Field-Ready · Lesson 6.2
Modbus
Older than BACnet, simpler than BACnet, and still on almost every mechanical system you will touch.
What Modbus Is and Why It Survived
Modbus was developed in 1979 by Modicon for communicating with programmable logic controllers on factory floors. It was not designed for buildings. It predates BACnet by about 15 years. It has no concept of objects, no device discovery, no COV subscriptions. It is about as simple as a communication protocol can be.
And it is still everywhere. Energy meters, VFDs, chillers, boilers, power monitors, cooling towers, generators: the mechanical equipment world runs on Modbus because it is simple, robust, and universally understood. Every PLC, every industrial drive, and a huge fraction of commercial HVAC equipment supports it.
Why it survived
Simplicity is a feature. Modbus has a small footprint, requires no configuration beyond a device address and baud rate, and works reliably on cheap hardware. Every engineer and tech in every country knows it. There is nothing to configure that can break. You point at a register, you get a number. That reliability matters more than elegance when you need a chiller to talk to a JACE on a Tuesday morning.
As a BAS tech, you will use Modbus primarily to integrate equipment that speaks it natively (energy meters, drives, chillers) into a BACnet or Niagara head end using a Modbus driver. You need to know how the data is organized on the device side so you can map it to BACnet points on the supervisor side.