Chapter 14 Sequential Function Chart 14.2.7 Initial Step During the execution of a sequential function chart, every step is activated in turn and cyclically. It is necessary to define a step as an initial step which is activated when the program is executed for the first time. In ISPSoft, the initial step in a sequential function chart is drawn as a box with a double line. Besides, there is only one initial step in a sequential function chart. Any step in a sequential function chart in an AH/AS series CPU module can be defined as the initial step in the sequential function chart.
When a sequential function chart in an AH/AS series CPU module is executed for the first time during the operation of the CPU module, the initial step in the sequential function chart is the step which is activated first. If there is a sequential function chart in a DVP series PLC, the sequential function chart will not be executed until the instruction SET used in another POU activate a step in the sequential function chart. Users can not define a step in a sequential function chart in a DVP series PLC as the initial step in the sequential function chart, and the first step in a sequential function chart in a DVP series PLC is always the initial step in the sequential function chart.
The real initial step in a sequential function chart in a DVP series PLC is the device or the symbol driven by the instruction SET used in another POU. An example of executing a sequential function chart in DVP series PLC is shown below. Users have to determine which step in a sequential function chart is the initial step in the sequential function chart, declare the step in the global symbol table, and apply the instruction SET to the step in another POU. In the program code where the instruction SET is applied to the step, the condition contact or the logical output preceding the step must be prevented from becoming ON again during the execution of the program.
Otherwise, the step will be ON, and the sequential function chart will be executed incorrectly. The special relay M1002 is highly recommended. The characteristic of M1002 is that M1002 is ON during the first scan cycle. 14-21