Declaration Rules for Variables/FB Instances
This topic contains information on the following:
The declaration rules listed below apply when declaring variables
by inserting and editing a declaration in a variables worksheet.
A variable/FB instance declared this way can be inserted afterwards into the code.
using the 'Variable' dialog while editing code.
This way, declaration and insertion are done within one step.
Variables are declared in accordance with the rules defined by the standard IEC 61131-3.
Since declarations are entered in a variables worksheet in EcoStruxure Machine Expert - Safety instead of entering plain text declaration blocks, the rules and restrictions given by the standard are verified by the grid based editor. Syntactical errors and violations of rules such as nesting errors of declaration blocks or incorrect data types are prevented. The variables compiler verifies each entered attribute immediately after leaving the grid input field. Fields that contain an error are displayed red.
Further Information
Refer to the topic "Variables Worksheets" for details.
The different variables have to be declared as follows:
Local symbolic variables have to be declared in the local variables worksheet using the keyword ('Usage') VAR.
Local variables cannot be connected to process data items (physical inputs and outputs). This is restricted to global I/O variables.
Input parameters of FB POUs are declared in the local variables worksheet using VAR_INPUT.
Output parameters of FB POUs are declared in the local variables worksheet using VAR_OUTPUT.
When initializing variables, the following rules must be observed.
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".
Function block instances are automatically declared in the local variable declaration of the POU where the function block is used using the keyword VAR (as for local variables). The variable name represents the FB instance name and the data type corresponds to the FB type. For further information refer to the topic "Instantiation of FBs".
Global variables are always declared in the global variables worksheet of the project.
Two different types of global variables are distinguished:
Global symbolic variables. Like local variables, these are normal symbolic variables without a physical I/O address, i.e., not assigned to any device terminal.
Like local variables, these global variables can be inserted directly into the code using the 'Variable' dialog (declaration is auto-inserted in the global variables worksheet) or declared manually in the global variables worksheet of the project. Refer to the topic "Declaring and inserting variables".
Global I/O variables. In contrast to symbolic variables, I/O variables are assigned to a physical device terminal, i.e., to a process data item. This means that a value read from/written to an I/O variable is read from/written to the assigned physical address. The IEC 61131 standard designates these variables as located.
In EcoStruxure Machine Expert - Safety, global I/O variables cannot be created in the same way as symbolic variables.
A global I/O variable is automatically created in the global variables worksheet when dragging a process data item (i.e., device terminal) from the 'Devices' window into the code. (Process data items are passed from EcoStruxure Machine Expert to EcoStruxure Machine Expert - Safety together with the safety-related device information and data.)
This behavior applies to both safety-related and standard process data items.
Both global symbolic and I/O variables can be used in each graphical POU of the project (in contrast to local variables which can only be used within the particular POU where they are declared).
NOTE:
Global variables cannot be used in the programming language ST (Structured Text).
The following applies to global symbolic variables:
The compiler generates an error if a global symbolic variable
is written more than once.
is read before it has been written.
The compiler outputs a message if a global symbolic variable
is neither written nor read (unused global symbolic variable).
is written but not read.
is read and written within the same POU. In this case, a local variable should be used instead of the global symbolic variable.
When declaring a global symbolic variable, no 'Usage' can be selected because the 'VAR_GLOBAL' declaration keyword automatically applies to all global variables.
Naming conventions: EcoStruxure Machine Expert - Safety allows to use DIN qualifiers in IEC 61131 variable names.
Rules for using DIN qualifiers
According to the IEC 61131 standard, variable names can consist of letters, digits, and underscores. The identifier has to begin with a letter or an underscore. The use of any other character causes the compiler error "Illegal identifier".
This naming convention has been expanded in EcoStruxure Machine Expert - Safety in a way that IEC 61131 variable names may also contain DIN qualifiers:
The characters - + < > can be used at any position in the name and as last character. However, they cannot be used as first character of a variable name.
The DIN qualifiers / * # and the numbers 0 to 9 can be used at any position in the variable name.
Rules for using DIN qualifiers in IEC 61131 variable names
Variable names must at least contain one alphabetical character.
Variables must not have the name of an IEC 61131 data type, such as BOOL, INT, WORD, REAL, etc.
Variable names must not be defined as they are for literal values. Literals are used in the code by first specifying the literal data type, followed by a hash sign: <literal_prefix>#<value>. For example, SAFEINT#5 and WORD#32767 are literals. Therefore, a variable declaration such as safeint#MyVar would be invalid.
Literal prefixes are not case-sensitive and include the following keywords:
BOOL, REAL, LREAL, SINT, USINT, INT, UINT, DINT, UDINT, LINT, ULINT, BYTE, WORD, DWORD, LWORD, TIME, T, DATE, D, TIME_OF_DAY, TOD, DATE_AND_TIME, DT, STRING, TIMEDATE48, WEIGHT, ANALOG, UNIFRACT, BIFRACT200, FIXED, BOOLEAN2, BCD4, ENUM4, SAFEBOOL, SAFEBYTE, SAFEDWORD, SAFEINT, SAFEDINT, SAFETIME, SAFEWORD
Further Information
Further information can be found in the topics "Columns (Properties) in Variables Worksheets" and "IEC 61131 Implementation - Variables".
Safety-related and standard variables in FBD/LD code
Safety-related and standard code is strictly distinguished in EcoStruxure Machine Expert - Safety. Therefore, also safety-related and standard variables, or more precise, safety-related and standard data types, are distinguished. It is, for example, not possible to connect a variable with a standard data type to a formal parameter which expects a safety-related variable.
NOTE:
Safety-related and standard variables can be mixed in FBD/LD networks. In such mixed networks, leading safety-related signal paths are visually distinguished. Some rules and restrictions must be observed.
Safety-related and standard variables in ST
Safety-related and standard variables can be mixed within one ST statement if particular rules are observed. Generally, safety-related variables can be assigned to standard variables but not vice versa.
Further Information
Refer to the topic "Mixing safety-related and standard variables in ST" for details.
For easier distinction of standard and safety-related variables, they are visually distinguished in the ST code editor. Safety-related variables are underlined in red, standard variables are not underlined.
