PLC, PAC, and Industrial PC Architectures for Automation

PLCs, PACs, and industrial PCs form the core of new industrial automation infrastructure

The industrial automation landscape is defined by deterministic control logic and the increasing data-intensiveness of digital transformation. To support these modern infrastructure needs, various controller architectures are used, such as programmable logic controllers (PLCs), programmable automation controllers (PACs), and industrial PCs.

However, as technology advances, the boundaries between these architectures have blurred. This creates a gray zone in which industrial controller selection depends more on the specific engineering constraints of the deployment. This blog does not aim to declare the best controller for industrial automation infrastructure, but rather to provide a decision framework guided by engineering requirements and trade-offs.

Understanding the Execution Model

The execution model is at the core of an industrial automation system, controlling how processors interact with external physical triggers. The choice among scan-based, task-based, and event-driven models will determine the degree of determinism and their resilience to timing drift.

PLCs operate on a scan-based execution model: They scan the input, execute the program, and update the output. This model provides cyclical determinism, ensuring that each logic instruction is executed exactly once per cycle and that input and output (I/O) states remain consistent throughout execution.

The main advantage of the scan-based execution model is its predictability. For instance, a control engineer can calculate the exact maximum response time for the system. However, a limitation is that the logic execution is tied to the slowest part of the cycle. If communication tasks are not isolated or prioritized appropriately, they can extend the effective scan time and affect control responsiveness.

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