From AC31's Backbone to the AC500 Bridge: The Tale of PM 581 and PM 581-ETH

From AC31's Backbone to the AC500 Bridge: The Tale of PM 581 and PM 581-ETH

Q: In the landscape of industrial automation, what made the ABB PM 581 such a foundational component within the AC31 series?

A: The PM 581 was far more than just a CPU—it served as the computational heartbeat of the AC31 platform. Engineered to deliver deterministic performance in demanding environments, it executed control logic, managed complex data, and seamlessly coordinated I/O and special function modules. Its robust design and consistent reliability made it the preferred choice for mid- to high-complexity applications, from manufacturing lines to process control systems, establishing it as a trusted core within countless automated operations over decades of service.

The PM 581-ETH, while sharing its predecessor’s name, fulfills a distinctly different role in ABB’s modern AC500 ecosystem. Designed as a high-performance communication co-processor, it extends system capabilities through integrated Ethernet connectivity, enabling advanced peer-to-peer communication and large-scale fieldbus data handling. Rather than replacing the classic PM 581, it represents an evolution in functionality—bridging legacy reliability with modern network demands.

At its core, the classic PM 581 embodied durability and precision. Its modular design allowed secure integration into AC31 racks, where it operated via a dedicated backplane bus. Using ABB’s Control Builder software, engineers deployed logic—from ladder diagrams to structured text—that the PM 581 executed through continuous scan cycles. It supported extensive I/O configurations across local and expansion racks and, with add-on modules like the TX 373, could interface with early Ethernet and fieldbus networks for supervisory control.

Balancing processing power, memory capacity, and cost, the PM 581 excelled in sequential control, PID regulation, and data tasks. While more advanced CPUs like the PM 591 were later introduced, the PM 581 remained the optimal choice for a wide spectrum of applications—cementing not only the success of the AC31 platform but also a legacy of steadfast performance that continues to resonate in industrial automation today.

To clearly delineate the fundamental differences between the classic AC31 CPU and its modern AC500 namesake, the following table compares their key parameters

It is precisely because the original PM 581 was so successful that ABB chose to reuse the identifier in its next-generation platform. The PM 581-ETH, however, is not a direct replacement part but a strategic nod to the past, carrying the legacy model number into a new era of networked automation. Users familiar with the robustness associated with the PM 581 name would find a corresponding level of performance and reliability in the communication domain with the PM 581-ETH module. This creates a technical and sentimental bridge, easing the transition for engineers moving from the venerable AC31 systems to the modern, feature-rich AC500 world.

In summary, the market position and importance of the original PM 581 cannot be overstated. It was a definitive module for the ABB AC31 series, representing its peak in terms of widespread application and user base reliability. As the pragmatic, powerful heart of so many installed systems, the PM 581 became synonymous with the AC31 platform itself for a generation of automation engineers. Its enduring presence in the field, even years after discontinuation, is a testament to its solid design. The subsequent introduction of the PM 581-ETH for the AC500 series serves as a deliberate homage, strategically linking the trusted performance of the past with the connected, future-proof capabilities of ABB's current automation portfolio.

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