onsemi DM74AS646NT

Overview

DM74AS646NT from Fairchild Semiconductor is an octal bus transceiver and register IC with totem-pole 3-STATE outputs, edge-triggered D-type flip-flops, and dual-bus bidirectional data routing. It operates at VCC = 4.5–5.5 V, supports up to 90 MHz clock frequency, and delivers ±48 mA output drive (IOL/IOH), enabling direct bus-line driving in industrial control backplanes and legacy TTL systems.

For engineers reviewing the DM74AS646NT datasheet, pinout, applications, or equivalent options, key selection criteria include real-time vs. stored data multiplexing via SAB/SBA controls, make-before-break select logic, G/DIR-enabled four-mode operation, and compatibility with LS TTL pinouts and timing.

Technical Context

The DM74AS646NT implements two independent 8-bit data paths (A↔B) with synchronous storage on positive clock edges (CAB/CBA). Its 3-STATE outputs support high-capacitance bus loading without pull-ups, and its "make before break" SAB/SBA switching eliminates glitches during real-time/stored data transitions.

It features advanced oxide-isolated, ion-implanted Schottky TTL process technology, guaranteeing switching specs at 50 pF across full temperature (0°C to +70°C) and supply (4.5–5.5 V) ranges. The device provides isolation mode when G is HIGH, retaining input functionality while disabling outputs.

Key Specifications

Pinout & Package

DM74AS646NT is housed in a 24-lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300" wide (Package Number N24C), with through-hole mounting and standard 0.1" pin pitch.

Key Features

Applications

Equivalent & Alternatives

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

Compared with SN74AS646N and DM74LS646N, the DM74AS646NT offers superior drive strength and speed while maintaining full pin compatibility with LS counterparts-making it optimal for performance-retrofitting legacy systems without layout change.

Availability

DM74AS646NT is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy system bus buffering, and embedded controller I/O expansion requiring stable component supply and long-term obsolescence management.

Supply support for DM74AS646NT 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

Fairchild Semiconductor was a leading designer of high-performance analog and discrete semiconductors, acquired by ON Semiconductor in 2016; its legacy AS-family TTL devices remain widely deployed in industrial and aerospace systems.

The DM74AS646NT belongs to Fairchild's Advanced Schottky TTL (AS) logic family, engineered for high-speed, low-power bus interfacing in mission-critical control and instrumentation systems.

FAQ

What is the primary function of the DM74AS646NT?

The DM74AS646NT is an octal bus transceiver with integrated edge-triggered D-type register, enabling bidirectional data transfer between two 8-bit buses (A and B) with optional real-time or registered forwarding. Its core function is synchronized bus interfacing in TTL-based systems, supporting four operational modes via G and DIR controls, and eliminating glitches during data-source switching using make-before-break SAB/SBA logic. The DM74AS646NT delivers 48 mA output drive and 90 MHz clock capability for demanding industrial backplane applications.

Does the DM74AS646NT support true bidirectional data flow between Bus A and Bus B?

Yes, the DM74AS646NT supports true bidirectional data flow: with G LOW and DIR LOW, data transfers in real time from Bus B to Bus A; with G LOW and DIR HIGH, data flows from Bus A to Bus B. Direction is controlled solely by the DIR pin when G is active, and both directions operate with identical timing specs (tPLH/tPHL ≤ 9 ns). The DM74AS646NT does not require external direction-control logic or bus-turnaround delays, making it ideal for synchronous dual-bus architectures.

How does the DM74AS646NT handle clocking for register storage?

The DM74AS646NT uses separate positive-edge clock inputs CAB and CBA: CAB triggers storage of Bus A data into the internal register, while CBA triggers storage of Bus B data. Both registers are edge-triggered D-type flip-flops, capturing data on the LOW-to-HIGH transition. The DM74AS646NT allows independent or simultaneous registration-e.g., storing A and B data on the same clock edge-and supports subsequent retrieval via SAB/SBA controls without affecting real-time transceiver operation.

What is the significance of the "make before break" behavior on SAB and SBA pins?

The "make before break" behavior on SAB and SBA ensures that the newly selected data path (real-time or stored) is established before the previous path is disconnected-preventing momentary open-circuit or undefined states during transitions. This eliminates output glitches that would otherwise corrupt bus data in time-sensitive applications like test equipment waveform routing or PLC scan-cycle handshaking. The DM74AS646NT implements this internally; no external timing constraints or delay circuits are required to achieve clean mode switching.

Is the DM74AS646NT pin-compatible with standard LS TTL equivalents?

Yes, the DM74AS646NT is functionally and pin-for-pin compatible with the 74LS646, sharing identical pin assignments, logic functions, and DC electrical characteristics (VIH/VIL, VOH/VOL). This allows direct replacement in existing LS-based designs to improve speed (90 MHz vs. 35 MHz) and drive strength (48 mA vs. 24 mA) without PCB modification. The DM74AS646NT maintains full backward compatibility while delivering AS-family performance gains.