General

The program for a mono task application is associated with a single user task, the master task.

You can choose the type of master task execution:

  • cyclic

  • periodic

Illustration

The following illustration shows the operating cycle.

Description of the Different Phases

The table below describes the operating phases.

Phase

Description

Acquisition of inputs

Writing to memory of the status of the data on the inputs of the discrete and application-specific modules associated with the task,

These values can be modified by forcing values.

Program processing

Execution of application program, written by the user,

Updating of outputs

Writing of output bits or words to the discrete or application-specific modules associated with the task depending on the state defined by the application.

As for the inputs, the values written to the outputs can be modified by forcing values.
NOTE: During the input acquisition and output update phases, the system also implicitly monitors the PLC (management of system bits and words, updating of current values of the real time clock, updating of status LEDs and LCD screens (not for Modicon M340), detection of changes between RUN/STOP, etc.) and the processing of requests from the terminal (modifications and animation).

Operating Mode

PLC in RUN, the processor carries out internal processing, input acquisition, processing of the application program and the updating of outputs in that order.

PLC in STOP, the processor carries out:

  • internal processing,

  • input acquisition (1),

  • and depending on the chosen configuration:

    • fallback mode: the outputs are set to fallback position.

    • maintain mode: the last value of the outputs is maintained.

(1) For Quantum PLCs, input acquisition is inhibited when the PLC is in STOP.

NOTE: For information about inhibiting and activating tasks using system bits refer to Task Control.