How does a programmable logic controller work?

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A Programmable Logic Controller repeats the same scan cycle over and over so that industrial equipment reacts without delay. The cycle divides into four fixed stages that finish within a few milliseconds. The Input Scan stage arrives - the PLC examines every sensor, switch and field device connected to its terminals. It records each on/off or voltage value in a reserved section of memory called the input image table. The Program Execution stage follows - the central processor retrieves the stored ladder logic or function block diagram. It tests the AND, OR besides NOT conditions against the freshly saved input states. When a rung or block evaluates as true, the processor sets the corresponding bit that commands an output. After the decision logic ends, the Output Update stage begins. The processor copies the new output bit pattern into the output image table. Electric signals leave the output modules plus reach actuators, motors, valves or alarm devices within the plant. The fourth stage is Housekeeping - the controller runs built in diagnostics, answers network requests and verifies its own hardware. When those checks finish, the sequence returns to the Input Scan stage and repeats forever. Because the loop duration stays almost constant, engineers predict the exact response time for each machine. Analog sensors feed voltage or current loops into I/O modules - the modules translate those levels into 16-bit or 32-bit digital numbers. The CPU stores data in RAM but also holds the operating system in ROM. A metal enclosure, wide temperature components and continuous self-monitoring let the unit operate in dust, heat and vibration while it detects its own faults.

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