What are Function Block Diagrams (FBDs) in BMS?

A Function Block Diagram (FBD) is a graphical programming language used extensively in Building Management Systems (BMS) to represent control logic. It is one of the programming methods defined by the IEC 61131-3 standard for programmable controllers.

In an FBD, control functions are represented by interconnected "blocks" that perform specific operations. These blocks can range from simple logic gates (AND, OR, NOT) and mathematical functions (add, subtract) to more complex control algorithms like PID loops, timers, counters, and comparators. Inputs flow into the left side of a block, and outputs emerge from the right, with lines connecting the output of one block to the input of another, forming a visual flow of data and control.

FBDs are popular in BMS for several reasons:

• Visual Clarity: They offer a highly intuitive way to visualise and understand complex control sequences, making it easier for engineers to design, debug, and maintain systems.
• Ease of Use: Programmers can quickly assemble control logic by dragging and dropping pre-defined function blocks and connecting them.
• Troubleshooting: The graphical representation helps in tracing the flow of logic and identifying issues during commissioning or fault diagnosis.
• Standardisation: Being part of an international standard, FBDs promote consistency across different controller manufacturers and platforms.

Engineers use FBDs to program DDC controllers for various applications, such as controlling HVAC equipment (AHUs, chillers), lighting systems, and other building services, by defining how inputs (like sensor readings) are processed to generate outputs (like actuator commands).

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Related Topics

• What is BMS integration? — how a BMS connects with VFDs, energy meters, BACnet/Modbus devices and other building systems
• How to design a BMS system step by step — the complete BMS design methodology covering site survey, IO list, controller selection, sequence of operations
• What is a Building Management System (BMS)? — fundamentals of BMS controls and architecture for HVAC, lighting, energy and access
• What is BMS commissioning? — the disciplined commissioning process that turns a BMS install into a working building brain
• Browse all Control Loops & FBD topics — more from this section of the EnSmart BMS Library