A PLC (Programmable Logic Controller) is a control computer designed for the control of machines and processes in the industrial environment. It reads the signals coming from the field, makes decisions according to the program loaded into it, and drives its outputs accordingly. It became widespread by replacing relay-based control panels and has today become the fundamental control layer of automation.
What distinguishes a PLC from an industrial computer
The hardware of a PLC is designed for a different goal than an office computer: uninterrupted operation, resistance to electrical noise, a wide temperature range, and deterministic behavior. “Deterministic” is the critical word here — how long an operation will take to complete must be predictable. A general-purpose operating system does not provide this guarantee.
Basic components
CPU (processor module): Runs the program, manages memory and communication. The capacity of the program memory and data memory varies with the CPU level.
Power supply: Usually 24 V DC. Supplies the modules. The supply requirement of the field devices must also be included in this calculation.
Digital input/output modules: On/off type signals. Button, sensor, and limit switch inputs; contactor, valve, and lamp outputs.
Analog input/output modules: Continuously varying quantities. Signal types such as 0–10 V, 4–20 mA, and PT100. The resolution (number of bits) directly determines the measurement precision.
Communication modules: Data exchange with other PLCs, HMIs, drives, and higher-level systems. Most modern CPUs have a built-in Ethernet port.
Special modules: Modules that carry out operations such as high-speed counting, motion, weighing, and temperature control independently of the CPU.
Scan cycle
The PLC program does not run once and stop; it processes within a continuously repeating loop. The classic cycle consists of the following steps:
1. Reading the input image — All input states are copied to memory at once. 2. Program execution — The program runs on this memory image. 3. Writing the output image — The calculated output states are transferred to the physical outputs. 4. System tasks — Communication, diagnostics, watchdog check.
This structure has an important consequence: even if an input changes in the middle of the program, the PLC sees the old value throughout that cycle. The change is only reflected in the next cycle. For this reason the scan time determines the lower bound of the system’s response speed. Fast signals (high-frequency encoder pulses, short-duration sensor signals) are not left to the scan cycle; a hardware high-speed counter input or an interrupt structure is used.
Programming languages
The IEC 61131-3 standard widely defines five languages:
- LD (Ladder Diagram) — Similar to a relay schematic, the language most easily
- FBD (Function Block Diagram) — Connecting function blocks.
- ST (Structured Text) — Text-based, similar to a high-level language.
- IL (Instruction List) — Assembly-like. Not preferred
- SFC (Sequential Function Chart) — Defines sequential
read by electricians. Common for control logic.
It is legible for analog processing and process control.
Suitable for calculation, loops, and data processing.
in new projects.
processes with a step/transition structure.
In practice it is not necessary to choose a single language; in the same project the control logic can be written in ladder and the calculations in ST.
Criteria examined when selecting a PLC
I/O count and type: As much as the current requirement, plus a margin for future expansion. The number of analog channels and the signal types are listed separately.
Scan time requirement: The function of the machine that must respond the fastest is decisive. In applications requiring sub-millisecond response, the controller level and hardware support are evaluated together.
Motion requirement: The number of axes and the need for synchronization between axes (cam, electronic gearing) directly determines the controller class.
Communication protocol: Which protocol do the existing devices in the field support? A choice such as EtherCAT, Profinet, or Modbus TCP is decided together with the network topology.
Memory and data management: The number of recipes, production data recording, and logging requirements increase the memory requirement.
Service and spare parts: In long-life machines, the product’s life cycle and the availability of spare parts are as important as the technical specifications.
Software environment and license: The accessibility of the programming software determines whether the maintenance team can later intervene in the machine.
Points to watch in application
The compatibility of the sink/source structure of the inputs with the field device, whether the outputs are relay or transistor (switching speed and load type), shielded cable and single-point grounding on analog signals, thermal management inside the enclosure — these are matters that must be decided during the project phase and are costly to correct later.
Related product families: PLC, Digital I/O, Analog I/O, Remote I/O, Communication Modules, HMI
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