-Rob Frohock

See also: Modbus RTU verses Modbus TCP/IP: What’s the Difference?

In industrial settings, Modbus is one of the oldest protocols for devices to communicate their data digitally to a central controller.  It allows transmitters to send integer data to controllers…it allows controllers to send integer data to SCADAs…and it is very easy to use.  BACnet is another protocol but started in the building automation sector and while it to can send integer data, BACnet was design to send many other types of data. 

BACnet stands for “Building Automation Control Network”.  As the name implies, BACnet began as a communication standard for devices communicating to a building automation system.  These devices originally performed simple tasks such as measuring flows through pipes and air ducts, even thermostats in commercial buildings.  Today much more complex devices, such as variable frequency drives and boiler control panels, use this protocol to transmit hundreds of data points to the central control system. 

BACnet started by using a RS-485 serial communication backbone (called BACnet MS/TP) similar to many other protocols.  It allows 31 devices to be daisy-chained on a single twisted wire pair that can be almost a mile long.  Over the years the speed (or “baud rate”) increased to allow more data to flow back to the central control system.  More recently, a variant protocol was created to use ethernet (called BACnet/IP).  Ethernet is much faster than serial communications and allows hundreds of devices on the same network.  It uses the same backbone as ethernet found in office buildings.  It operates at 10 Mps, 100 Mps, or 1 Gps and can use the same devices that any ethernet network uses, such as managed switches, fiber optic cable, etc.

BACnet is different from Modbus is several ways.  First, the data that BACnet sends can be much more complex.  The data is organized in “objects” and each object has “properties”.  For example, a pressure transmitter might send one object called “Process Pressure”.  This object would have a property called “Object Type” set as “Analog Input”; a property called “Current Value”; a property called “High Alarm Setpoint”; a property called “Current Status”, etc.  Each one of these properties has a value which could be an integer, a Boolean (True or False), a string of text, or other forms of data.  The central control system can be programmed to read all the properties, or just a few.  Each device can transmit many objects and each object can have many properties, so the data available can be vast and complex.  Even video can be an object on BACnet/IP.

What also makes BACnet different from Modbus is a feature called “dynamic device binding.”  Think of this as the central control system shouting to the whole network “Who is out there?” and all the devices responding with their name, address, and objects available.  This makes installing and commissioning a BACnet network much easier and less prone to errors in syntax when mapping the data from objects.  To provide all this functionality, manufacturers must program their devices exactly to the BACnet standard which is maintained by ASHRAE, but the BACnet Testing Laboratory provides a third-party service to ensure the product will perform correctly.

If you are providing equipment that will communicate to the central control system at site, you will be informed of the protocol that is required.  If you are choosing a central control system, the choice of protocol is an important one, as it controls the type of data that will travel on the network.  However, if you have devices that only communicate via Modbus, but need to be installed on a BACnet, there are protocol converters available to allow this.  Note these protocols need to be programmed to convert the data from one protocol to another.

 In summary, Modbus and BACnet will continue to be more popular in the sectors from with they started, but as industry and building automating continue to overlap, it is important to have a basic understanding of both of these protocols and know their advantages.