Rules for the order

1. First logi.CAD 3 checks whether there are networks that can be evaluated according to the →IEC-standard.
   Networks can be evaluated, if none of its inputs depends on a network not yet evaluated. Mind the following: Only if inputs for a network are used by other networks as output, the network cannot be evaluated at first.
   If all inputs of the network are part of an explicit →feedback loop within the network, this network is put on hold regarding its evaluation after the evaluation of all networks that can be processed. Vice versa the following applies: logi.CAD 3 does not put a network on hold, if this network contains at least one input that can be evaluated or one input that is part or an implicit feedback loop (see example 3 below).
   

   Quoting the IEC-standard

   • No element of a network shall be evaluated until the states of all of its inputs have been evaluated.

   • The evaluation of a network element shall not be complete until the states of all of its outputs have been evaluated.

   • The evaluation of a network is not complete until the outputs of all of its elements have been evaluated, even if the network contains one of the execution control elements.

   Implementer-specific realization

   The IEC-standard defines that i t shall be possible for the user to utilize an implementer-specific means to determine the order of execution of the elements in an explicit →feedback loop, for instance by selection of a feedback variable.

   Therefore, logi.CAD 3 attempts to resolve a feedback loop using feedback variables. But if there is no variable in the feedback loop, the calls are used to resolve the feedback loop.

   Subsequently, logi.cals recommends that you use such feedback variables in feedback loops. On the one hand, you can better control the execution order of the network elements by using a feedback variable. And on the other hand, you remain compliant with the IEC-standard, since feedback loops without a feedback variable are not considered in the IEC-standard.

   Mind the following: If a variable is used as array index itself or within the array index, logi.CAD 3 considers this variable as an input as well.

2. If logi.CAD 3 determines that

   • only one of the networks can be evaluated, the statements of this network are evaluated.

   • several networks or not one of the networks can be evaluated, logi.CAD 3 determines the graphical position of these networks. Networks are ordered from "top left to bottom right". logi.CAD 3 evaluates the statements of the network with the statement most top/left (top is applied before left).
     Mind that there are 2 types of statements:

   • Call statements (short: →calls): Calls are created by positioning a →function block or a →function.

     Here you see calls of the SUB function, AND function and TON function block respectively:
     
     For calls, the crucial factor is the upper left corner of the call (without a possibly displayed instance name).

       Examples

     The call of the upper NOT block ranks before the lower one (top is applied before left):
     The call of the left NOT block ranks before the right one:

     In principle, SFC elements of an →SFC network behave like calls within the network. But observe that SFC elements are evaluated according to predefined rules of evolution

   • Assignment statements (short: →assignments): Assignments are created by positioning a value field and by connecting it with a call or another value field.

     Here you see assignments to the variable var3 and var5 respectively:
     
     For assignments, the crucial factor is the position of the input of the value field.

       Examples

     The value field with var1 is the statement most top/left. Hence, the statements of the network with the pink value fields are evaluated before the ones of the network with the blue value fields.
     The value field with var4 is the statement most top/left. Hence, the statements of the network with the blue value fields are evaluated before the ones of the network with the pink value fields.
     The value field with var5 is the statement most top/left. Hence, the statements of the network with the blue value fields are evaluated before the ones of the network with the pink value fields.
     The value field with var5 is still the statement most top/left although the value field with var8 seems to be positioned more top. The crucial factor is that the input of the value field with var5 is most top/left. Hence, the statements of the network with the blue value fields are evaluated before the ones of the network with the pink value fields.

After the network has been evaluated completely, the rules are applied again to determine the next network to be evaluated (starting with the 1. step).

Examples for the order

The order of the networks can be deduced from the execution order (= number displayed within the red rectangle) that is shown for the statements of the networks in the examples.