PLC behavior can be altered either by user intervention or by a fault. In this case, the process control channels follow a preset downgraded functioning mode. The following PLC operating modes can alter process control operations.
On PLC power-up, the system searches for a valid application in the user memory space.
If the application is valid, the system changes to a state of configuration and each process control channel is called. The channel context is then set on initial values which can be used on application.
If the application is not valid, the system changes to standby mode to wait for a reconfiguration request.
In RUN mode, the processor carries out successively on each cycle:
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the reading of input channels
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the execution of setpoint programs
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the execution of process control loops
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the processing of sequential programming
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the writing of outputs
All process control channels are called in each task cycle:
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Calculation of the process value, (PV), calculation of the Feed Forward (OUT_FF), alarm management, operating modes, setpoint programmers and generation of Manu or Tracking mode commands are all carried out on each cycle.
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Loop commands in Auto mode are generated and setpoints are calculated during the sampling period.
The process control channels do not directly initiate the STOP mode for the processor or the task. Switching to STOP mode means that all the current functions in progress stop.
The process control channels are no longer carried out. They refuse all commands (in Auto or Manu, etc.). The calculation results remain in their current state. Physical outputs take on the fallback value set in the configuration.
Inputs are always refreshed. Parameters may therefore be altered. The validity check is carried out on the next start-up.
A cold restart can be initiated in several ways:
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Changing the cartridge (cold start)
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A reconfiguration ( by loading a program, transferring a new application, etc.)
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A new configuration
A cold restart is indicated by the %S0 system bit.
The process control channels monitor their configuration, and initialize their parameters and status as from the first cycle. Algorithm processing is executed from the second cycle.
All commands generated in a sequential operation from the first cycle are accepted, except for orders to track or autotune on the loop controller. The command is refused.
A warm restart happens on a power cut followed by a power restart.
At the moment of the power cut, parameters are saved. The system and application contexts (e.g. application data, operating modes) are retained.
Any autotuning in progress will be aborted. The process control channels are carried out as from the first cycle.
