Expressions

One of the fundamental capabilities required in a controller is its ability to compute expressions. An expression is structured text that represents a calculation and has a specific syntax. The expressions provide information for process operators so that they can make control decisions. This section provides an overview of the functions, operators, and syntax used for expressions.

To write an expression, use the Expression Editor. The Action, Calc/Logic, and Condition function blocks, as well as the Sequential Function Charts, allow you to enter expressions for execution in the controller.

CAUTION!

When writing expressions using an external editor, only use a plain ASCII text editor (Notepad is recommended). Using other editors can cause the expression to change during the load and save process.

Expressions can be used for the following applications:

• Analog Tracking Block Expressions — The expressions in the block define the tracking conditions evaluated that determine the block's behavior.

• Discrete Control Condition Block Expressions — The expressions in the block define the interlock conditions, permissive conditions, and force setpoint conditions that determine the block's behavior.

• Action Block Expression — The expression in the action block allows you to evaluate an equation and assign the result of the evaluation to a parameter within the DeltaV system.

• Condition Block Expression — Used with Boolean-valued expression. To set the condition, you must specify an expression as TRUE. Note that the timed part of condition definitions should not be part of the expression, but rather part of the definition of the condition.

• Calculation/Logic Block Expression — Used with a collection of expressions whose results can be assigned to function block parameters or module parameters.

  For example, for an expression in a Calc/Logic block (CALC1) in the composite, COMPOS1, that is in the composite, COMPOS2, that is a block in the module, MOD, you can:

  • Reference a CALC1 parameter within the block, IN1.CV.
  • Reference a COMPOS1 parameter within a composite, ^/ATTR.CV.
  • Reference a COMPOS2 parameter outside a composite, /COMPOS2/PARAM.CV.
  • Reference a Module (MOD) parameter, /MSTATUS.CV.
  • Reference another Module parameter, //FIC101/PID/SP.CV.
  • Reference a diagnostic parameter from the module, //NODEX/OINTEG.
  • Reference a DST parameter for the module, //DST/FIELD1_VAL_PCT.

• SFC Step Actions — Used to assign the result of an expression to a database item, which can be an SFC step, state variable, or a parameter.

• SFC Step Transitions — Used with a Boolean-valued expression. When it is evaluated as TRUE, the transition is triggered and associated steps are enabled or disabled.

• SFC Step Mode — Used to allow operators (with the appropriate privileges) complete control over SFC execution by requiring that all transitions be forced. The operator also has the power to redo a given step before proceeding or to restart the sequence from a different point by changing the active step. The SFC can be placed into Step mode from Control Studio Online or Debug views.