TwinCAT 3 CONSTANT initialization

[kalabdel]

Hello gentlemen,


Why does this work:

VAR CONSTANT
    SIZE_OF_dutEVENT : UDINT := SIZEOF (DUT_Event);
    MAX_SIZE_OF_dutEVENT : UDINT := 5;
    NUMBER_OF_BYTES_FOR_ALL_dutEVENTS : UDINT := MAX_SIZE_OF_dutEVENT * SIZE_OF_dutEVENT;
    
END_VAR

And this doesn't work, I get the error :Initialisation of constant variable 'MOTOR_DATA_LOWER_BOUND ' not constant

MOTOR_DATA_LOWER_BOUND : DINT := LOWER_BOUND(ioaMotors,1);

What am I missing?




Thanks

 

[drbitboy]

from here:

The operators LOWER_BOUND and UPPER_BOUND can be used to determine the index limits of the array that is actually used at runtime.

Quantities determined "at runtime" cannot be constants.


Perhaps it could be run the other way? Onstead of using a deco,a; value for the lower bound of the declaration of the array, declare the constant as the desired lower bound and then use the constant in the declaration of the array. I.e. instead of

 VAR
   ioaMotors : ARRAY [4..10] of DINT;
END_VAR
VAR CONSTANT
   MOTOR_DATA_LOWER_BOUND : DINT := LOWER_BOUND(ioaMotors,1);
END_VAR

do this

VAR CONSTANT
   MOTOR_DATA_LOWER_BOUND : DINT := 4;
END_VAR
VAR
   ioaMotors : ARRAY [MOTOR_DATA_LOWER_BOUND..10] of DINT;
END_VAR

Which appears to be valid cf. here.

 

[kalabdel]

from here:
Quantities determined "at runtime" cannot be constants.


Perhaps it could be run the other way? Onstead of using a deco,a; value for the lower bound of the declaration of the array, declare the constant as the desired lower bound and then use the constant in the declaration of the array. I.e. instead of

 VAR
   ioaMotors : ARRAY [4..10] of DINT;
END_VAR
VAR CONSTANT
   MOTOR_DATA_LOWER_BOUND : DINT := LOWER_BOUND(ioaMotors,1);
END_VAR

do this

VAR CONSTANT
   MOTOR_DATA_LOWER_BOUND : DINT := 4;
END_VAR
VAR
   ioaMotors : ARRAY [MOTOR_DATA_LOWER_BOUND..10] of DINT;
END_VAR

Which appears to be valid cf. here.

Thanks drbitboy. You are correct, I missed that. I will have to look for a different approach.
ioaMotor_Data[*] is In_Out variable length array as per your first link. I will rework the structure to avoid constants altogether. Or some other way so it is calculated at compile time.