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

4.3 Function Block Diagram (FBD)
4.3.1 General
This subclause defines FBD, a graphic language for the programming of programmable controllers
which is consistent, as far as possible, with IEC 60617-12. Where conflicts exist between this
standard and IEC 60617-12, the provisions of this standard shall apply for the programming of
programmable controllers in the FBD language.
The provisions of clause 2 and subclause 4.1 shall apply to the construction and interpretation of
programmable controller programs in the FBD language.
Examples of the use of the FBD language are given in annex F.
4.3.2 Combination of elements
Elements of the FBD language shall be interconnected by signal flow lines following the conventions
of 4.1.2.
Outputs of function blocks shall not be connected together. In particular, the “wired-OR” construct of
the LD language is not allowed in the FBD language; an explicit Boolean “OR” block is required
instead, as shown in figure 24.
a)
b)
| a c |
+---||--+--()--+
| b | |
+--||---+ |
| |
+-----+
a---| >=1 |---c
b---| |
+-----+
Figure 24 - Boolean OR examples
a) “Wired-OR” in LD language
b) Function in FBD language
4.3.3 Order of network evaluation
When a program organization unit written in the FBD language contains more than one network, the
manufacturer shall provide implementation-dependent means by which the user may determine the
order of execution of networks.
IEC 2501/02
Copyright International Electrotechnical Commission 
Provided by IHS under license with IEC
Not for Resale
No reproduction or networking permitted without license from IHS
--``````-`-`,,`,,`,`,,`---

---

– 144 –
61131-3 
 IEC:2003(E)