BACnet Protocol. Frequently Asked Questions
David Fisher
PolarSoft® Inc.
The author of this article teaches BACnet courses in the US and is a member of the ASHRAE
BACnet SSPC-135 committee
What is BACnet?
BACnet stands for Building Automation Control network and is a communication protocol for building automation developed by ASHRAE (ANSI/ASHRAE standard 135-2001) and recently ISO 16484-5. The main goal of the BACnet protocol is to standardize the interaction between building automation devices from different manufacturers, allowing for the exchange of information and the joint operation of equipment.
Why was the BACnet protocol developed?
ASHRAE recognized that building automation systems needed a common, standardized protocol. This was due to the fact that this field, by its nature, tends to use proprietary protocols that do not allow existing systems from different manufacturers to work together and interact.
In 1987, ASHRAE attempted to develop a protocol (set of rules) that would govern the interaction between various devices used in building automation systems. The BACnet standard has now been adopted by ANSI (American National Standards Institute) and ASHRAE, and has received international recognition and has been adopted as a national standard in a number of countries. This laid the foundation for the future development of this industry.
What is BACnet equipment?
BACnet devices are similar in physical form to other standard building automation devices you may be familiar with, but their physical form is not the point, because BACnet is simply a set of rules for how devices in a building automation system communicate with each other. The microprocessors in these devices are programmable, which means they can “understand” each other and comply with the general requirements of the BACnet protocol. The physical nature of the device itself remains unchanged.
Does BACnet offer anything special that DDC (Direct Digital Control) systems cannot?
Yes, it does. BACnet gives you the ability to choose the optimal equipment from the manufacturer you prefer for a specific area of work. Instead of being tied to a specific brand of building automation system manufacturer that is already installed in the building. Since you have more options, this allows for a more efficient installation of equipment to optimize its future operation. For both new buildings and those being renovated, BACnet provides easy future modification and expandability. With the right approach, new devices can easily be integrated with a previously installed BACnet system.
The more BACnet devices are developed, the easier it will be to solve the problems of integrating complex systems (for example, access control, security or fire protection systems) and direct cooperation between companies will become an easily solved everyday issue.
Is the BACnet system easily expandable?
Scalability of systems was the main driving force behind the creation of the BACnet protocol. As a result, this standard is truly open. The protocol allows you to choose from a larger number of equipment manufacturers. The correct selection of equipment allows not only to make the system more scalable, but also more efficient to use. Newly developed products for building automation systems can be freely integrated into existing BACnet systems, thereby providing for the requests and requirements that may arise for these systems in the future.
What types of buildings and their sizes are best suited for the installation of BACnet systems?
As with standard DDC systems, any size building can be equipped with a BACnet control system. These systems can be simple, consisting of just a few devices, or quite complex. BACnet is an open standard as long as strict criteria for device compatibility are met. Therefore, it is cost-effective for even small facilities to adopt BACnet control systems. As equipment manufacturers strive to develop their products to meet the needs of the BACnet market, more and more devices will appear for various building automation applications.
Does a BACnet system provide a more efficient HVAC control solution than conventional DDC systems?
Not necessarily. BACnet is basically a set of communication rules for building automation equipment. This is not to say that it automatically provides a better building management system. However, what differentiates building automation systems in performance is how accurately complex information is processed and distributed among the controlled devices. The BACnet standard was designed to be open to future changes and extensions of the system. Even allowing devices to have exclusive (closed) functions while demonstrating BACnet compliance.
For example, Manufacturer A may provide a complete BACnet building management system that can accept and integrate devices from Manufacturers B-Z. However, Manufacturer A's system may include several exclusive mappings that will only be available to Manufacturer A's own equipment. This does not mean that equipment from Manufacturers B-Z will not work in Manufacturer A's BACnet system. Manufacturer A has simply added some additional features to help them further advance their products. And you may want to carefully consider what additional features, exclusive to one manufacturer, a system includes before deciding to purchase it.
What is the benefit to a building owner of using a BACnet system?
If for some reason the building owner is dissatisfied with the product, service, cost of replacement equipment, or other aspects related to the specifics of a particular vendor that has installed a BACnet-compliant system, he will always be able to find a suitable replacement from another manufacturer. The building owner can rest assured knowing that the agreed-upon BACnet equipment will work in the system regardless of the manufacturer. In addition, if the manufacturer stops producing BACnet equipment, the building owner will not have to replace the entire system or repair obsolete devices.
Here are some more benefits that building owners can expect:
— The ability to select and add more sophisticated devices to their system when they become available.
— Potential cost savings due to the ability to select lower cost equipment.
— Easy integration with pre-installed BACnet controllers on various equipment (such as boilers or chillers).
What benefits will a property developer receive from using a BACnet system in the building they manage?
