The Expression action is a mathematical
operation that takes a variable number of input arguments
and returns a single output argument. An expression is any
legal combination of symbols that represent a value. The
system has rules for what symbols are legal and
illegal.
Every expression consists of at least two operands with one or more operators. Operands are values, whereas operators are symbols that represent particular actions. For example, In the expression X + Y - 10. X, Y, and 10 are operands, and + and - are operators.
The following lists the valid operators.
| Operator | Description |
|---|---|
| + | Addition |
| - | Subtraction |
| * | Multiplication |
| / | Division |
| ^ | Exponentiation (for example: 2^3 = 8) |
| % | Modulus (for example: 5%2 = 1) |
| ( ) | Force precedence of evaluation |
| >> | Bitwise shift right |
| << | Bitwise shift left |
| & | Not implemented as an operator, see the and function below. |
| | | Not implemented as an operator, see the or function below. |
| ^ | Not implemented as an operator, see the xor function below. |
| ~ | Not implemented as an operator, see the not function below. |
The following lists the valid logical operators for conditional operations:
| Logical Operator | Description |
|---|---|
| == | Equal |
| != | Not Equal |
| < | Less Than |
| <= | Less Than or Equal |
| > | Greater Than |
| >= | Greater Than or Equal |
| && | And (for example: x==5&&y==10 returns true if x is 5 and y is 10) |
| || | Or (for example: x==5 || y==10 returns true if x is 5 or y is 10) |
| ! | Not (for example: !(x==5) returns false if x is 5) |
There are also a number of built-in functions used in an expression, such as sin(X) and log(X).
Parameter descriptions
The top of the Expression action provides the Formula and Functions parameters that accommodate creating an expression.
| Formula | Use the Formula
pane to type an expression. As you type in the Formula box, the logical variables used in the expression are automatically added to the Input tab. In the example expression, x==y (x is equal to y), the logical variables x and y are added to the input tab. |
| Functions | You can use built-in functions
within an expression. To add a built-in
function, double-click the function in
the Functions
list. The function is added to the Formula box. Note that the default input operand for the sin( ) function is X. You can change this to any value you want. When using the trig functions, the angle is specified in radians. For example, sin(1.0) = 0.841471. The 1.0 is equivalent to 1 radian: where 2 pi radians equals 360 degrees |
The following table lists the built-in functions that you can use within an Expression action.
| Function | Description |
|---|---|
| abs(X) | Absolute value |
| acos(X) | Trigonometric arcosine |
| and(X,Y) | Bitwise AND "&" |
| asin(X) | Trigonometric arcsine |
| atan(X) | Trigonometric arctangent |
| avg(X1, X2...,Xn) | Average of a set of values |
| ceil(X) | Ceiling (round up) |
| cos(X) | Trigonometric cosine |
| cosh(X) | Hyperbolic cosine |
| exp(X) | e to the power X |
| floor(X) | Floor (round down) |
| ln(X) | Natural log (base e) |
| log(X) | Natural log (base e) |
| log10(X) | Log base 10 |
| max(X1, X2...,Xn) | Maximum of a set of values |
| min(X1, X2...,Xn) | Minimum of a set of values |
| not(X) | Bitwise NOT "~" |
| or(X,Y) | Bitwise OR "|" |
| sin(X) | Trigonometric sine |
| sinh(X) | Hyperbolic sine |
| sqrt(X) | Square root |
| sum(X1, X2...,Xn) | Sum of a set of values |
| tan(X) | Trigonometric tangent |
| tanh(X) | Hyperbolic tangent |
| xor(X,Y) | Bitwise XOR "^" |
Input tab
The following describes the Input tab
available from the bottom of the
Expression action pane. In the example
expression, X + Y - 10 will be evaluated. However, before
the trigger can evaluate this expression, values need to be
assigned to both X and Y.
The Input tab lets you assign values to
the expression using trigger variables.
The following describes the columns on the Input tab.
| Column | Description |
|---|---|
| Name | This column provides the expression variables. The expression is automatically added when you type the expression in the Formula box or double-click a built-in expression function from the Functions list. The variables used in the expression are just placeholders. They are not local variables, static variables, or device variables. |
| Logical | By default, all variables used in an
expression have the data type
FLOAT8 which is an 8
byte floating point number. Depending on your system, you might see LREAL as the default data type. LREAL is also an 8 byte floating point number. When you assign the target value to hold the result of the expression, you can convert this FLOAT value to a different data type. |
| Value | The column lets you assign the
trigger variable you want to use as
input for the expression
variable. Select the Value column to select from a list of device variables, or a user defined local or static variable. For this example, the actual PLC device variable Local CPU 1.D[10] is assigned to X, and Local CPU 1.D[11] is assigned to Y  |
| Type | When you specify Value, the default data type of the variable is automatically added to the Type column. You can override the default data type by selecting a different data type from the list. |
When the trigger executes and evaluates this expression, the following actions will occur:
- Get the value of the device variable data type INT2 Local CPU 1.D[10].
- Get the value of the device variable data type INT2 Local CPU 1.D[11].
- Add these 2 values together.
- Subtract 10 from the value.
The last step is to specify the location to store the result of the expression.
Output tab
The Output tab for an Expression action lets you specify the location to store the result of the expression as follows:
Variables
Use the Variables parameter to assign
the variable that will store the result of the expression
evaluation.
Select the Value column and then use
the down-arrow to display a list of trigger
variables.
The list includes all started devices (for example Local CPU1), Local Variables, Static Variables, and Event Variables.
When you assign a device variable, the Type drop-down list becomes available with the associated data type of the variable.
Type
Use the Type drop-down list if the data
type of the device variable needs to be changed.
Only valid data types are listed. For this example, PLC device variable Local CPU1.D[15] is assigned to hold the result of the expression.
To review. When the trigger executes, the expression is evaluated and the following will occur:
- Add Local CPU1.D[10] to Local CPU1.D[11]
- Subtract 10 from the value
- Store the result in Local CPU1.D[15]
Specifying a static variable to store the result of the expression
You can also specify a static variable as the recipient of an expression evaluation. A static variable retains its value while the trigger is in a started state. It is assumed that you have defined the static variable using the Static Variables tab on the New Trigger window.
When the static variable is defined, its initial value is
created when the trigger is started. The value (and
possibly modified value) continues to exist while the
trigger is in a started state and is lost when the trigger
is stopped.
The steps to add a static variable to the Output tab for the expression are the same as a device variable, except you select a static variable from the list.
The Value column will contain the static
variable.
Notice the Type drop-down list is
unavailable. You cannot change the data type of a static
variables from this list.