Selecting a PLC Platform for Modern Industrial Automation
(Source: Mia/stock.adobe.com)
Published June 3, 2026
Selecting a programmable logic controller (PLC) platform is one of the most consequential decisions in the life cycle of an industrial automation system. Engineers often base this decision on immediate capital expenditure constraints and legacy vendor relationships. The repercussions of this choice can extend beyond decades of operational expenses, maintenance issues, and scalability bottlenecks.
Today, control system engineers face a paradox of specifying hardware for a production environment that will continue to change over several years. The industrial controller selected for a greenfield facility must operate reliably in the plant-floor hardware environment and remain relevant in an era of rapid convergence of information technology (IT) and operational technology (OT).
In our previous blog in this PLC series, we laid out the constraints that guide engineers in evaluating different industrial controller architectures. Here, we outline an engineering-driven decision framework for selecting a PLC platform that meets a project’s specific control requirements, integration needs, and long-term constraints.
Control Requirements
The fundamental requirement of any industrial controller is determinism. This ensures that the control system responds to a specific set of inputs with a corresponding set of outputs within a defined, predictable time window. Designers must consider how the PLC’s runtime schedules tasks and synchronizes input/output (I/O) updates. In practice, PLCs may use a main scan task (e.g., continuous scan) plus periodic and/or event-driven tasks, so it’s important to understand the execution model and timing guarantees.
The scan task is a repeating loop that reads inputs, executes logic, and writes outputs at a cycle time determined by instruction execution time and system overhead. For instance, if the code contains conditional jumps that change with process state, scan time will vary. Even when a continuous task interacts with a high-frequency input without synchronization, aliasing occurs. To achieve tighter determinism, high-performance architectures rely on time-driven or periodic task execution.
In periodic execution, the real-time operating system (RTOS) uses a preemptive scheduler to interrupt lower-priority tasks and execute critical control loops at a fixed interval. However, the risk in periodic architectures is overrun. For example, if a task is scheduled every 10ms but inefficient programming or memory latency results in a worst-case execution time of 11ms, the watchdog timer may trip. This requires the engineer to budget execution time and leave headroom to account for worst-case execution paths.
For applications that demand an immediate reaction to physical input, such as detecting a product on a conveyor, the PLC must support low-latency hardware interrupts. To meet this requirement, event tasks are triggered by specific hardware events rather than by clock cycles. This approach offloads detection from the main scan and allows the central processing unit (CPU) to interrupt execution immediately to run the corresponding reaction logic.
Programming Environment
Engineering workflow efficiency in new industrial automation infrastructure depends on how developers write, simulate, debug, and maintain code. These factors have become some of the most significant variables in the total cost of ownership.
Modern PLCs can be programmed using the IEC 61131-3 standard languages, which include Ladder Diagram (LD), Function Block Diagram (FBD), and Structured Text (ST). LD, also known as ladder logic, is popular for discrete logic because it is easy to understand, while ST is a high-level, code-like language used for array handling and complex logic. FBDs are ideal for process controls and are often used in safety logic configurations.
Engineers must verify which languages are available on the specific PLC under consideration and confirm if there are any limitations. For example, a smaller micro-PLC might support only LD or a vendor-specific functional block language, limiting the complexity of the algorithms implemented. Not every PLC supports all languages, but having at least LD, ST, and FBD provides sufficient flexibility.
There are times when the PLC’s integrated development environment (IDE) becomes more important than the raw CPU specifications. Ideal PLC development software should provide online monitoring, stepping, breakpoints, and simulation capabilities. Because these factors can affect development time and troubleshooting, engineers must consider how easy it is to debug the PLC in the field.
Scalability Constraints
When defining CPU and memory requirements, engineers must not only meet but exceed the immediate requirements within reasonable bounds. The aim should be to run the PLC at a moderate load to provide sufficient headroom for transient spikes and future additions. If the PLC is near 100 percent usage at startup, even small changes could lead to scan overruns.
As systems evolve with tighter timing requirements and additional sensors, diagnostics, and communications, the PLC may not be able to handle the load. Under-specifying the CPU is a common long-term mistake engineers make during development. For example, older PLCs may have been sufficient for basic discrete logic, but when engineers try to add proportional-integral-derivative (PID) control or data logging, the CPU cycle time increases significantly.
Memory constraint is another important aspect affecting scalability. Engineers must choose a memory size that is much larger than the initial requirement. Most control systems have a 10- to 20-year life cycle, and engineers often want to add improvements over time. Therefore, it is much easier to implement such additions if the PLC has spare performance capacity.
Engineers must also determine whether the PLC can be upgraded to a higher-performance CPU without requiring a full system replacement, and whether the controller supports multithreading. The goal is to understand how the PLC handles multiple tasks and distributed processing to enable the addition of advanced sensors or an artificial intelligence (AI) module. These additions become increasingly relevant in large industrial automation systems.
Integration and Ecosystem Fit
No PLC operates in isolation but is part of a larger automation infrastructure. Nearly every PLC communicates with an operator interface, whether it’s a simple text display, a complete supervisory control and data acquisition (SCADA) system, or an Industrial Internet of Things (IIoT) gateway. Engineers must ensure that the PLC supports common communication protocols for these upper layers. Standard protocols include Open Platform Communications Unified Architecture (OPC UA), Modbus Transmission Control Protocol (TCP), and proprietary protocols such as EtherNet/IP and PROFINET.
Therefore, choosing a PLC that is interoperable with other PLCs is essential. For example, the project might need to integrate a new PLC with an old one that uses only Modbus remote terminal unit (RTU). The new PLC should be able to host a serial Modbus module and act as the master to the old PLC to receive data.
Vendor lock-in is another aspect to consider, and it can have practical implications. Proprietary platforms can limit expansion and support opportunities. Alternatively, a PLC that adheres to widely used industry standards can integrate components from different vendors and migrate if needed.
Conclusion
The PLC has become a core functional unit of the control system. Therefore, the controller must be task-specific, operationally reliable, and resistant to future bottlenecks. By considering factors such as raw specifications, programming environment, performance headroom, and integration, engineers can select a PLC platform that will reliably support the industrial automation systems of tomorrow.
In the final part of this PLC blog series, we look at virtual PLCs and how they enhance scalability by eliminating vendor lock-in.