Yes. BACnet makes building management system manufacturers more competitive, giving end users a wide range of choices and the possibility for future expansion. This allows the development company not to be tied to only one vendor. Like any DDC (direct digital control) system, the BACnet protocol provides the ability to remotely monitor. For small projects, this service can save money, since the management company can monitor several objects at once from a single control point. One operator interface can be used for several systems.
An additional advantage is that you can choose one operator interface to control a system with equipment from different vendors. Your choice can be based on graphical capabilities, hourly data collection, or other features of specific manufacturers.
Are there any cost-saving advantages when comparing a BACnet system to a conventional DDC control system?
Of course there are. The easier it is to integrate the various services in a building, the more the cost of monitoring and managing the building will be reduced. Here is what you can expect in this case:
— higher quality and variety of information that can be obtained from the systems during the operation of the building;
— interconnected assessments of the parameters of various systems and their analysis;
— better financial planning and management of building operations.
The better the integration with energy suppliers, the more accurate data on energy supply and consumption will be available to the building management service. Ultimately, building energy management will be more cost-effective, and coordination between energy suppliers and consumers will be improved.
Can BACnet equipment be used in building renovations?
Yes. One of the greatest strengths of BACnet is its openness and ability to integrate different interfaces. As more equipment manufacturers support BACnet, a variety of devices will appear on the market to take advantage of this capability. Some devices allow existing non-BACnet systems to interface with BACnet devices. Once the right interface is selected, other BACnet products can be used in conjunction with existing components.
Previously, the only chance to make a significant addition or extension to a building management system was to completely redesign it. With the advent of the BACnet protocol, this can now be avoided.
How can consulting engineers be involved in the implementation of the BACnet protocol?
A very important role is given to professionals responsible for selecting equipment for BACnet systems. Engineers must constantly improve their knowledge of the BACnet protocol, BACnet equipment, and networks. When writing specifications for facilities, it is important to follow the development of the BACnet protocol and not simply say “the system must be BACnet compliant.”
As the developer of the BACnet protocol, ASHRAE is an excellent source of training materials. As the building management system integrates with access control/security, transportation, fire suppression, and other systems, engineers will need knowledge of BACnet.
Since the protocol requirements are the same for all equipment manufacturers, the prices for BACnet devices will be competitive. This is beneficial for engineers who want to get as many different functions as possible in a building management system for as little money as possible.
BACnet systems are flexible and easily expandable by design. The more new ideas that are used in industry, the less closed systems (with internal protocols) will be in demand.
Given the inherent nature of BACnet communication between devices, a lot of equipment has been designed with remote access capabilities. It is already quite common to have a connection between an engineering company's office and a building automation system. This access to BACnet system management means time and money saved for the consulting company, given that there is no need to install monitoring equipment.
Does the operating organization need to retrain to effectively monitor and manage a BACnet system?
Probably not. If the operating company is familiar with the vendor's end equipment, it will likely require some retraining when switching to a BACnet system.
The communication component of the BACnet system will be transparent to the operator. The system is displayed to the end user in the same way as all systems from any manufacturer. Typically, monitoring and control points with their corresponding values will be displayed with their specifications. The operating terminal of the selected equipment manufacturer can interact with control systems from other manufacturers. This means that if the operator is familiar with the control system from one manufacturer, he can continue to use it, despite the fact that controllers from other manufacturers are used.
However, as new products from equipment manufacturers come to market, the operator may need to have more in-depth knowledge of the system installation and programming. Of course, this is true for any new building management system.
Does BACnet provide the means to manage more than one building?
Yes, internetworking is built into BACnet equipment. For example, campus buildings can be connected to existing or new local area networks (LANs). Buildings not directly connected to LANs can be controlled remotely via dial-up.
Is there an independent agency that certifies BACnet equipment?
Yes. The organization is called BTL — BACnet Test Laboratories, which tests and certifies BACnet equipment. BTL was formed by the BACnet Manufacturers Association (BMA) to provide independent testing for BACnet devices. In February 2001, BTL began testing equipment based on the ASHRAE 135.1P standard.
Is it true that extensions in proprietary OEM systems to BACnet make devices incompatible?
Not necessarily. There are really three areas where BACnet can be extended: Object Properties, Services, and Enumerated Values. If a device extends its functionality in these three areas, it will also be able to interoperate with other BACnet devices that use the standard Objects, Properties, and Services.
If a device uses non-standard object types or non-standard properties of standard objects, it can still effectively interact with other objects and properties. BACnet object properties are a specific data type. BACnet defines 12 so-called «primitive» or «application» data types, including real numbers (floating point), fractional and integer numbers, signed notations, bit notations, etc. BACnet allows «constructed» data types, which are collections of primitive and other constructed data types. If a property of an object uses a primitive data type, then it is possible to interact with the device without special translation of the context of the values that appear. Simply put, those objects that use simple (primitive) data types for their properties can interoperate more universally than others. If this rule is followed, equipment manufacturers have a huge freedom of action in the application of equipment without any additional development.Non-standard services used in a device generally require the presence of special software. And this is usually used in areas where the ability to interact between different systems requires additional approvals and permissions. If a BACnet device heavily depends on non-standard services, it will be limited in its interaction with other devices and systems.
