Standard functions: ABS, SQRT, MIN, MAX, LIMIT, SEL, MUX

Functions every controller already has

The standard ships a library you neither declare nor import. Using it is shorter and clearer than a hand-written IF, and faster to execute.

Numeric functions

Function	What it does	Example
`ABS(x)`	absolute value	`ABS(-7.5)` is `7.5`
`SQRT(x)`	square root	`SQRT(16.0)` is `4.0`
`MIN(a, b)`	the smaller one	`MIN(80.0, Level)`
`MAX(a, b)`	the larger one	`MAX(0.0, Level)`
`LIMIT(mn, x, mx)`	clamp between two bounds	`LIMIT(0.0, Level, 100.0)`

Mind the argument order of LIMIT: minimum first, then the value, then the maximum. It replaces four lines:

(* instead of this *)
IF Out > 100.0 THEN Out := 100.0;
ELSIF Out < 0.0 THEN Out := 0.0;
END_IF

(* write this *)
Out := LIMIT(0.0, Out, 100.0);

SQRT belongs in flow measurement: a differential pressure flow transmitter gives a signal proportional to the square of flow, so real flow is the root of the reading.

Flow := FlowMax * SQRT(LIMIT(0.0, DP, 100.0) / 100.0);

Careful: SQRT of a negative number is a runtime fault. A MAX(0.0, x) in front protects you from noise that pushes the reading slightly below zero.

Selection functions

Function	What it does
`SEL(G, IN0, IN1)`	returns `IN0` when `G` is false, `IN1` when true
`MUX(K, IN0, IN1, ... )`	returns argument number `K`

(* vendor code: this does NOT run in the lab on this site *)
SetPoint := SEL(NightMode, 70.0, 40.0);
Recipe   := MUX(RecipeNo, 0.0, 25.0, 50.0, 75.0);

Conversion and rounding

TRUNC cuts the fraction off, ROUND (called REAL_TO_INT by several vendors) goes to the nearest integer:

Expression	Result
`TRUNC(7.8)`	`7`
`ROUND(7.8)`	`8`
`TRUNC(-7.8)`	`-7`

To send a value to an HMI field with one decimal, the usual trick is Disp := ROUND(Level * 10.0); and the HMI places the point.

A word on string functions

When you build a message or a batch number you need the string library:

Function	What it does
`LEN(s)`	length of the string
`CONCAT(a, b)`	join two strings
`FIND(a, b)`	position of `b` inside `a`, zero if absent
`LEFT` `RIGHT` `MID`	take part of a string

String handling is expensive on a controller. Put it in a slow task, or move it to the HMI or SCADA where it belongs.

What runs in the lab

Runs	Does not run
`ABS` `MIN` `MAX` `LIMIT` `SQRT`	`SEL` `MUX` `TRUNC` `ROUND` and all string functions

This lesson's program uses only the five supported ones. The rest is here so you recognise it when you open your vendor's environment.

Try it in the lab

The lab runs in your browser: an ST editor, a virtual process that suits the lesson, the same program as a live ladder diagram, a watch table, and export to Siemens SCL.

What to try

Run the program, press START and watch Error and AbsError: the first changes sign, the second stays positive.

The lamp lights while the level is within 5 of 50%.

Watch Command: it never goes above 100 or below 0, whatever the error.

Change SetPoint to 30.0 and download again.

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