With the EN (enable)-input at blocks it can be specified whether an operation shall be executed or not. Having function- and function block-calls this means whether the function/the function block is called or not.
If the EN-input is connected the operation will only be executed if a ‘1’ is next to it. How this is executed internally you will see easiest if you compare the FBD with the STL-display:
A I 0.0
JNB _002
L L#90
L L#67
+D
T MD 10
_002: NOP 0
If the input I 0.0 is not ‘1’ it will be jumped to the mark _002 because of the JNB, that means nothing happens. The NOP 0 is necessary here because after a jump label there has to be at least the no-operation.
If the EN-input stays unconnected the operation is executed absolutely. You can detect an unconnected input due to the fact that the operand starts with a ‘?’. If you have defined a symbol that starts with a ‘?’ you will have to put it in double inverted commas. Here is the comparison between the FBD- and the STL-display.
L L#90
L L#67
+D
T MD 10
NOP 0
Here no jump exists, so the addition will be executed in any case. The NOP 0 is only necessary here because of the retranslation from STL to FBD.
See also: