PLCs

Programmable Logic Controllers (PLCs) are rugged digital computers used for automation of electromechanical processes, such as control of machinery on factory floors. They monitor inputs from sensors and devices, execute control instructions, and drive outputs to actuators. PLCs are designed for real-time operation and high reliability in harsh industrial environments.

A typical PLC consists of a central processing unit, memory, input/output (I/O) modules, power supply, and communication interfaces. I/O modules provide digital and analog signals; digital inputs sense on/off states; digital outputs switch relays or power devices; analog modules handle variable voltages or currents.

Operation and programming: PLCs operate on a scan cycle: read inputs, execute user program, update outputs. They support multiple programming languages standardized in IEC 61131-3, including Ladder Diagram, Function Block Diagram, Structured Text, and Sequential Function Chart; Instruction List is deprecated. Programs run in cyclic tasks with real-time constraints; scheduling supports multitasking and interrupts on some models.

Networks and interoperability: PLCs communicate via industrial protocols such as Modbus, Profibus, EtherNet/IP, PROFINET, DeviceNet, and OPC UA. They can be integrated with HMIs, SCADA, and distributed control systems; safety PLCs provide redundant processing and fault detection per standards like IEC 61508 and ISO 13849 for functional safety.

Applications and history: Originating in the late 1960s to replace large relay-based control systems, PLCs have evolved into compact controllers and distributed architectures used in manufacturing, packaging, robotics, water and waste treatment, energy, and process industries.