Expressions

Overview

An expression is a construction which after its evaluation returns a value. This value is used in instructions.

Expressions are composed of operators, operands, and/or assignments. An operand can be a constant, a variable, a function call, or another expression.

Examples

33

(* Constant *)

ivar

(* Variable *)

fct(a,b,c)

(* Function call*)

a AND b

(* Expression *)

(x*y) / z

(* Expression *)

real_var2 := int_var;

(* Assignment, see below *)

Order of Operations

The evaluation of an expression is performed by processing the operators according to certain rules. The operator with the highest order of operation is processed first, then the operator with the next operating level, and so on, until all operators have been processed.

Below you find a table of the ST operators in the order of their ordinal operating level:

Operation

Symbol

Operating Level

placed in parentheses

(expression)

highest order

function call

function name (parameter list)

..............

exponentiation

EXPT

.............

negate

–

............

building of complements

NOT

...........

multiply

*

..........

divide

/

.........

modulo

MOD

........

add

+

.......

subtract

–

......

compare V

<,>,<=,>=

.....

equal to

=

....

not equal to

<>

...

boolean AND

AND

..

boolean XOR

XOR

.

boolean OR

OR

lowest order

Assignment as Expression

As an extension to the IEC 61131-3 standard (ExST), assignments can be used as an expression.

Examples: 

int_var1 := int_var2 := int_var3 + 9;

(* int_var1 and int_var2 both equal to the value of int_var3 + 9*) 

real_var1 := real_var2 := int_var;

(* correct assignments, real_var1 and real_var2 will get the value of int_var *) 

int_var := real_var1 := int_var;

(* a message will be displayed due to type mismatch real-int *) 

IF b := (i = 1) THEN
i := i + 1;
END_IF

(*Expression used inside of IF condition statement: First b will be assigned TRUE or FALSE, depending on whether i is 1 or not, then the result value of b will be evaluated.*)