A function block diagram, or FBD, is one of the five PLC programming languages defined by IEC 61131-3. Instead of rungs of ladder logic, FBD represents a program as a set of blocks wired together, with signals flowing from inputs on the left to outputs on the right. It reads like a signal-flow schematic, which makes it a natural fit for analog and process control.
Function Block Diagram in one line: A function block diagram (FBD) is a graphical PLC programming language, defined in IEC 61131-3, in which logic is built from interconnected function blocks - each performing an operation such as a timer, PID loop, or comparison - with data flowing along the connecting lines.
Each block in an FBD performs a defined operation - a logical AND, a timer, a counter, a scaling calculation, or a PID controller. The block has input pins on the left and output pins on the right. You wire an output of one block into an input of the next, and the value flows along that connection when the program executes. The visual layout mirrors how the data actually moves through the logic.
This makes FBD especially readable for continuous and analog control. A loop that reads a pressure, scales it, compares it to a setpoint, and drives a valve is drawn almost exactly as an instrument engineer would sketch it. Complex logic that would sprawl across many ladder rungs often collapses into a compact, legible block diagram.
FBD shines for process control, signal processing, and any logic that is fundamentally about moving and transforming analog values. PID loops, filtering, and analog computations are cleaner in FBD than in ladder. It also encourages reuse: a well-defined function block - say, a standard pump-control block - can be dropped into many programs, tested once, and trusted everywhere.
In oil and gas, FBD commonly appears in compressor control, metering, and any skid where analog loops dominate. Ladder still rules for discrete interlocks and permissives, so real projects frequently mix the two. Whatever the internal language, the PLC exposes its results as tags over its native protocol, and a SCADA platform like Merobix reads those tags to trend and alarm on them without needing to see the FBD itself.
Ladder logic is drawn as relay-style rungs and excels at discrete on/off interlocks; FBD is drawn as connected blocks with signals flowing left to right and excels at analog and process control such as PID loops. Both are IEC 61131-3 languages and are often mixed in one project.
FBD is a strong choice for continuous control, signal processing, and analog computations - anything where data flows through a series of transformations. It is also good when you want reusable, self-contained blocks like a standard PID or pump-control block.
A function block is a single reusable unit of logic with defined inputs and outputs; a function block diagram is the language in which you wire those blocks together into a program. FBD is the diagram; function blocks are its building pieces.
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