646NT is available at Aetrix Electronics and suitable for industrial PLC backplane interfaces, legacy system bus extenders, test equipment data capture modules, and redundant communication controllers requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for SN74HC646NT 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 for industrial, automotive, and communications markets.

The SN74HC646NT belongs to TI's 74HC high-speed CMOS logic family, engineered for robust noise immunity, low power consumption, and interoperability with TTL and CMOS systems in demanding industrial control applications.

FAQ

What is the maximum clock frequency supported by SN74HC646NT?

The SN74HC646NT supports a maximum clock frequency of 31 MHz at VCC = 4.5 V and TA = 25°C, as specified in its timing characteristics table. This value decreases to 27 MHz over the full industrial temperature range (–40°C to +85°C). The limit applies to both CLKAB and CLKBA inputs and assumes proper signal integrity, load capacitance ≤50 pF, and clean power supply decoupling. Exceeding this frequency may result in setup/hold violations or metastable register states.

How does SN74HC646NT handle bus contention during direction switching?

The SN74HC646NT prevents bus contention through its 3-state output architecture and controlled enable sequencing. When OE is high, both A and B ports enter high-impedance state regardless of DIR or clock activity, physically isolating the buses. During active transfer, DIR must be stabilized before OE is asserted low, and timing parameters (ten/tdis ≥61 ns at 4.5 V) ensure non-overlapping driver activation. No internal arbitration logic exists - external control logic must guarantee exclusive bus ownership.

Can SN74HC646NT operate reliably at 3.3 V supply voltage?

Yes, SN74HC646NT operates reliably at 3.3 V supply voltage, as confirmed by its recommended operating conditions (VCC = 2 V to 6 V) and electrical characteristics tables. At VCC = 3.3 V, VOH is guaranteed ≥2.5 V and VOL ≤0.45 V under 4-mA load, meeting standard 3.3 V CMOS logic thresholds. Propagation delay increases to ~15 ns (typical), and output drive reduces to ±4.2 mA - still sufficient for driving 10–12 LSTTL loads or typical PCB traces.

What is the purpose of the NC pins on SN74HC646NT?

SN74HC646NT has no NC (No Connect) pins in its PDIP-NT package - all 24 pins are assigned functional roles as defined in the pinout table. The confusion may arise from alternate packages like SOIC-DW, which includes NC pins (e.g., Pins 4, 12, 13, 19, 20, 25, 26, 27, 28 in some variants), but those do not apply to SN74HC646NT. For SN74HC646NT, every pin from 1 to 24 connects to internal circuitry and must be properly terminated per datasheet guidelines.

How does the isolation mode function in SN74HC646NT?

In isolation mode (OE = high), SN74HC646NT disables all A and B port outputs, placing them in high-impedance state, while preserving the contents of both internal 8-bit registers. Data previously latched into the A-register via CLKAB or B-register via CLKBA remains intact and unaffected by bus activity. This allows downstream logic to be safely disconnected or reconfigured without losing context - a key feature for fault-tolerant systems, hot-plug interfaces, or multi-master bus arbitration schemes.