The Building Automation and Control network (BACnet) protocol is a standardized set of rules that governs how DDC systems exchange information
The use of BACnet (or any open protocol) may result in a loss of system performance and functionality. This is because BACnet is designed to provide the tasks that the daily operator needs to accomplish. It does not address system programming, configuration, or setup. These are all functions that will continue to be achieved in unique manners by each supplier. This means that when BACnet is applied correctly, it can deliver an interoperable system. However, the Owner will still need unique software packages in order to edit or reconfigure the system.
What is communicated? BACnet represents common building automation and control functions as collections of information called objects. Presently BACnet defines 18 standard objects, including analog inputs and outputs, binary inputs and outputs, and schedules.
* BACnet defines methods for exchanging data. These methods are referred to as services and are used by system suppliers to exchange (read, write) information between systems. BACnet can work over a variety of commonly available network technologies used in the DDC industry, including Ethernet, ARCNET, EIA-232, MS/TP(EIA-485), and the LonTalk data link layers.
Engineer must first decide what interoperable functions should be provided.
* Data Exchange: The most basic level of interoperablity is support for data exchange. This allows one device (either a controller or PC) to view (read) or change (write) data that exists on another device. While this function is fairly basic, it can be used to achieve a number of tasks. It can be used to connect equipment from multiple suppliers in to a single system, to display information on a PC workstation graphic, or to collect data for reports and logs.
* Alarms and Events: This provides the ability to have a controller recognize when a problem exists and to communicate this to another device (typically a PC workstation). This is most commonly used to annunciate an alarm from a BACnet controller to a PC workstation, but also can be used to provide control interlocks.
* Scheduling: This allows for a schedule to be defined or modified at a BACnet workstation and executed in a controller.
* Trends: This function defines how data is to be stored in a controller so that it can be retrieved by a BACnet workstation.
* Network Management: This function covers the tasks associated with a network of controllers. For example, it allows for the time to be coordinated in all controllers.
Due to the flexibility and openness of the BACnet standard, all parties involved in the planning process will need to be focused on the desired performance and functionality of the system. Since the BACnet standard is not yet formally supported by a third-party testing and compliance organization, it is the design team’s responsibility to create a clear specification to ensure that interoperability is achieved among the various manufacturers’ DDC systems connected to the BACnet network. The ASHRAE BACnet committee (SSPC-135) is working to establish a testing and certification agency so users can be assured that two manufacturers’ systems will communicate with each other.
The BACnet standard currently defines an approach to specifying the above functionality. This approach is currently under revision by the ASHRAE SSPC-135 committee. Therefore, the specifier should consult the BACnet standard before specifying a BACnet solution.
The above summary was prepared by Sinclair Energy from the GPC-13P: Specifying Direct Digital Control Systems for more information please review this document.
DIRECT DIGITAL CONTROLS (DDC) – What is it and why should I be interested.
The fact that all technologies are converging could not be truer than in the case of large building and home DDC control. Both industries have discovered the power of networking. In the large building the requirement for indiviual room control and closer thermal zone control has created smaller , low cost , flexible and more powerful application specific controllers. Unlike standalone home devices with their own simple local interfaces the industrial DDC panels network their interface to powerful graphical interfaces. These virtual interfaces are extremely powerful and allow real time data to be superimposed on actual digital photographs or line art. This sophisticated interface is present at any dial up or networked location. The industrial controls generally have greater accuracy and more power in the area of control. Most have a control language that is like a blank page that allows any relationship to be created between any input or output point on that panel or any other panel connected to the network. Standard interface Input and Output voltage levels with easy software scalable ranges allows specialized sensors and outputs to be added to achieve control in all applications. The trending capabilities of most industrial DDC panel allow every input and output to be traced and quickly displayed on networked graphical display software. Access to this powerful feature can be as simple as a click on a icon on the relative graphical screen. Typically a floor plan or virtual panel interface. Other features like runtime totalization of output is normally an included feature.
I feel that in the near future both sides will learn from each other and the technology convergence will continue to provide smoother and seemless interfacing with our enviroment.
Ken is President of Enviromation Services Inc a virtual company that’s goal is to Automate your Environment .