An expression is made up of operands, operators, functions, constants, and comments. Each expression must follow a specific syntax to be valid. The basic rules for valid DeltaV syntax expressions include:
Calculation/Logic Block definitions have a temporary variable capability.
The Condition function block evaluates a multi-line expression. The assignment operator (:=) is not a valid operator.
The Action block requires the use of the assignment operator (:=). All other operators are valid in the Action block provided the final evaluation of the expression is an assignment.
Named set constants are represented by SetName:valueName.
The only data type supported is floating point.
An external (or database) reference is represented by a 'path' surrounded by single quotes.
The expression should end with a semi-colon (;). DeltaV software usually adds a semi-colon for you if one is not there.
Brackets used in an expression must be paired. Non-paired brackets can cause database migration errors.
The expression evaluator is stack oriented and allows a maximum of 32 operands and operators. The use of parentheses to organize the expression minimizes the stack usage. The equations are evaluated using RPN (Reverse Polish Notation).
Relative Notation
The following rules indicate the relative notation of parameters in an expression.
Notation |
Relates to Rule |
Rule |
|---|---|---|
// |
External |
References a parameter that is outside this module. |
/ |
Module relative |
References a parameter within the current module. |
^ |
Block relative |
References a parameter up one block level. |
/+/ |
Phase relative |
References a parameter within a batch phase. |
Assignment Statements
The := operator is an operator in DeltaV expressions. This operator allows for the assignment of calculated values to locations inside and outside of the current block. Examples of the assignment operator follow:
'Block1.mode.target' := MAN;RADIUS := .5; OUT1:= 5 *RADIUS;
Assignment statements can be in any of the following formats:
output := value;
or
'external reference path' := value;
or
temporary variable := value;
If you wanted to increment a module-level parameter called SYRUP, the expression might look like this:
'/SYRUP' := '/SYRUP' + 1;
The assignment operator is not valid in the Condition block. However, the assignment operator is required in the Action block.
If-Then-Else-End_if Statements
The IF-THEN-ELSE-END_IF structure allows you to execute conditional code in expressions. When a block tests a condition that evaluates to TRUE, it executes one set of the statements; otherwise, it executes a different set of statements. The following example illustrates the IF-THEN-ELSE-END_IF structure:
IF '/Block1.mode.ACTUAL' = MAN THEN
'/Block1.mode.TARGET':= AUTO;
ELSE
OUT1:= IN1;
END_IF;
DeltaV software allows you to use ENDIF or END_IF for your convenience. However, structured text typically requires the use of the keyword END_IF.
In the preceding example, the condition tested is whether 'Block1.mode.ACTUAL' is equal to manual. Notice that the '=' operator is not used as an assignment operator, but rather to test the two operands for equality. If the condition is TRUE, the 'Block1.mode.TARGET' is set to AUTO; otherwise, OUT1 is set to the value of IN1. Multiple statements can be placed between the THEN keyword and the ELSE keyword as well as between ELSE and the END_IF keyword.
It is not always necessary to use the ELSE portion of the statement. For example, if you wanted to set the parameter CALC block parameter OUT1 to TRUE when the process variable of PID1 in LIC-549 goes above 75, the expression would look like this:
IF '//LIC-549/PID1/PV.CV' > 75 THEN 'OUT1.CV' := TRUE; ENDIF;
The CV extension in this example stands for current value. If the choice exists, ST stands for status.
After you enter an expression, you can validate the expression syntax. The validation process identifies syntax problems with the expression and any unresolved parameters. The expression can be saved to the database with syntax errors, but the errors should be corrected before downloading the expression.
While-Do-End_While
The WHILE-DO-END_WHILE structure allows you to continue executing a group of statements while the value of an expression is True. This structure is available in the Calc/Logic and Action function blocks.
The following example illustrates the WHILE-DO-END_WHILE structure:
I := 1;
WHILE (I <= 5) DO
'^/PARAM1'[I][1]:= I + .12;
I := I + 1;
END_WHILE;
The indices are outside of the single quotes unless the .CV field is used. The parameter syntax must be exactly as shown in the example ('^/PARAM1'[I][1]). It is recommended to show all access to the matrix parameter (an input or output parameter defined as a floating point array type) using two dimensions where the second is always [1].
WARNING!The absolute limit of the number of iterations of a WHILE loop is 2000. However, nested loops, IF statements within loops, and Labels within loops may reduce the limit. If a WHILE loop reaches the limit, the loop stops, BLOCK_ERR and MSTATUS are set, and the loop is not executed until the module is re-downloaded.
Exit
This structure prematurely exits the innermost WHILE-DO loop currently being executed. The EXIT statement can only appear inside the statements of a WHILE_DO loop.
The following example illustrates using EXIT in a WHILE-DO loop:
I := 1;
WHILE (I <= 5) DO
'^/PARAM1'[I][1]:= I + .12;
IF ('^/PARAM1'[I][1] > 5) THEN
EXIT;
END_IF;
I := I + 1;
END_WHILE;
This command is only available in the Calc/Logic and Action function blocks because that is where the WHILE-DO construction is supported.
GOTO label
The GOTO label structure transfers processing to the statement identified by the specified label. A label definition is a string followed by a colon (:).
The following example illustrates the GOTO label construct:
REM This expression takes IN1 to the IN2 power cnt:=0; answer:=1; REM The following statement creates the label begin begin: IF cnt>=IN2 THEN GOTO end; END_IF; answer:=answer*IN1; cnt:=cnt+1; GOTO begin; end: OUT1:=answer;