According to the IEC 61131-3 standard, the safety logic in EcoStruxure Machine Expert - Safety is developed using variables instead of directly addressing inputs, outputs, or using flags.
Variables have to be declared in variables worksheets. This can be done either while inserting them into the code using the 'Variable' dialog ( simplified method as insertion and declaration are done in one step) or before using them in the code by manually entering the declaration into the variables worksheet. Refer to the topics "Variables: Inserting and Declaring in FBD/LD" and "Variables: Inserting and Declaring in ST" for details.
These variables worksheets are implemented as special grid-based editors. Refer to the topic "Variables Worksheets" for details.
Scope: Local and global
The scope of a variable determines in which POU(s) it is valid, i.e., where it can be used. According to the IEC 61131 standard, the possible scopes are local and global. The scope of a variable is defined by the location where it is declared (local or global variables worksheet) and by the variable keyword used for the declaration.
EcoStruxure Machine Expert - Safety supports local and global variables, each of them as safety-related and standard data type (see section below).
Local variables
A variable which can only be used in one POU, is called a local variable (its scope is local).
Local variables have to be declared in the variables worksheet relating to the POU, where the variable is going to be used, with one of the variable declaration keywords VAR, VAR_INPUT, or VAR_OUTPUT.
As local variables cannot be connected to process data items (physical inputs/outputs), they are called symbolic variables. EcoStruxure Machine Expert - Safety stores these variables to free memory areas in the Safety Logic Controller memory which are not known by the user. Symbolic variables may have an optional initial value.
FB instances are treated in the same way as local variables. Their instance names have to be declared using VAR.
In debug mode, local variables can be overwritten.
Global variables
A variable which can be used in each POU of the project is called a global variable. Variables with a global scope have to be declared in the global variables worksheet. Here, the column 'Usage' for selecting the IEC 61131-defined declaration keyword VAR_GLOBAL is not visible because it automatically applies to all global variables.
(Note for experienced IEC 61131 users: in EcoStruxure Machine Expert - Safety, global variables can be used without an additional local VAR_EXTERNAL declaration.)
Two different types of global variables are distinguished: Global symbolic variables (without assigned physical address) and global I/O variables which are assigned to a process data item (input or output signal). Refer to the topic "Declaration Rules for Variables/FB Instances" for details.
In ST, global variables cannot be processed.
Global variables connected to an output may have an optional initial value (see section "Initializing variables" below). In debug mode, I/O variables can be forced.
Safety-related and standard variables are distinguished
To fulfill the requirements of the IEC 61508 standard and as an extension to the IEC 61131 standard, 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.
Further Information
Refer to the topics "IEC 61131 Implementation - Data Types" and "Declaration Rules for Variables/FB Instances" for details.
Naming conventions for variables
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
According to the IEC 61131-3 standard, initial values can be assigned to variables. The initial value is used when processing the variable for the first time during runtime.
The following rules apply:
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".
According to IEC 61131 standard, variables are declared using variable declaration keywords. In EcoStruxure Machine Expert - Safety, variable declaration keywords are selected in the 'Usage' column.
The following IEC 61131-defined keywords are relevant in EcoStruxure Machine Expert - Safety:
|
Keyword |
declares... |
|
VAR |
|
|
VAR_INPUT |
variables which are input parameters of function block POUs. An input variable can only be a local (symbolic) variable. An input variable can only be read by the FB. According to the IEC 61131-3, write access to input variables is not allowed. However, EcoStruxure Machine Expert - Safety does not verify a possible write access while compiling the project due to compatibility reasons. |
|
VAR_OUTPUT |
variables which are outputs of function block POUs. An output variable can only be a local (symbolic) variable. It can be written and read by the FB. |
|
not visible in EcoStruxure Machine Expert - Safety VAR_GLOBAL |
global variables in the global variables worksheet. Although this keyword is defined in the IEC 61131 standard for declaring global variables, it is not visible in EcoStruxure Machine Expert - Safety. |
