The motor should shut off if any of the following three conditions occurs:
The block valve is closed.
The tank level is less than 100 gallons.
The regulatory valve is less than 5 percent open.
Now we need a way to specify these requirements. The condition blocks serve this purpose. Each condition will have associated with it an expression that identifies the condition precisely in mathematical or logical terms. Expressions are made up of operands, operators, functions, constants, and keywords.
Two guidelines for writing an expression for a condition are:
The expression must end with a semi-colon (;).
Parameter values must be enclosed by single quotes (').
The DeltaV system provides an Expression Assistant to help you define expressions. After you enter an expression, the Expression Assistant checks the syntax, indicates any problems, and identifies any unresolved parameters. You can access the Expression Assistant using either the Object menu or the right-click context menu for Action, Calculation/Logic, and Condition function blocks.
The Expression Assistant inserts the characters shown in the following table when constructing expressions. If you type the expressions without using the Expression Assistant, you need to use these characters in the same way.