onsemi DM74AS646WM

Overview

DM74AS646WM 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, delivers ±48 mA output drive, and is packaged in a 24-lead SOIC (JEDEC MS-013, 0.300" wide) for use in buffered I/O ports and multiplexed bus systems.

For engineers reviewing the DM74AS646WM datasheet, pinout, applications, or equivalent options, key selection criteria include real-time vs. stored data path control via SAB/SBA, G/DIR-enabled four-mode operation, 3-STATE bus driving capability without pull-ups, and compatibility with LS TTL logic families in space-constrained PCB layouts.

Technical Context

The DM74AS646WM integrates two independent 8-bit data paths: one for real-time bidirectional bus transfer (A↔B), and another for edge-triggered storage (CAB/CBA clocks). Its "make before break" SAB/SBA select logic prevents glitches during mode transitions between live and registered data.

Control is implemented through active-low enable (G), direction (DIR), and dual clock inputs (CAB, CBA), enabling four distinct operational modes - including isolated storage, real-time A→B/B→A transfer, and stored-data output to either bus - all while maintaining input functionality even when outputs are disabled.

Key Specifications

Pinout & Package

DM74AS646WM is housed in a 24-lead Small Outline Integrated Circuit (SOIC) package per JEDEC MS-013, 0.300" body width, with standard gull-wing leads and 1.27 mm 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 74F646PC, the DM74AS646WM uniquely combines SOIC packaging, integrated storage registers, and "make before break" select logic - making it the only option among the three that supports both real-time and registered data routing in surface-mount production designs.

Availability

DM74AS646WM is available at Aetrix Electronics and suitable for microprocessor bus interfacing, industrial I/O expansion, legacy computer backplane buffering, and test equipment data multiplexing requiring stable component supply and long-term obsolescence management.

Supply support for DM74AS646WM 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, power management, and logic devices before its acquisition by ON Semiconductor in 2016.

The DM74AS646WM belongs to Fairchild's advanced Schottky TTL logic family, designed specifically for high-speed, low-glitch bus interfacing in commercial-grade computing and industrial control systems.

FAQ

What is the primary function of the DM74AS646WM?

The DM74AS646WM is an octal bus transceiver and register IC that provides bidirectional data transfer between two 8-bit buses (A and B) with optional edge-triggered storage. It supports real-time transfer, isolated storage, and registered output via dedicated control pins (G, DIR, CAB, CBA, SAB, SBA), making it suitable for synchronized bus interfacing in microprocessor and industrial control systems. The DM74AS646WM implements true 3-STATE outputs and "make before break" select logic to prevent signal glitches.

Does the DM74AS646WM support both real-time and stored data paths?

Yes, the DM74AS646WM supports both real-time and stored data paths via the SAB and SBA control inputs. A LOW level on SAB selects real-time data from Bus A to Bus B; a HIGH level selects data previously stored in the internal register. The same applies to SBA for Bus B to Bus A. Its "make before break" design ensures glitch-free transitions between these modes, a feature confirmed in the device's function table and logic diagram. This dual-path capability is intrinsic to the DM74AS646WM and distinguishes it from basic transceivers.

What are the voltage and timing requirements for reliable DM74AS646WM operation?

The DM74AS646WM requires a VCC supply of 4.5 V to 5.5 V and operates over 0°C to +70°C. Valid input levels are VIH ≥ 2.0 V and VIL ≤ 0.8 V. Clock inputs (CAB/CBA) must meet tW ≥ 5 ns (HIGH) and ≥ 6 ns (LOW), with data setup time tSU = 6 ns and hold time tH = 0 ns relative to the positive clock edge. These values are guaranteed across temperature and voltage per the Electrical Characteristics table and apply directly to the DM74AS646WM in its specified SOIC package.

Is the DM74AS646WM pin-compatible with other 74-series transceivers?

The DM74AS646WM is functionally and pin-for-pin compatible with the LS TTL counterpart (e.g., 74LS646), as explicitly stated in the Fairchild datasheet. It shares identical pin assignments, logic behavior, and DC/AC electrical specifications within its operating range. However, it is not pin-compatible with the DM74AS648WM due to complementary output polarity - a distinction clearly documented in the "Different Modes of Control" section. Compatibility applies strictly to the DM74AS646WM and verified LS-family equivalents.

What package type is used for the DM74AS646WM and what are its mechanical dimensions?

The DM74AS646WM is supplied in a 24-lead Small Outline Integrated Circuit (SOIC) package designated M24B, conforming to JEDEC MS-013 with a 0.300" body width and 1.27 mm lead pitch. Its physical dimensions are 7.50 mm × 15.40 mm × 2.35 mm (L × W × H), with gull-wing leads suitable for reflow soldering. This package is distinct from the PDIP version (DM74AS646NT, N24C) and is the only package associated with the DM74AS646WM ordering code.