Variables/FB Instances: Declaring in Variables Worksheets
When editing code in the graphical IEC 61131 languages FBD or LD, variables and FB instances can be declared while inserting them into the code with the 'Variable' dialog. This way, declaration and insertion are done within one step. Nevertheless, it is also possible to first declare variables in the variables worksheet and then insert the declared variable into the FBD/LD code.
When programming in ST (Structured Text), variables/FB instances must first be declared in the variables worksheet and can then be inserted in the ST code.
This topic describes the manual declaration of variables and FB instances in the variables worksheet.
When editing a declaration, the system automatically verifies the grid line. If a particular declaration contains an error (e.g., the variable already exists or an initial value with an incorrect data type was defined), the corresponding grid entry is displayed in red.
Further Information
Refer to the related topics list below for further information on variables worksheets and the declaration rules that have to be observed while declaring or editing variables.
NOTE:
If a standard signal is connected to a physical input, the data type of the corresponding global variable must be modified from safety-related to standard (e.g., from SAFEBOOL to BOOL) to avoid an incorrect use of the signal in the code. The same applies if a safety-related signal is used only as standard signal in the code. Modifying the data type can either be done in the appropriate variables worksheet or using a type converter function.
How to declare a local variable, an FB instance or a global symbolic variable
What type of variable do you want to declare?
Local variable: open the local variables worksheet either by clicking the 'Toggle WS' icon on the toolbar while the related code worksheet is active or by double-clicking the respective variables worksheet icon in the project tree.
Global variable: open the global variables worksheet by clicking the 'Global decl.' icon on the toolbar.
If required, insert a new group.
Right-click on the group name into which the new declaration is to be inserted. Select 'New variable' from the context menu. This command is used for inserting both a variable and an FB instance declaration. Alternatively, select a particular group line and press <INS>.
Enter the variable/instance name. The name must be unique within its scope (local or global).
Select the data type in the 'Type' combo box of the new declaration.
Symbols in the combobox
The 'Data type' combo box provides the elementary data types as well as all available FB types for selection.
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Elementary standard (non-safety-related) data type |
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Elementary safety-related data type |
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User-defined function block used in the present project |
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Standard IEC 61131-defined function block |
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Safety-related function block |
When declaring a function block instance, the type of the instance must correspond to the FB type to be inserted.
Define the scope by selecting the 'Usage' from the corresponding combo box.
Symbols in the combobox
In the 'Usage' combo box, the variable declaration keyword can be selected. The list is context-sensitive. Only keywords are provided for selection which are allowed for the present POU type.
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VAR (local variable) |
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VAR_IN (input variable of a function block POU) |
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VAR_OUT (output variable of a function block POU) |
NOTE:
In the global variables worksheet, the 'Usage' column is not visible because the keyword is set to VAR_GLOBAL by default.
If you are declaring a function block instance only the keyword VAR is available.
When declaring a global symbolic variable in the global variables worksheet, the 'Usage' column is not visible because the keyword is set to VAR_GLOBAL by default.
By selecting the 'Feedback' checkbox you can declare a variable that can be used for programming an implicit feedback in the safety logic. The 'Feedback' flag can only be set in combination with 'Usage = VAR' and 'Scope = Local'.
When programming implicit feedbacks, the safety logic may be constantly preempted by the updating of the feedback, which may result in a complex timing of your entire application.
WARNING
UNINTENDED EQUIPMENT OPERATION
Verify the impact of programmed implicit feedbacks particularly with respect to their effect on the performance of your application.
Make certain that suitable organizational measures (according to applicable sector standards) have been taken to avoid hazardous situations if the safety logic application operates in an unintended or incorrect way.
Do not enter the zone of operation while the machine is operating.
Ensure that no other persons can access the zone of operation while the machine is operating.
Observe the regulations given by relevant sector standards while the machine is running in any other operating mode than "operational".
Use appropriate safety interlocks where personnel and/or equipment hazards exist.
Failure to follow these instructions can result in death, serious injury, or equipment damage.
Further Information
For further information, refer to the topic "Implicit feedbacks in FBD".
If desired, enter a short comment for the new declaration.
The comment entered here is also visible in the tooltip which appears when hovering the mouse over the respective object in the FBD/LD code.
Optional: Define an initial value.
Rules for initializing variables
The initial value is optional. If no initial value is specified, the variable is initialized with the default initial value of the data type as defined by IEC 61131-3.
Initial values can be specified for local (symbolic) variables and global output variables.
Global input variables (global variables which are assigned to a physical input) cannot be initialized.
The initial value has to match the selected data type.
For Boolean variables, the default initial value is 0 or FALSE.
For safety-related variables, initial values must be entered with preceding safety-related data type in the following format: SAFEINT#value, SAFEBYTE#value, SAFEWORD#value, SAFEDWORD#value, or SAFETIME#values.
(value represents the proper value, e.g., SAFEINT#13 or SAFETIME#1s.)
To initialize a safety-related Boolean variable with FALSE, you can either enter SAFEBOOL#0 or SAFEFALSE. Use accordingly SAFEBOOL#1 or SAFETRUE to preset a safety-related Boolean variable to TRUE.
Global variable acts as global constant: if a global symbolic variable has an initial value, it can be considered as a global constant with symbolic name. The initialized global symbolic variable is write-protected and the compiler treats it as a constant. As a result, it can be connected, for example, to a function block formal parameter which expects a constant.
Refer to the topic "Inserting Constants".
In case of a local output variable (VAR_OUT), you can set the 'DIAG' flag if you want the variable to be written to the DQ buffer in order to initiate an implicit diagnostic.
This flag is only available for local output variables (usage VAR_OUT) of the data type WORD. If set, an implicit diagnostic is performed for this particular variable.
Background: Safety-related firmware function blocks as well as user-defined FBs within a safety-related project may have a diagnostic output which allows polling of FB status information. This diagnostic information can be written to a reserved output buffer area (DQ buffer) while compiling and it can be further processed, for example, using a diagnostic tool. To add a local output variable (FB output) to this buffer, the flag 'DIAG' must be selected.
