onsemi DM74AS646WMX

Overview

DM74AS646WMX 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 direction control. It supports real-time or stored data transfer between Bus A and Bus B, operates at up to 90 MHz clock frequency, delivers ±48 mA output drive, and functions across 0°C to +70°C. It is used in buffered I/O ports and bidirectional bus interfaces in industrial control backplanes.

For engineers reviewing the DM74AS646WMX datasheet, pinout, applications, or equivalent options, key selection considerations include its 24-pin SOIC package, 3-STATE bus-driving capability, clock-edge storage behavior, and compatibility with LS TTL logic families in multiplexed data path designs.

Technical Context

The DM74AS646WMX integrates eight independent D-type flip-flops clocked on the positive edge of CAB or CBA, enabling synchronous capture of bus data. Its SAB/SBA select pins implement a "make-before-break" transition to prevent glitches during real-time ↔ stored data switching.

Four operational modes are controlled via G (enable) and DIR (direction): real-time A→B, B→A, storage from either bus, or retrieval to either bus. When G is HIGH, both buses enter high-impedance isolation while retaining input functionality for storage triggering.

Key Specifications

Pinout & Package

DM74AS646WMX is housed in a 24-lead Small Outline Integrated Circuit (SOIC), JEDEC MS-013, 0.300" wide (Package Number M24B), with standard surface-mount footprint and thermal resistance θJA = 81.5°C/W.

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 74F646PC, DM74AS646WMX provides optimal balance of SOIC manufacturability, 90 MHz capability, and 48 mA drive in commercial-temperature industrial systems-without requiring layout changes or increased power delivery infrastructure.

Availability

DM74AS646WMX is available at Aetrix Electronics and suitable for industrial backplane interfaces, legacy system I/O expansion, and buffered data acquisition nodes requiring stable component supply and long-term obsolescence management.

Supply support for DM74AS646WMX 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 U.S.-based semiconductor company specializing in analog and mixed-signal ICs, acquired by ON Semiconductor in 2016; its legacy logic portfolio remains widely deployed in industrial and automotive systems.

The DM74AS646WMX belongs to Fairchild's advanced Schottky TTL (AS) logic family, designed for high-speed, low-glitch bus interfacing in control systems where LS TTL performance limits were reached.

FAQ

What is the primary function of the DM74AS646WMX?

The DM74AS646WMX is an octal bus transceiver and register that enables bidirectional data transfer between two 8-bit buses (A and B), with optional edge-triggered storage of bus data. It supports four operational modes-real-time A↔B transfer, storage from either bus, and retrieval to either bus-controlled by G, DIR, SAB, SBA, CAB, and CBA signals. Its totem-pole 3-STATE outputs drive bus lines directly without external components, making DM74AS646WMX ideal for buffered I/O and multiplexed data paths in industrial control systems.

Does DM74AS646WMX support true bidirectional data flow without external direction logic?

Yes, DM74AS646WMX implements full bidirectional control internally via the DIR and G pins. When G is LOW, DIR determines real-time direction: DIR = HIGH enables A→B transfer, DIR = LOW enables B→A transfer. When G is HIGH, both buses enter high-impedance isolation while retaining input functionality-allowing clock-triggered storage without direction conflict. This eliminates need for external direction-control gates, simplifying PCB layout and reducing component count in DM74AS646WMX-based designs.

What is the significance of the "make-before-break" behavior on SAB and SBA pins in DM74AS646WMX?

The "make-before-break" design of SAB and SBA ensures that the newly selected data path (stored or real-time) is established before the previous path is disconnected. This prevents momentary bus floating or glitches during mode transitions-critical in synchronous systems where unintended logic states could corrupt data or trigger false interrupts. In DM74AS646WMX, this behavior is implemented in silicon and guaranteed across temperature and voltage, distinguishing it from discrete mux solutions requiring external timing compensation.

Can DM74AS646WMX replace 74LS646 in existing designs?

Yes, DM74AS646WMX is functionally and pin-for-pin compatible with 74LS646, sharing identical pin assignments, DC voltage thresholds (VIH = 2.0 V, VIL = 0.8 V), and logic behavior. It improves upon LS TTL with higher speed (90 MHz vs. ~30 MHz), stronger output drive (48 mA sink vs. 8 mA), and tighter propagation delays (≤9 ns vs. ~25 ns). No PCB changes are needed-DM74AS646WMX drops into LS sockets or footprints, delivering immediate performance uplift in legacy 74LS646 applications.

What are the thermal limitations of DM74AS646WMX in continuous operation?

DM74AS646WMX is rated for 0°C to +70°C free-air operating temperature and has a junction-to-ambient thermal resistance (θJA) of 81.5°C/W in its SOIC (M24B) package. At maximum rated ICC (211 mA) and VCC = 5.5 V, worst-case power dissipation is ~1.16 W, implying a theoretical junction rise of ~94°C above ambient-exceeding safe limits. Therefore, derating is required: sustained operation above 50°C ambient demands airflow, copper pour, or reduced loading. DM74AS646WMX must be applied within manufacturer-recommended conditions to ensure reliability and avoid thermal shutdown or parametric shift.