Chapter 14 Sequential Function Chart When users create a transition program, a symbol name which is the same as the transition program name is added to the program code of the transition program. The users only have to create a conditional operation. Owing to the fact that the actions and the transitions in a sequential function chart share the same local symbol table, there are not local symbol tables in the program editing windows for the transitions. Users can not create a complex transition program.
If a transition condition must undergo a complex operation, users can create the complex operation in an action associated with a step, create a condition flag in the action, and assign the flag to the transition following the step. Although users do not need to declare the symbols in the transition programs, the system assigns memories which function as flags to the symbols. As a result, users have to make sure that the symbols in the transition programs can be assigned a specific logic state. Otherwise, an error will occur if the program is executed. Take the transition program which is created by means of a structured text below for example.
During the execution of the transition program for the first time, Tran4 will be ON if the result of ( M0 AND M1 ) is ON. Since Tran4 is ON, it passes control to the next step. After the control is passed to the step preceding the transition statement following THEN will not be executed. Tran4 will still be ON because it is not updated. The system will program again, the transition program will be executed again. If the result of ( M0 AND M1 ) is OFF, the evaluate the transition condition as TRUE, and the control will be passed to the next step. Adding an ELSE section to the transition program below is a better way to ensure that the symbol Tran4 is assigned a specific logic state.
Users also have to pay attention to the same problem in a CASE sttructure. 14.2.3 Simultaneous Divergence and Divergence of Sequence Selection Users can create divergent paths in a sequential function chart. There are two types of divergences. One is a simultaneous divergence, and the other is a divergence of sequence selection Simultaneous divergence A simultaneous divergence is shown below. The divergent steps are connected by a double line. The characteristic of a simultaneous divergence is that the divergent steps share the same transition. When the transition is true, the steps are active. When Tran0 is true, STEP_0 becomes inactive, and STEP_1 and STEP_2 become active. 14-9