Named constants

Special input values can be used for inputs on function blocks of the type MC_LREAL (see the chapter MC_LREAL/Special input values). The MC3 uses only the standard data type LREAL instead of MC_LREAL and supports the use of so-called named constants.

Named constants are silent NaN values according to IEEE 754, used to indicate uninitialized or invalid data. The mantissa is encoded in a Beckhoff-specific bit pattern. The advantage is improved readability, for example, when viewed online. For example, the previous default value of 0 can be interpreted as a placeholder for the default dynamics or as the actual value 0. In this case, misinterpretations or incorrect mathematical operations are possible.

The following points rank among the main properties of NaN values:

All arithmetic operations that use NaN as input data return NaN as the result.
All relational operators =, !=, > < >= <= always return the value False if at least one of the operands is NaN.
The Standard C functions isnan() and _isnan(), as well as the PLC functions LrealIsNaN() and RealIsNaN() (Tc2_Utilities library), return True if the argument has the value NaN.
The expression isnan(a) is equivalent to the expression !(a == a) or NOT(a = a).

The fact that NaN values propagate when used in further calculations has the advantage that invalid values cannot be overlooked.

CAUTION

Software malfunctions

NaN values can lead to potentially dangerous malfunctions of the software concerned!

Only use expressly approved NaN values, specifically as control values in motion control and drive control functions.

For further information, see the chapter on NaN values.

The syntax for use in the PLC is STRING_TO_LREAL('[Wert]') or the shorthand form TO_LREAL('[Wert]'). The following constants can be used for the MC3:

Value	Meaning	Example
#Invalid	Invalid value	Default input value for `MC_MoveVelocity.Velocity`, since this input must be defined when using the function block.
#Default	Can be used, for example, to use the axis's default dynamics as input for function blocks.	`MC_MoveAbsolute.Velocity := TO_LREAL(`‘`#Default`’`);`
#Ignore	Can be used to disable inputs or boundary conditions.	Disabling the software limit switches on one side. Define the position derivatives as input or boundary conditions for the MotionFunctionPoints (camming) to be ignored.
#Maximum	Can be used, for example, to use the maximum dynamics of the axis as input for function blocks.	`MC_MoveVelocity.Velocity := TO_LREAL(`‘`#Maximum`’`);`
#MaximumNegative	Can be used, for example, to use the inverted maximum dynamics of the axis as input for function blocks.	`MC_MoveVelocity.Velocity := TO_LREAL(`‘`#MaximumNegative`’`);`
#DefaultNegative	Can be used, for example, to use the inverted default dynamics of the axis as input for function blocks.	`MC_MoveVelocity.Velocity := TO_LREAL(`‘`#DefaultNegative`’`);`

The syntax and usage of named constants have been optimized in TwinCAT 3.1 Build 4026.25 and are described below.

	Available from TwinCAT 3.1 Build 4026.25

Special values, such as NaN (Not a Number), are now provided as type-safe constants. The LREAL_CONST namespace provides access to the following variables:

Constant	Meaning
DEFAULT	Application-specific default value (positive)
IGNORE	Marker value “do not evaluate”
INVALID	Marker value for invalid size
MAXIMUM	Largest finite positive value
DEFAULT_NEGATIVE	Application-specific default value (negative)
MAXIMUM_NEGATIVE	Largest finite negative value
NAN	Not a Number
INFINITY	Positive infinity
INFINITY_NEGATIVE	Negative infinity

Example: Using

VAR
    fNumerator : LREAL;
    fDenominator : LREAL;
    fResult : LREAL
END_VAR
IF fDenominator = 0 THEN
    fDenominator := LREAL_CONST.NAN;// invalid: division by zero
END_IF
    fResult := fNumerator / fDenominator;