Texas Instruments SN74HCT646NTG4

Overview

SN74HCT646NTG4 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). It operates at 4.5 V to 5.5 V, delivers ±6-mA output drive, features typical propagation delay of 12 ns at 5.5 V, and supports real-time or registered data transfer in industrial bus interface applications.

For engineers reviewing the SN74HCT646NTG4 datasheet, SN74HCT646NTG4 pinout, SN74HCT646NTG4 application, or SN74HCT646NTG4 equivalent, key selection criteria include clock-controlled register storage (CLKAB/CLKBA), independent A/B bus direction control (DIR), multiplexed select logic (SAB/SBA), and high-current 3-state outputs capable of driving up to 15 LSTTL loads.

Technical Context

The SN74HCT646NTG4 integrates dual 8-bit D-type flip-flop registers with a bidirectional transceiver core, enabling simultaneous storage and transparent transfer. Clock inputs CLKAB and CLKBA trigger low-to-high edge capture on respective buses, while DIR selects data direction (A→B or B→A) during active output enable (OE low).

SAB and SBA inputs determine whether real-time or stored data appears at outputs, supporting four bus-management modes: isolation, real-time transfer, stored-data transfer, and concurrent A/B register loading. All control inputs (OE, DIR, SAB, SBA, CLKAB, CLKBA) are TTL-voltage compatible and exhibit ≤1 µA max input current.

Key Specifications

Pinout & Package

SN74HCT646NTG4 is housed in a 24-pin plastic dual in-line package (PDIP-NT), with 0.3-inch body width and through-hole mounting. Thermal resistance θJA is 67°C/W, suitable for natural-convection environments in industrial control panels.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74HCT245N and SN74ACT646N, the SN74HCT646NTG4 uniquely combines register storage, multiplexed data routing, and TTL-compatible 5-V operation-making it the only option among the three that supports both real-time and registered transfer modes within industrial voltage and noise constraints.

Availability

SN74HCT646NTG4 is available at Aetrix Electronics and suitable for industrial PLC backplane interfaces, legacy system bus extensions, test equipment data capture modules, and automated manufacturing fixture controllers requiring stable component supply across extended product lifecycles.

Supply support for SN74HCT646NTG4 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 SN74HCT646NTG4 belongs to TI's HCT logic family-designed specifically for 5-V systems requiring TTL-input compatibility, low power consumption, and robust noise immunity in harsh industrial environments.

FAQ

What is the function of the SAB and SBA pins on the SN74HCT646NTG4?

The SAB (Select A to B) and SBA (Select B to A) pins on the SN74HCT646NTG4 control multiplexing between real-time and stored data paths. When SBA is low and OE is active, stored B-bus data is routed to the A bus; when SAB is low, stored A-bus data is routed to the B bus. This enables glitch-free bus arbitration without requiring external logic, and is a defining capability of the SN74HCT646NTG4 not found in basic transceivers like the SN74HCT245N.

Does the SN74HCT646NTG4 support operation below 4.5 V?

No, the SN74HCT646NTG4 has a specified minimum supply voltage of 4.5 V per its recommended operating conditions. Operation below this threshold may result in undefined logic states, increased propagation delay, or failure to meet TTL input compatibility requirements. The SN74HCT646NTG4 is not rated for 3.3-V or mixed-voltage operation-designs requiring lower VCC should consider the SN74LVC646A or similar LVCMOS alternatives.

How does the SN74HCT646NTG4 handle power-up sequencing to avoid bus contention?

The SN74HCT646NTG4 ensures high-impedance outputs at power-up by requiring OE to be tied to VCC via a pull-up resistor. With OE held high during ramp-up, all A/B outputs remain in 3-state mode regardless of DIR, CLKAB, or SAB/SBA states. This prevents unintended bus driving during initialization-a critical safeguard explicitly documented in the SN74HCT646NTG4 datasheet and essential for reliable industrial system startup.

Can the SN74HCT646NTG4 simultaneously store data on both A and B buses?

Yes-the SN74HCT646NTG4 supports concurrent storage: asserting both CLKAB↑ and CLKBA↑ in the same cycle latches current A-bus data into the A register and B-bus data into the B register. This dual-register architecture allows independent, synchronized capture of bidirectional traffic-enabling applications such as real-time protocol monitoring where both upstream and downstream data must be timestamped and preserved.

What is the thermal performance limitation of the SN74HCT646NTG4 in its NT package?

The SN74HCT646NTG4 in the PDIP-NT package has a junction-to-ambient thermal resistance (θJA) of 67°C/W. At maximum rated ICC (80 µA) and worst-case output loading, self-heating remains negligible-but under sustained 6-mA per-pin drive across all eight outputs, power dissipation can approach 200 mW, raising die temperature by ~13°C above ambient. This makes the SN74HCT646NTG4 suitable for convection-cooled industrial enclosures up to 70°C ambient without derating.