Extended enumerations can appear in several areas of BACnet system communication. A common area is in alarm and event reporting. Devices can be designed to report extended event types that are not defined in the BACnet standard and use vendor-specific event types. This does not mean that interoperability between devices and systems is impossible. It just may not be optimal in some cases. For example, an operator workstation might report an «Event Type 456» alarm if no meaningful meaning for that event is set.
Are standard objects really preferable to objects in vendor-proprietary systems?
In general, no. Standard objects have the advantage of behaving as already described in the BACnet standard. Objects in proprietary systems (we prefer to call them «non-standard objects») cannot be used in any way unless you know they exist and have a description of what they do. Assuming you know what non-standard objects are and what they can do, there is no reason not to take advantage of this opportunity.
The mechanism of non-standard objects and/or properties is one of the most important features of the BACnet standard. In general, it cannot be said that it is easier or more difficult to read and write the properties of standard objects than non-standard ones. The stumbling block is that for non-standard object properties, in order to have easier interaction with other devices and systems, it is necessary to limit their scope of application with the so-called «primitive» data types. Otherwise, the properties of objects that do not take this limitation into account cannot be interpreted from the general context and will require special software that will «understand» the specifics of non-standard data types. As a rule, BACnet devices do not have these non-standard data types, unless some solution becomes popular and many equipment manufacturers begin to copy it.
In summary, it is important to remember that if an equipment manufacturer has designed a custom object with the prospect of its interaction with other systems and devices, in many cases it can be used more efficiently and easily than standard objects. Consider, for example, a special type of controller that includes 50 parameters. One way to implement BACnet in this controller might be to represent the value of each parameter as a Present_Value property of a BACnet object's binary or analog value. The advantage of this way is that everyone knows how to use BV and iW objects. However, even with the minimum set of required properties, BV and iW objects have a number of additions related to memory space and required functionality. In contrast, those 50 parameters can be represented more efficiently than 50 properties of a custom object (or 5 objects with 10 properties, or 2 with 25, etc.). In this case, no modifications are required. And the explanations of how these parameters work could be much simpler.
Why not use OPC instead of BACnet?
By definition, OPC is a centralized gateway solution. OPC defines a software interface so that workstation applications, such as an operator workstation, can use a common interface with any number of «drivers» that have detailed information about the specifics of the communications and protocols. So everything about OPC depends on how the «client» software talks (on the same computer) to the «server» software. There are a few downsides to this.
The only internal communication between system components must occur within the centralized workstation software. This is slow, limits scalability, and is unreliable.
Because it is centralized, you cannot take advantage of system expansion, part replacement, or flexibility at the subsystem and component level.
This architecture implies a huge dependence on the reliability of the computer and the Windows operating system. And this combination cannot be called reliable in terms of long-term operation.
If a project involves a large number of equipment manufacturers, there is a big potential problem with support and maintenance, because individual components of vendors change and are updated. And the corresponding OPC servers must also track these changes, hence additional maintenance costs appear, which the Customer will clearly not like.
The OPC interface is an «oriented control process», gravitating towards a memory array architecture for data and centralization of control and monitoring. This condition greatly limits the capabilities of the system in comparison with BACnet, which has a high degree of distribution and object-oriented design. In this case, maintenance is more expensive when you
bind each data object to a fixed architecture. Since this structure changes over time, all moments with access attributes to it must be tracked and changed. There is no such problem with BACnet objects.
It can be said that OPC is an open technology in the sense that anyone can freely use it. You can find even greater openness in the international BACnet standard, which, unlike OPC, is not permanently dependent on the Windows operating system.
The cost of the workstation software is the same whether you use OPC or something else. The more important question is: which interface is supported more widely in the environment of the devices you want to automate? Almost every manufacturer of building automation equipment can provide a BACnet interface at many levels. Why limit your entire system design to OPC?
Most third party workstation software has both OPC and BACnet options, depending on the software drivers.
So don't be fooled. BACnet has a very wide range of capabilities and options for OEMs, and that range is growing as BACnet has become an international standard. By looking at the system from the core (instead of the workstation), the Customer has more flexibility in choosing the components of the system at each level, creating true distributed collaboration, and a higher cost lag. And none of the functionality of their workstation will be changed.
By reducing the entire system to a single OPC solution, this is not possible. The Customer will eventually run into cost barriers to expanding and maintaining the system. A
Translated by A.A. Golovin
Executive Director of the BIG-RU Association.
The original article is available on the website
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