onsemi MC74AC646DWR2

Overview

MC74AC646DWR2 from onsemi is an octal registered bus transceiver with non-inverting 3-state outputs, designed for bidirectional data flow control between A and B buses in high-speed digital systems. It integrates dual D-type flip-flop registers (A and B), independent clock inputs (CAB/CBA), transmit/receive controls (SAB/SBA), and output enable logic (DIR/G). Key confirmed parameters: 5.0 V max supply voltage, ±24 mA output drive, 12 ns typical bus-to-bus propagation delay at 5 V, and SOIC-24 package.

For engineers reviewing the MC74AC646DWR2 datasheet, pinout, applications, or equivalent options, this page delivers verified functional architecture, real-time vs. registered transfer modes, 3-state timing behavior, DC/AC electrical limits, and direct alternatives for bus interface redesigns in industrial control and legacy computing systems.

Technical Context

The MC74AC646DWR2 implements two independent 8-bit register banks synchronized to separate clock edges: CAB loads A-bus data into A-register, CBA loads B-bus data into B-register. Each register feeds a non-inverting 3-state output buffer controlled by DIR (direction) and G (global output enable).

Four operational modes are defined by SAB/SBA and DIR/G states: real-time A→B or B→A transfer, stored-A→B or stored-B→A transfer, and full isolation. Input thresholds meet AC logic levels (VIH = 2.1 V min at VCC = 3.0 V), and outputs guarantee VOH ≥ 2.9 V / VOL ≤ 0.1 V at ±24 mA load under recommended conditions.

Key Specifications

Pinout & Package

MC74AC646DWR2 is housed in a Pb-free SOIC-24 wide-body package (Case 751E), 15.5 mm × 7.6 mm footprint, 2.65 mm height, 1.27 mm pitch, with gull-wing leads suitable for reflow soldering.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MC74AC646DWR2, SN74ACT646DW adds TTL input compatibility at no timing cost, while 74LCX646M trades 5 V operation for faster 3.3 V performance-making MC74AC646DWR2 the optimal choice for mixed-voltage or 5 V legacy bus extension where robustness and voltage flexibility are prioritized.

Availability

MC74AC646DWR2 is available at Aetrix Electronics and suitable for industrial PLC backplanes, automotive body control hubs, and legacy computer bus extenders requiring stable component supply across extended product lifecycles.

Supply support for MC74AC646DWR2 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 specializing in energy-efficient power management, analog, logic, and sensing solutions for automotive, industrial, and cloud infrastructure markets.

The MC74AC646DWR2 belongs to the 74AC logic family, engineered for high-speed, low-power registered data transfer in noise-sensitive industrial control and computing environments where bus integrity and timing predictability are critical.

FAQ

What is the maximum clock frequency supported by MC74AC646DWR2?

The MC74AC646DWR2 does not specify a maximum clock frequency in its datasheet; instead, it defines minimum clock pulse width (tw = 3.5 ns min at 5.0 V) and setup/hold times (ts = 4.0 ns, th = 0.5 ns). These values support reliable operation up to approximately 140 MHz in registered mode, assuming proper PCB layout and 50 pF load. The actual usable frequency depends on system-level timing margins and board parasitics-not an intrinsic clock limit of the MC74AC646DWR2 itself.

Does MC74AC646DWR2 support hot-swapping or live insertion?

The MC74AC646DWR2 is not explicitly rated for hot-swap operation. Its absolute maximum ratings allow VCC = −0.5 V to +7.0 V and input voltages from −0.5 V to VCC + 0.5 V, but no current-limiting or precharge circuitry is integrated. Safe live insertion requires external protection (e.g., series resistors, TVS diodes) and controlled power sequencing. The device's 3-state outputs can isolate the bus during insertion, but the MC74AC646DWR2 itself lacks built-in hot-swap features like slew-rate control or undervoltage lockout.

Can MC74AC646DWR2 operate at 3.3 V supply while interfacing with 5 V logic?

MC74AC646DWR2 operates correctly at 3.3 V supply (VCC = 3.3 V), with guaranteed VIH = 2.1 V and VOL ≤ 0.1 V at ±24 mA. However, its outputs swing only to ~3.2 V (VOH ≈ VCC − 0.1 V), which falls below the 3.5 V minimum VIH required by standard 5 V TTL or CMOS inputs. Direct 3.3 V → 5 V interfacing is not safe without a level translator. The MC74AC646DWR2 is intended for same-supply-domain bus bridging-not mixed-voltage translation.

How does the DIR pin function in MC74AC646DWR2 compared to standard 74-series transceivers?

In the MC74AC646DWR2, DIR controls data flow direction *only* when outputs are enabled (G = LOW); it selects whether A-bus data appears on B outputs (DIR = LOW) or B-bus data appears on A outputs (DIR = HIGH). Unlike basic transceivers, DIR does not gate register loading-CAB/CBA clocks always update their respective registers regardless of DIR state. This separation of data path control from storage control is a defining feature of the MC74AC646DWR2 architecture.

Is MC74AC646DWR2 pin-compatible with MC74ACT646DWR2G?

Yes, MC74AC646DWR2 and MC74ACT646DWR2G share identical SOIC-24 pinouts, register structure, clock/control logic, and 3-state timing. The sole functional difference is input threshold compatibility: MC74ACT646DWR2G accepts TTL-level inputs (VIH = 2.0 V), while MC74AC646DWR2 requires CMOS-level inputs (VIH = 2.1 V at 3.0 V). No PCB changes are needed for substitution-only verify upstream driver compatibility before replacing MC74AC646DWR2 with MC74ACT646DWR2G.