Funktionsblockdiagram

Funktionsblockdiagram, or function block diagram (FBD), is a graphical programming language used to implement control strategies on programmable logic controllers (PLCs). It is standardized as part of IEC 61131-3, the international standard for PLC programming languages. In an FBD program, logic and control functions are represented by interconnected function blocks. Each block exposes input and output pins; blocks can be basic (AND, OR, NOT, timers, counters) or user-defined, and can be organized into libraries for reuse. Wires convey data between blocks, forming a dataflow network.

Programs are executed by the PLC in cyclic scans. Blocks read their input values, compute outputs, and propagate results along wires. Some blocks retain internal state (for example, timers or counters). The graphical nature of FBD makes it well suited for implementing discrete controls, sequencing, and process control logic.

FBD emphasizes modularity and readability; it is often preferred for visualizing signal flow in complex control schemes. It contrasts with ladder logic in presentation, though both languages can implement similar logic, and many PLCs support both within the same project. Compared with text-based languages such as structured text, FBD can be easier to interpret for relay-logic and state-driven designs, though large diagrams can become difficult to manage.

Industry practice typically relies on libraries of standardized function blocks and tool-supported simulation and code generation. FBD remains a core part of IEC 61131-3 and is widely supported in industrial automation software and hardware.