Variable: `VAR_GENERIC CONSTANT`

Generic Constant Variable

A generic constant is a variable in the VAR_GENERIC CONSTANT scope of a function block. The variable is defined only when the function block instance is declared.

Generic constants are allowed to be used in methods. This can be particularly helpful in the VAR_IN_OUT or VAR_IN_OUT CONSTANT scope.

Syntax of the type declaration

Syntax of the type declaration of a function block with generic constants

FUNCTION_BLOCK <function block name>
VAR_GENERIC CONSTANT
    <generic constant name> : <integer data type> := <initial value> ; //Initial value will be overwritten
END_VAR

`<function block name>`	Name of the function block
`VAR_GENERIC CONSTANT` `END_VAR`	Scope for generic constants You can declare any number of generic constants in the `VAR_GENERIC CONSTANT` scope.
`<generic constant name>`	Variable name of the generic constants
`: <integer data type>`	Construct which types the generic constant. An integer data type or the sub-scope of an integer data type is permitted. Example: `maxlen1 : INT := 4;` `maxlen2 : INT(5..10) := 5;`
`:= <initial value>`	Optional Initialization Important The initial value is needed only for compile checks. At runtime, the value is overwritten.

The generic constant can be used as usual in the implementation of the function block. These kinds of constants can be used, for example, for the indices of arrays or the lengths of strings.

Syntax of the variable declaration

Syntax of the variable declaration (instantiation of the function block) with a specified constant

The following options are possible:

PROGRAM <program name>
VAR
    <fb instance name> : <function block name> < <literal> > ;
    <fb instance name> : <function block name> <( <expression> )> ;
    <fb instance name> : ARRAY [ <index start > .. <index end> ] OF <function block name> < <array length> > ;
END_VAR

Table 9. Variable declaration

`<scope>` `END_...`	Scope
`<fb instance name>`	Variable; name of the function block instance
`: <function block name>`	Types the variable with the function block This is a function block with generic constants.
`< <literal> >`	Optional Assigns a specific value to the data type which is valid only for this function block instance. The value is formally inserted `in angle brackets` as an suffix to the data type. Example: `fbMyString1 : FB_MyString<100&>;` If multiple generic constants have been declared for the type-defining function block, then a comma-separated list of values is passed in angle brackets. There is one value for each constant. Example: `fbPou : FB_Pou<100, 1000>;`
`<( <expression> )>`	Optional Assigns a specific expression to the data type which is valid only for this function block instance. The expression is inserted `in angle brackets` as an suffix to the data type. The expression itself must be enclosed in round brackets . The round brackets are necessary because it is permitted to use symbols such as `<` or `>` in an expression. Otherwise, the uniqueness of the code is no longer guaranteed. Example: `fbMyString2 : FB_MyString<(2 * cconst)>;`
`ARRAY [ <start index start> .. <end index> ] OF <function block name>`	Optional Types the variable with an array of function blocks. The length of the array is: `<end index>` - `<start index>` + `1` This is a function block with generic constants. The constants are used as array indices. Example: `arrMyString`
`< <array length> >`	Assigns a specific array length to the data type which is valid especially for this function block instance. The specified array length is formally inserted in angle brackets as an suffix to the data type. The specified value must match the array length as defined by the start index and end index. Example: `arrMyString : ARRAY[0..5] OF FB_MyString<6>;`

Examples

Example 92. Example of using multiple generic constants

FUNCTION_BLOCK FB_Pou
VAR_GENERIC CONSTANT
    lnstring : DINT := 10;
    numstring : DINT := 100;
END_VAR
VAR
    arraystring : ARRAY[0..numstring-1] OF STRING(lnstring);
END_VAR
;

PROGRAM PLC_PRG
VAR
    fbPou : FB_Pou<100, 1000>;
END_VAR

Example 93. Example: Generic constant of a sub-scope type

FUNCTION_BLOCK FB_SrString 
VAR_GENERIC CONSTANT
    maxlen2 : INT(5..10) := 5;    //subrange data type
END_VAR
VAR
    arrTest : ARRAY[0..maxlen-1] OF BYTE;
END_VAR
;

PROGRAM SrMain
VAR CONSTANT
    cconst: INT(5..10) := 5;
END_VAR
VAR    
    fbMyString1 : FB_SrString<5>;
    fbMyString2 : FB_SrString<(2 * cconst)>;
    arrMyString : ARRAY [0..5] OF FB_SrString<6>;
END_VAR

Example 94. Example: Generic function block with parameterizable array variable

The following code demonstrates how to define a function block which can process arrays of arbitrary length. The function block has an array with a generic but constant length. "Constant" means that, although each function block instance varies in its array length, it is constant during the lifetime of the object.

