Iec 61131-3 Second edition 2003-01 Programmable controllers – Part 3: Programming languages

2.5 Program organization units
The program organization units defined in this part of IEC 61131 are the function, function block, and
program
. These program organization units can be delivered by the manufacturer, or programmed by
the user by the means defined in this part of the standard.
Program organization units shall not be recursive; that is, the invocation of a program organization unit
shall not cause the invocation of another program organization unit of the same type.
The information necessary to determine execution times of program organization units may consist of
one or more implementation-dependent parameters.
2.5.1 Functions
For the purposes of programmable controller programming languages, a function is defined as a
program organization unit which, when executed, yields exactly one data element, which is considered
to be the function result, and arbitrarily many additional output elements (VAR_OUTPUT and
VAR_IN_OUT). As for any data element, the function result can be multi-valued, for example, an array
or structure. The invocation of a function can be used in textual languages as an operand in an
expression. For example, the SIN and COS functions could be used as shown in figure 4.
a)
VAR X,Y,Z,RES1,RES2 : REAL; EN1,V : BOOL; END_VAR
RES1 := DIV(IN1 := COS(X), IN2 := SIN(Y), ENO => EN1);
RES2 := MUL (SIN(X), COS(Y));
Z: = ADD(EN := EN1, IN1 := RES1, IN2 := RES2, ENO => V);
b)
+-----+ +------+ +------+
X ---+-| COS |--+ -|EN ENO|-----|EN ENO|--- V
| | | | | | | |
| +-----+ +---| DIV |-----| ADD |--- Z
| | | | |
| +-----+ | | +-| |
Y -+---| SIN |------| | | +------+
| | | | +------+ |
| | +-----+ |
| | |
| | +-----+ +------+ |
| +-| SIN |--+ -|EN ENO|- |
| | | | | | |
| +-----+ +- -| MUL |---+
| | |
| +-----+ | |
+---| COS |------| |
| | +------+
+-----+
a) Structured Text (ST) language - see subclause 3.3
b) Function Block Diagram (FBD) language - see subclause 4.3
NOTE This figure shows two different representations of the same functionality. It is not required
to support any automatic transformation between the two forms of representation.