Until recently, there was no standard industry network protocol for building automation, and users had to choose between many different systems from different manufacturers. Proprietary communications were a result of no off-the shelf communication solution. Today, we have reached a place where there are three inter-operable standard network protocols to choose from: BACnet, LonWorks and Modbus. All three are widely used, and according to a Building Operating Management survey in 2011, 62% of respondents had at least one BACnet application; for LonWorks the percentage was 40%, while for Modbus the number was 30%.
BACnet
Building Automation Controls Network (BACnet) is a network protocol specifically used for multiple devices to communicate across building automation systems by system users and building system manufacturers. It was developed by ASHRAE and is used for applications like boiler control and tank level measurements.
Advantages of BACnet include:
- Scalability between cost, performance and system size
- Endorsement and adoption by nearly every major vendor in North America and many other countries
- Robust internetworking including multiple LAN types and dial-up
- Unrestricted growth and the ability to add new innovations and new features anytime
Disadvantages of BACnet include:
- Limited the number of field devices that can connect to a master station except Ethernet TCP/IP
- MT/TP-Wire Length
- Ethernet-Infrastructure
- New standard has security standard but not implemented in all devices
Other key features of BACnet:
- Ethernet, IP, MS/TP, Zigbee transmission modes
- Meets ANSI/ASHRAE Standard 185; ISO-16484-5; ISO-16484-6
- Low cost; No charge for usage or licensing fees
Modbus
Modbus is a network protocol best used for industrial automation systems specifically for connecting electronic equipment. Although Modbus is best for industrial applications, its simplicity allows it to be a useful tool for building automation as well. It was developed by Modicon Inc. for tasks such as request temperature reading, send status alarm, or fan schedule.
Advantages of Modbus include:
- Easy connection to Modicon
- Suitable for small/medium volumes of data (≤255 bytes)
- Data transfer designed for industrial applications
- Openly published and royalty-free
- Easy to deploy and maintain
- Moves raw bits or words without placing restrictions on vendors
Disadvantages of Modbus include:
- Limited the number of data types; Large binary objects are not supported.
- No standard method for a node to find the description of a data object, i.e. finding a register value represents a temperature between 30◦ and 175◦.
- No security against unauthorized commands or interception of data
- Transmissions must be contiguous which limits the types of remote communications devices to those that can buffer data to avoid gaps in the transmission.
- Great amount of configuration and programming required
- Protocol is not common in the SIMATIC family
Other key features of Modbus:
- ASCII, RTU, TCP/IP transmission modes
- Meets IEC 61158 standard
- Low cost; No charge for usage or licensing fees
LonWorks
LonWorks is a communication network protocol useful for building automation applications designed on a low bandwidth, for networking devices through power lines, fiber optics, and other media. It was designed by Echelon Corporation/ Motorola for Security, lighting systems, HVAC, machine control, manufacturing, and metering.
Advantages of LonWorks include:
- Web based tool; saves time and cost
- Numerous developers of Lonworks products in the market
- Less Architecture at device level
Disadvantages of LonWorks include:
- Outdated
- Controlled devices & variables are connected to a separate control device. (Not recommended due to network interruptions producing system failures)
- Extensions are allowed only through the LonMark Consortium.
- Hardware specific, and requires the Neuron chip for network movement of the protocol.
- Close to “plug & play” ability, yet still far from achieving interconnectivity using Microsoft Windows
Other key features of LonWorks:
- MS/TP, network, SNVT transmission modes
- Meets ANSI/EIA 709.1; ISO/IEC 14908-1, 14908-2, 14908-3, 14908-4 standards
- High cost; Limited users (exclusive to actual members; mostly manufacturers)