onsemi MC74ACT646DW

Overview

MC74ACT646DW from onsemi is an octal registered bus transceiver with 3-state outputs and non-inverting data paths, designed for bidirectional data flow between A and B buses in synchronous digital systems. It features independent A/B registers, real-time and stored-data transfer modes, TTL-compatible inputs, ±24 mA output drive, and operates at 4.5–5.5 V. It is used in address/data bus isolation and buffering in industrial control backplanes.

For engineers reviewing the MC74ACT646DW datasheet, pinout, applications, or equivalent options, key selection criteria include clock-controlled register loading (CAB/CBA), dual-directional transmit/receive control (SAB/SBA), output enable logic (G/DIR), and SOIC-24 package compatibility with legacy 74ACT logic families.

Technical Context

The MC74ACT646DW implements two independent 8-bit D-type flip-flop registers-one per bus-with asynchronous 3-state output control via G (global enable) and DIR (direction select). Data loads on LOW-to-HIGH transitions of CAB (A→B) or CBA (B→A), enabling synchronized capture before transmission.

Its logic supports four distinct operational modes: real-time A-to-B or B-to-A transfer, stored-A-to-B or stored-B-to-A transfer-each selected by combinations of SAB, SBA, DIR, and G. All inputs are TTL-compatible; outputs drive ±24 mA with VOH ≥ 4.4 V and VOL ≤ 0.44 V at VCC = 4.5 V.

Key Specifications

Pinout & Package

MC74ACT646DW is housed in a 24-pin SOIC wide-body (SOIC-24 WB) package per Case 751E, with 300-mil width and Pb-free finish. Pin 1 is marked by a notch or dot; leads are gull-wing, 1.27 mm pitch.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar octal registered transceiver applications.

Compared with SN74ACT646N, MC74ACT646DW offers surface-mount reliability and thermal performance; compared with MC74ACT245DW, it adds critical clocked register staging for synchronous data handshaking-making it indispensable in DMA, burst-mode, or time-critical bus arbitration systems.

Availability

MC74ACT646DW is available at Aetrix Electronics and suitable for industrial backplane isolation, microcontroller peripheral expansion, and legacy system bus bridging requiring stable component supply and long-term obsolescence management.

Supply support for MC74ACT646DW 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

onsemi (formerly ON Semiconductor) is a global semiconductor supplier delivering energy-efficient, intelligent power and sensing solutions for automotive, industrial, cloud, medical, and IoT applications.

The MC74ACT646DW belongs to the 74ACT advanced CMOS logic family, engineered for high-speed, low-noise, TTL-compatible operation in industrial control, instrumentation, and legacy system upgrades.

FAQ

What is the maximum clock frequency supported by MC74ACT646DW?

The MC74ACT646DW does not specify a maximum clock frequency in its datasheet because it uses edge-triggered D-type registers-not a free-running oscillator. Its usable data rate depends on propagation delays and setup/hold times: with tPLH/tPHL ≤ 16 ns and ts/th ≥ 7 ns/2.5 ns at VCC = 5.0 V, it reliably supports synchronous bus operation up to approximately 50 MHz in well-terminated systems. The MC74ACT646DW's performance is validated under these AC operating conditions, not a fixed clock limit.

Does MC74ACT646DW support hot insertion or live bus swapping?

No, the MC74ACT646DW is not hot-swap rated. Its absolute maximum ratings do not include live-insertion stress testing, and the device lacks built-in bus-fault protection or power-up sequencing control. Applying VCC or signals before establishing proper GND can exceed IIK/IOK limits (±20/±50 mA), risking latchup or permanent damage. For hot-swap applications, external protection circuitry or dedicated hot-swap controllers must be used alongside the MC74ACT646DW.

Can MC74ACT646DW operate at 3.3 V supply voltage?

No, MC74ACT646DW is not specified for 3.3 V operation. Its recommended VCC range is strictly 4.5–5.5 V, and DC/AC characteristics (e.g., VIH = 2.0 V min, VOH ≥ 4.4 V) are guaranteed only within that range. At 3.3 V, input thresholds become marginal, output drive collapses below TTL compatibility, and timing parameters degrade beyond specification. Use MC74AC646DW (2.0–6.0 V) or 74LVC646 for true 3.3 V compatibility instead of MC74ACT646DW.

How does the DIR pin interact with SAB and SBA in MC74ACT646DW?

In MC74ACT646DW, DIR controls data direction *only when outputs are enabled* (G = LOW); SAB and SBA determine whether real-time or stored data is transferred. When DIR = LOW, data flows from A bus to B bus (if SAB active) or from B bus to A bus (if SBA active); when DIR = HIGH, the direction reverses. Both DIR and SAB/SBA must be asserted correctly per function table entries-DIR alone does not override SAB/SBA logic. This dual-control scheme ensures precise, glitch-free bus arbitration in the MC74ACT646DW.

Is MC74ACT646DW pin-compatible with MC74AC646DW?

Yes, MC74ACT646DW is pin-compatible and functionally identical to MC74AC646DW in the same SOIC-24 package, differing only in input threshold specifications: MC74ACT646DW has TTL-compatible inputs (VIH = 2.0 V), while MC74AC646DW uses CMOS thresholds (VIH = 3.15 V at VCC = 4.5 V). Both share identical pinout, timing, drive strength, and register architecture-so MC74ACT646DW may substitute MC74AC646DW in TTL-driven systems without layout change, but not vice versa without verifying input drive levels.