Sunday, January 23, 2011

PLC Models with Implicit Scan Cycle

By an implicit scan cycle we mean that the cyclical behavior is modeled, but the duration of each scan cycle is not considered. We consider as the following examples:
1. It s the case that each program execution takes exactly the same time for older PLCs. There is program memory for a fixed instructions number that are all executed in each scan cycle. The remaining space is filled with skip instructions for shorter programs.
2. A similar behavior can be found in modern PLCs, an alternative program execution mode to the cyclical one as described above is a periodical execution mode. The execution program is started periodically with fixed time intervals, where the interval should be longer than one program execution.
3. Worst case behavior is relevant for some applications, in the sense that only maximal delays could do harm. The worst case assumption is that each scan cycle takes maximal time. The worst case analysis then can be done on a model with fixed scan cycle time of maximal length.
4. An operation mode typically of PLCs is that there are several programs running in parallel on one PLC. In this case the execution mechanism of the parallel programs is synchronous, e.g. in each scan cycle program is executed once. For the analysis of the behavior synchronous execution the length of each scan cycle is irrelevant. Synchronous model approaches are well suited to analyze this kind of applications.

For these implicit scan examples assumption can be applied only if the program execution is considered alone without combining it with an environment model. If the PLC behavior and environment has to be analyzed, then the environment model has to contain an explicit clock tick at which it synchronizes with the program. For instances above this clock tick is in fixed time intervals, in case the time between clock tick may possibly vary.

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