What Is a Function Block Diagram?

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How a Function Block Diagram Works

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.

Where FBD Fits Best

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.

Execution Order, Feedback, and One-Scan Delays

An FBD network looks parallel but executes sequentially: the runtime evaluates blocks in a defined order each scan, generally following data flow from inputs toward outputs, with the programming tool assigning explicit execution numbers where the order is ambiguous. Most of the time you never think about it. It starts to matter the moment a wire feeds backward - an output routed into an earlier block's input. That feedback value is necessarily the result from the previous scan, so a latch or filter built with feedback carries a one-scan delay by construction.

The practical rules are simple. When logic depends on order, set the execution order deliberately instead of trusting the tool's default, and document why. When you see feedback, ask what the one-scan-old value does on the first scan after a restart, and initialize it explicitly if the answer matters to the process. Edge-detection and latching blocks from the standard library are usually safer than hand-rolled feedback for exactly this reason: their first-scan behavior is defined and documented rather than an accident of layout.

A Worked Example: A Scaled High Alarm With a Delay

Take a pressure high alarm, drawn entirely in FBD with symbolic values. The analog input block delivers raw counts; a scale block converts counts to engineering units using the range configured for the transmitter; a comparison block tests the scaled value against a high setpoint H; an on-delay timer requires the comparison to stay true for a delay time T before passing it on; and the timer's output sets the alarm bit the SCADA system reads.

Reading the network left to right:

1. Raw counts enter from the input channel.
2. The scale block outputs the value in engineering units.
3. The comparator goes true whenever the value exceeds H.
4. The on-delay timer ignores excursions shorter than T, filtering measurement blips.
5. The timer output drives the alarm bit, and the same signal can drive a lamp or feed an interlock.

The point of the example is how little translation the diagram needs: the instrument engineer's sketch, the loop narrative, and the executable program are nearly the same drawing. Change H or T and the structure is untouched; swap the comparator for a low-limit test and you have the matching low alarm. That structural stability is what makes FBD programs pleasant to maintain years after the original author has moved on.

Mixing FBD With the Other 61131-3 Languages

Real programs rarely commit to one language. Discrete interlocks and permissives usually stay in ladder logic, where electricians and technicians read them fastest; heavy math and data handling drop into structured text, often inside the body of a function block that FBD then wires into the diagram; stepwise sequences belong in sequential function charts. The IEC 61131-3 standard explicitly allows a project to mix languages per program unit, so choose per problem, not per project.

A rough map of where each language earns its place:

Language	Strongest at
Ladder (LD)	Discrete interlocks, permissives, motor logic
Function block diagram (FBD)	Analog signal flow, PID loops, reusable blocks
Structured text (ST)	Math, loops, data handling inside blocks
Sequential function chart (SFC)	Stepwise sequences, batch phases, startup logic
Instruction list (IL)	Legacy code only; deprecated in the current edition of the standard

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