Glossary · PLC and control programming
Function block diagram (FBD)
Also known as: FBD, FUP
German: Funktionsplan (FUP)
In IEC 61131-3, the function block diagram (FBD) is a graphical programming language in which functions and function blocks are represented as boxes connected by signal lines, similar to a circuit diagram of logic gates.
- PLC programming
- Standards
In one sentence
FBD is a graphical IEC 61131-3 language that connects function and function block boxes with signal lines, like a logic diagram.
Example
An interlock is programmed in FBD as an AND box whose inputs are GuardClosed and PressureOK and whose output enables the pump.
How it applies
- Engineering: FBD is popular for Boolean logic, interlocks and signal processing because the data flow is visible. Many safety PLC environments use FBD or a restricted variant of it for safety logic.
- Commissioning: Online monitoring shows the signal state on every line, which makes it easy to see which condition blocks an output.
- Maintenance: Electricians and technicians who are used to logic diagrams often read FBD more easily than text code.
- Documentation: When program printouts are handed over, FBD networks need titles and comments that explain the purpose in plain language. For interlocks, the network title should match the interlock name in the specification.
FBD vs. LD
Both are graphical IEC 61131-3 languages. The Ladder diagram (LD) draws logic as contacts and coils between power rails, derived from relay circuits. FBD draws logic as boxes and signal lines, derived from logic and signal flow diagrams. Many tools can switch simple networks between the two views. German tools call FBD Funktionsplan (FUP).