- The operating system starts cycling and monitoring the time.
- The CPU begins reading data from the input modules and checks the status of all inputs.
- The CPU begins executing user or application programs written in relay ladder logic or any other PLC programming language.
- Next, the CPU performs all internal diagnostic and communication tasks.
- Based on the results of the program, all outputs are updated by writing data to the output module.
- This process continues as long as the PLC is in Run mode.
There is an agitator motor set up to automatically agitate the liquid in the vessel when the temperature and pressure reach preset values. In addition, a separate manual pushbutton station is used to operate the motor. The process is monitored by pressure sensor switches and temperature sensor switches. These switches close their respective contacts when the conditions reach their preset values.
Input field devices such as pressure switches, temperature switches, and manual pushbuttons are hardwired to the appropriate input modules, while output devices such as motor start coils are hardwired to the appropriate output modules of the PLC. The system continuously scans all inputs for preset values according to the program in the PLC and updates the outputs accordingly, i.e., when both the pressure and temperature switches are energized or the pushbuttons are energized, the PLC automatically energizes the coils for the motor starters so that agitation action can occur.