onsemi MC74ACT646N

Overview

MC74ACT646N from onsemi is an octal registered transceiver with 3-state outputs and non-inverting data paths, implementing dual-directional bus interface logic with independent A/B register storage. It supports real-time or stored-data transfer between 8-bit buses, features TTL-compatible inputs, drives ±24 mA per output, and operates at 4.5–5.5 V for industrial-grade digital systems.

For engineers reviewing the MC74ACT646N datasheet, pinout, applications, or equivalent options, this device is selected for bidirectional bus buffering in legacy TTL/CMOS mixed-signal backplanes, memory address/data multiplexing, and synchronous I/O expansion where clocked register hold and 3-state isolation are required.

Technical Context

The MC74ACT646N integrates two independent 8-bit D-type flip-flop registers (A and B), dual clock inputs (CAB, CBA), and dual transmit/receive controls (SAB, SBA) to enable four distinct data-handling modes: real-time A→B, real-time B→A, stored-A→B, and stored-B→A. Its DIR and G inputs jointly manage output direction and 3-state enablement.

It uses standard CMOS process with TTL-compatible input thresholds (VIH = 2.0 V min, VIL = 0.8 V max at VCC = 4.5–5.5 V), supports 50 pF load capacitance, and meets AC timing requirements including 7.0 ns minimum setup time (ts) and 2.5 ns minimum hold time (th) for bus-to-clock synchronization at 5 V.

Key Specifications

Pinout & Package

MC74ACT646N is housed in a 24-lead 300-mil plastic dual in-line package (PDIP), pin-compatible with industry-standard SOIC-24 footprints but with through-hole mounting. Thermal resistance θJA is 59.8 °C/W.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MC74ACT646N, SN74ACT646N offers identical performance and drop-in compatibility, while MC74ACT245N trades register capability for faster real-time throughput-making it suitable only where clocked data retention is not required.

Availability

MC74ACT646N is available at Aetrix Electronics and suitable for memory interface design, industrial backplane expansion, test equipment signal routing, and legacy microprocessor system upgrades requiring stable component supply and long-term obsolescence management.

Supply support for MC74ACT646N 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 focused on energy-efficient power management, analog, sensors, and logic solutions for automotive, industrial, and cloud infrastructure markets.

The MC74ACT646N belongs to the 74ACT advanced CMOS logic family, designed specifically for high-speed, TTL-compatible interfacing in industrial control, instrumentation, and legacy computing systems requiring robust noise immunity and 5 V operation.

FAQ

What is the operating voltage range for the MC74ACT646N?

The MC74ACT646N operates strictly within 4.5 V to 5.5 V. It is not rated for 3.3 V or lower supply voltages, and its TTL-compatible inputs require VIH ≥ 2.0 V and VIL ≤ 0.8 V at 5 V - attempting to run outside this range risks undefined logic states or damage. The MC74ACT646N datasheet specifies no guaranteed operation below 4.5 V, and maximum ratings prohibit VCC < −0.5 V or > +7.0 V.

Does the MC74ACT646N support both real-time and stored data transfer modes?

Yes, the MC74ACT646N supports four distinct data-handling functions via SAB/SBA control: real-time A→B, real-time B→A, stored-A→B, and stored-B→A. Data is loaded into internal D-type registers on LOW-to-HIGH clock transitions (CAB or CBA), and the SAB/SBA inputs determine whether outputs reflect live bus values or registered content - a capability confirmed in the Function Table and Figures 1–4 of the MC74ACT646N datasheet.

What is the maximum clock frequency supported by the MC74ACT646N?

The MC74ACT646N does not specify a maximum clock frequency directly, but its AC characteristics define worst-case propagation delays (e.g., tPLH/tPHL ≤ 16.0 ns clock-to-bus) and minimum pulse widths (tw ≥ 7.0 ns). Based on these, reliable operation is achievable up to approximately 30 MHz in well-terminated 50 pF load conditions - though actual system frequency depends on board layout, fanout, and timing margins. The MC74ACT646N is not characterized for operation beyond its specified AC limits.

Can the MC74ACT646N be used in place of the MC74AC646N?

No - the MC74ACT646N and MC74AC646N are not interchangeable without design review. While pinout and basic function match, the MC74ACT646N has TTL-compatible inputs (VIH = 2.0 V min), whereas the MC74AC646N requires higher VIH (3.15 V min at 4.5 V) and operates down to 2.0 V supply. Substituting MC74ACT646N into an AC-designed system may cause false triggering; substituting MC74AC646N into an ACT system risks input undervoltage failure. The MC74ACT646N is optimized for 5 V TTL interfacing only.

Is the MC74ACT646N available in surface-mount packaging?

Yes - the MC74ACT646N designation refers specifically to the 24-pin PDIP (plastic dual in-line package) variant. However, onsemi also offers the same logic function in SOIC-24 (MC74ACT646DWG) and tape-and-reel (MC74ACT646DWR2G) formats. These surface-mount versions share identical electrical specifications and timing with the MC74ACT646N but differ mechanically - PCB layout must accommodate SOIC-24 footprint and thermal profile, not the through-hole PDIP pattern used by MC74ACT646N.