Monitoring temperature, pressure, and fluid levels in chemical tanks, using feedback loops to adjust valves and heaters.
The PLC scan cycle consists of four distinct steps performed repeatedly:
: The brain that executes software instructions and manages memory.
I/O modules bridge the gap between the PLC’s processor and the physical world. Handles ON/OFF signals from devices like pushbuttons, limit
Handles ON/OFF signals from devices like pushbuttons, limit switches, and proximity sensors.
+----------------------------------------+ | 1. Input Status Scan | | (Reads physical inputs into memory) | +----------------------------------------+ | v +----------------------------------------+ | 2. Program Execution | | (Evaluates logic top-to-bottom) | +----------------------------------------+ | v +----------------------------------------+ | 3. Diagnostics & Communications | | (Checks hardware, updates networks) | +----------------------------------------+ | v +----------------------------------------+ | 4. Output Status Scan | | (Updates physical devices from memory) | +----------------------------------------+ | +--- Loops back to Step 1
: Timing functions, counters, and basic math and logic operations. Program Execution | | (Evaluates logic top-to-bottom) |
Ladder logic uses two vertical rails representing power and a series of horizontal lines called "rungs." The primary symbols mimic electrical contacts:
Programmable Logic Controllers (PLCs) are the backbone of modern industrial automation. Whether you are a student, technician, or engineer, mastering PLC principles is crucial for designing and troubleshooting automated systems. One of the most foundational and highly regarded textbooks in this field is .
+-------------------------------------------------------+ | 1. Input Scan | | (Reads the status of all connected input devices) | +-------------------------------------------------------+ | v +-------------------------------------------------------+ | 2. Program Scan | | (Executes the logic starting from the top to bottom) | +-------------------------------------------------------+ | v +-------------------------------------------------------+ | 3. Output Scan | | (Updates the state of all connected output devices) | +-------------------------------------------------------+ | +--<-- Loop Repeats Continuously proximity sensors (inputs)
Where the application code (e.g., ladder logic) and status files (input/output states, timers, counters) are stored. This is typically backed up by batteries or stored in non-volatile flash memory to prevent data loss during power outages. Input/Output (I/O) Modules
Implementing sequencing, sensors, and sorting logic.
Handles binary signals (ON/OFF). Examples include pushbuttons, limit switches, proximity sensors (inputs), and solenoid valves, indicator lights, motor starters (outputs).
The two vertical lines representing the power source.