Emerson manual Rosemount 848L, Reference Manual, 00809-0100-4696,Rev AA September

Reference Manual

00809-0100-4696, Rev AA September 2004

Rosemount 848L

Logic Functions

A function has a name and a set of one or more arguments contained within a closed set of parentheses. The seven channel reference functions (IN, ICF, ICR, PS, DO, EQ, and OUT) have been described above. These are the only functions that take a channel number as an argument. The other functions require functions for all arguments unless the last argument is a constant number.

When a function is evaluated, it leaves its true or false value behind to be evaluated by the next function or used as the result of evaluating the equation. This is the result of using a simple and fast evaluation method known as Reverse Polish Notation (RPN). The RPN method requires nesting the functions like OR(IN(1),IN(2)) rather than using operator notation like IN(1) IN(2). This can lead to the following:

AND(IN(1),OR(IN(2),AND(IN(3),OR(IN(4),AND(IN(5),OR(IN(6),AND(IN(7),

(IN(8))))))));

The equation is evaluated by evaluating the deepest functions first, IN(7) and (IN(8). If they are both true then the AND function evaluates to true. Then IN(6) is evaluated, then the OR evaluates to true, and so on working up from the deepest level in reverse order until the first (and top level) AND can be evaluated. The result is stored in the channel specified by EQx, which contains the text of the equation as explained above.

Drawn as a ladder logic, the equation would look like the figure below:

848L/LOGIC_FUNC_EXAMPLE.JPG

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