Expression syntax of internal signals

An expression signal computes its value from other signals with a formula, for example {T101.Level} * 0.01 * 120. The server evaluates it each time one of its inputs changes. How to create one is described in Internal signals.

Expressions on screens, which animate objects, use a different syntax; this page applies only to expression signals.

Signal references

Form	Example	Note
In braces	{Line1.Pump1.Current}	Always works, also for path parts that start with a digit, such as {1A.Feeder}
Bare path	Line1.Pump1.Current	Must start with a letter or _

A path consists of parts of letters, digits and _, separated by dots. A name directly followed by ( is a function, not a signal. An expression cannot refer to its own signal.

Literals

Kind	Examples
Number	12, 1.5, .5, 1.5e3
Text	'AUTO', "AUTO" (no escape characters)
Logical	true, false (any capitalisation)

Operators

From lowest to highest precedence:

Precedence	Operators	Meaning
1	or, ||	Logical or
2	and, &&	Logical and
3	not, !	Logical not
4	==, !=, <, <=, >, >=	Comparison
5	+, -	Addition, subtraction; + with a text joins the texts
6	*, /, %	Multiplication, division, remainder
7	- (in front)	Negative value

not binds less tightly than a comparison: not {A} == 1 means not ({A} == 1). Use parentheses when in doubt.

Functions

Function names may be written in any capitalisation.

Function	Arguments	Result
if(c, a, b)	exactly 3	a if c is true, otherwise b; only the chosen branch is calculated
min(x, …)	1 or more	The smallest value
max(x, …)	1 or more	The largest value
abs(x)	1	The absolute value
floor(x)	1	Rounded down
ceil(x)	1	Rounded up
round(x), round(x, n)	1 or 2	Rounded to n decimals (0 by default, at most 15)

Type rules

• In calculations a logical value counts as 1 or 0, and a text is read as a number. A text that is not a number is an error.
• Two texts are compared alphabetically; otherwise values are compared as numbers. If a text cannot be read as a number, == is false and != is true.
• As a condition, a number is true when it is not 0, and a text is true when it is not empty, 0 or false.
• The result is converted to the data type of the signal. Whole-number types round it; a result outside the range of the type counts as an error.

Missing and bad inputs

• If any input has no value or a bad quality, the expression is not calculated. The signal keeps its last good value with a Bad quality, or has no value if it never had one. This applies to all inputs, also to those in the branch of if that is not used.
• Division or remainder by zero and a result that is not a finite number also count as an error, with the same effect.

Limits

Limit	Value
Length of the expression	4096 characters
Different input signals	64
Nested parentheses and functions	32 levels
Depth of the formula	64 levels, for example 63 operators in one chain

Errors

The configuration editor checks the expression when you validate or apply the configuration. The message names the character position and the problem, for example an unknown function sqrt at character 12.