This kind of construct is beneficial, for example, to a library programmer who wants to implement a generic library POU.

FUNCTION_BLOCK FB_MyString 
VAR_GENERIC CONSTANT
    maxlen : UDINT := 1;
END_VAR
VAR
    test : ARRAY[0..maxlen-1] OF BYTE;
END_VAR
;

PROGRAM PLC_PRG
VAR CONSTANT
    cconst: DINT := 1000;
END_VAR
VAR    
    fbMyString1 : FB_MyString<100>;
    fbMyString2 : FB_MyString<(2 * cconst)>;   
    arrMyString : ARRAY[0..5] OF FB_MyString<6>;    
END_VAR
;

Inheritance

A function block can inherit from a base function block with a generic constant (EXTENDS). The inheriting function block requires its own generic constant. A specific value can then be transferred externally.

Syntax:

FUNCTION_BLOCK <function block name> 
VAR_GENERIC CONSTANT        
    <generic constant name> : <integer data type> ; 
END_VAR 
EXTENDS <function block base> < <generic constant name> >

A function block with a generic constant can implement an interface (IMPLEMENTS). The interface declares a property (PROPERTY) with which the specified value can be accessed externally. The interface itself must not declare any generic constants or local variables. Interfaces have no implementation.

Syntax:

FUNCTION_BLOCK <function block name>
VAR_GENERIC CONSTANT
    <generic constant name> : <integer data type> ;
END_VAR
IMPLEMENTS <interface name>

Tip

When coding, make sure that the declaration of the generic constants is inserted first, followed by EXTENDS and IMPLEMENTS. This takes some getting used to, but the reason is that generic constants can also be used with base classes.

Example 95. Example

Definition of the IString interface for a generic function block.

It is strongly recommended to define an interface for generic function blocks. The interface should allow the generic function block instance to be used without knowledge of the generic constants.

The Length property enables access to the generic constant.

INTERFACE IString 
METHOD Append : BOOL 
VAR_INPUT     
    strAppend : IString; 
END_VAR 

METHOD Assign : BOOL
VAR_INPUT     
    stringIn : STRING; 
END_VAR 

METHOD ToString : STRING
VAR_INPUT 
END_VAR 

PROPERTY Length : DINT

Declaration of the function block FB_MyString with the generic constant maxlen

FUNCTION_BLOCK FB_MyString 
VAR_GENERIC CONSTANT        
    maxlen : UDINT; 
END_VAR 
IMPLEMENTS IString

The function block FB_LongString is an extension of the specified function block FB_MyString.

FUNCTION_BLOCK FB_LongString EXTENDS FB_MyString<1000>

Declaration of the function block FB_MySpecialString with the generic constant maxlen2 as an extension of the specified function block FB_MyString. The function block is extended by the method METH_ToLatin.

FUNCTION_BLOCK FB_MySpecialString 
VAR_GENERIC CONSTANT        
    maxlen2 : UDINT:= 1; 
END_VAR 
EXTENDS FB_MyString<maxlen2>

METHOD METH_ToLatin : STRING
VAR_INPUT
END_VAR

Instantiation of the function blocks with specific constants

PROGRAM PLC_PRG 
VAR CONSTANT     
    cconst: DINT := 1000; 
END_VAR
VAR     
    string1 : FB_MyString<100>;
    string2 : FB_MyString<(2 * cconst)>;
    derived1 : FB_LongString;
    derived2 : FB_MySpecialString<100>;
END_VAR

Calls

string1.METH_Assign ('Welt'); 
string2.METH_Assign ('Hallo '); 
string2.METH_Append(string1); 
derived2.METH_ToLatin('Hello World');