The pragma defines the initialization order of programming blocks and global variable lists.
Variables in a list (GVL or POU) are initialized from top to bottom.
If there are several global variable lists, then the initialization order is not defined.
The initialization does not apply for the initialization of literal values, for example
1, 'hello', 3.6, or constants of base data types. However, you must define the initialization order
yourself if there are dependencies between the lists. You can assign a defined initialization
slot to a GVL or POU with the 'global_init_slot' attribute.
Constants are initialized before the variables and in the same order as the variables. During initialization, the POUs are sorted according to the value for <slot>. Then the code for initializing the constants is generated and afterwards the code for initializing the variables.
Syntax:
{attribute 'global_init_slot' := '<slot>'}
<slot>: Integer value that defines the position in the call order. The default value for
a POU (program, function block) is 50000. The default value for a GVL is 49990. A
lower value means an earlier initialization. Caution: If several blocks or GVLs receive
the same value for the 'global_init_slot' attribute, then the initialization order remains undefined.
Insert location: The pragma always affects the entire GVL or POU and therefore it
must be located above the VAR_GLOBAL or POU declaration.
If several programming blocks have got assigned the same value for the attribute 'global_init_slot', the order of their initialization remains undefined.
Example
The program includes two global variable lists GVL_1 and GVL_2, as well as a PLC_PRG program that uses variables from both lists. GVL_1 uses the variable B for initializing a variable A, which is initialized in GVL_2 with a value of 1000.
GVL_1
VAR_GLOBAL //49990 A : INT := GVL_2.B*100; END_VAR
GVL_2
VAR_GLOBAL //49990 B : INT := 1000; C : INT := 10; END_VAR
PLC_PRG
PROGRAM PLC_PRG //50000 VAR ivar: INT := GVL_1.A; ivar2: INT; END_VAR ivar:=ivar+1; ivar2:=GVL_2.C;
In this case, the compiler prints an error because GVL_2.B is used for initializing GVL_1.A before GVL_2 has been initialized. You can prevent this by using the global_init_slot attribute to position GVL_2 before GVL_1 in the initialization sequence.
In this example, GVL_1 must have at least one slot value of 49989 in order to achieve the earliest initialization
within the program. Every lower value has the same effect:
GVL_2
{attribute 'global_init_slot' := '100'}
VAR_GLOBAL
B : INT := 1000;
END_VAR
Using GVL_2.C in the implementation part of PLC_PRG is also not critical even without using a pragma because both GVLs are initialized
before the program in either case.