A PLC scan cycle executes a ladder logic program continuously by reading input signals, processing the logic, updating output states, and repeating the sequence within milliseconds. This rapid cycle ensures reliable and real-time control of industrial equipment.
Motor Start/Stop Operation
In a typical motor control circuit, pressing the Start pushbutton energizes the contactor coil, causing the motor to start. A seal-in (holding) contact then closes in parallel with the Start button to keep the coil energized even after the button is released. When the Stop pushbutton is pressed, the circuit opens, de-energizing the contactor coil, breaking the seal-in path, and stopping the motor immediately.
PLC Scan Cycle Sequence
The scan cycle follows a logical sequence of input scanning, program execution, output updating, and repeat scanning. Each cycle evaluates the ladder logic, updates the motor status, and responds instantly to changes in the Start or Stop pushbuttons, ensuring safe and efficient machine operation.
Applications
This scan cycle is widely used in motor starters, conveyor systems, pumps, compressors, manufacturing automation, and industrial process control where dependable Start/Stop functionality is essential.