Data Communication Protocols
In Building Automation Systems
– An Overview
BACNet and KNX both are reliable and robust systems, independent and open systems, and hence are supported by a large number of manufactures across an exhaustive product range...
- Aneesh Kadyan
Building Automation Systems (BASs) provide means to control as well as monitor various operational aspects of buildings. Activities that typically require human interface such as data logging, equipment health monitoring, data analysis etc. can be automated by a BAS system to reduce human error – as well as enhance efficiency and accuracy of data. With the cost of manpower increasing, and the need to operate and manage buildings at the lowest possible cost, BASs are finding their ways in more and more building designs. A BAS while costing more at the build stage helps lower operating costs for the owners and the payback periods for the Capex are in the range of 2 to 4 years.
A typical BAS installation enables equipment and systems of a building such as the generators, HVAC plants, domestic water systemsetc. to be 'connected' to a management system through sensors. Sensors capture system data such as pressure, temperature, flow etc., as well as the state of the equipment – on or off, speed etc. The sensors convert the data to electrical signals that are transmitted to the analysis device, usually a microprocessor or a computer where the data is again converted to enable the device to 'read' the data. The output of this data collection activity is the physical value the operator sees on the computer screen of the controller. Data can be logged in the computer’s memory or output to standard printers for review.
A BAS can be a simple monitoring tool where only system data is captured and analysed, but no active control of the equipment is provided. Advanced BAS have both monitoring and control abilities, which enable remote start/stop as well as operations of the systems.
BAS communications protocols
While a BAS has a number of components, the sensors, actuators and the controllers are the most important and critical elements. The sensor senses the physical quantity and the controllers manipulate the system through actuators to the required end points. There are a plethora of manufactures of both sensors and controllers, and hence, for the sensors, actuators and controllers to function correctly in a system, a common method of understanding data is essential. A communication protocol is a means for a device manufacturer to make sensors that can send out data in a way that the controller is able to read the data and take suitable action.
There are two main classifications of communication protocols – Proprietary and Open protocols. The proprietary systems are designed by manufactures that have the entire gamut of BAS systems inhouse or where there are specific building requirements. Open systems are the more common ones as they allow a larger number of options for the end users. The most commonly used protocols in the building automation industry are the BACnet, Modbus and LONWORKS.
BACNet (Building Automation and Control Network)
This protocol was specifically designed for the building automation sector, and the HVAC systems, and hence is one of the most commonly used protocols. The BACNet protocol is an ASHRAE/ANSI standard (135P) and was introduced in 1995. BACnet devices have a microprocessor that associated software to use the BACnet protocol for system application. The BACNet protocol uses 'Objects,' 'Properties' and 'Services' to represent data of the system.
- Objects: Each object has an identifier and a number of properties associated with it. The properties are used to monitor and control the object. 54 standard objects are defined in the protocol, such as Analog Input, Analog Output etc.
- Properties: The properties define a BACNet object. An Object can have multiple properties and each property has an identifier and a value. The property can be read only, where other objects read the value of the objects property and read/write where the property value can be changed – e.g. changing set point of a VAV controller.
- Services: This is the process by which one BACnet device request for information or gives instructions to other BACNet devices to carry out tasks. The BACNet protocol has a list of 32 standard services and categories into Object Access, Device Management, Alarm and Event, File Transfer and Virtual Terminal.
The BACNet protocol is very versatile and widely used in the industry as it can be used on most common network systems such as Ethernet, ARCNET and IP. The IP protocol allows BACNet devices to connect over standard internet networks, thus, allowing remote monitoring and operations. A typical BACNnet system is shown in figure 1.
Fig. 1: Typical BACNet Network...
The KNK communication protocol is an approved European (EN) and International (ISO) Standard, which is also widely found in building automation system applications. The protocol can be used over simple Twisted Pair lines as well as RF and IP networks.
The KNK protocol has two configuration modes which manufactures provide
- S Mode (System Mode), which is used for complex installations requiring a high level of customisation and features.
- E Mode (Easy Mode) provides basic functionalities to the devices and can be programmed without any specialised knowledge and tools. This mode is used for simple systems.
A KNX system has two main components – The sensor and the actuator. Data between Sensors and Actuators are shared in the form of 'data telegrams.' These telegrams are sent out to Group Addresses which are logical names for 'topics.' The topics link the output of a sensor to the input of an actuator. Sensors emit the telegram and multiple sensors listen into the topics.
A KNX system can be used without a controller during normal operation – as a processor is not required for the group to listen to the telegrams. This allows the system to be robust as well as reduces cost.
An Engineering Configuration Tool (ETS) is used to configure KNX devices that use the S Mode configuration. Figure 2 shows a typical KNX installation.
KNX has been developed for use on a large number of building management systems and covers:
- blinds & shutters
- Heating, Ventilation And Air Control (HVAC)
- Audio/Video Control (AV)
- operation and visualisation
- remote access
Fig. 2: Typical KNX Network...
BAS are essential components of modern buildings, with many Green certification systems mandating them. The BAS systems enable better control of the various equipments and systems in a building, resulting in higher efficiencies and reliability. Which system to use, BACNet or KNX? Both are reliable and robust systems, independent and open systems, and hence are supported by a large number of manufactures across an exhaustive product range.
The choice of communication protocol to be used is usually decided by the equipment and control elements that are installed and the level of flexibility that is required. Building automation products typically use the BACNet protocols – while the building management systems tend to deploy the KNX protocol.
Author is from CBRE South Asia Pvt Ltd.
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