Archive for the ‘BACnet MS/TP’ Category

BACnet – Introduction

Monday, June 15th, 2009

BACnet® has turned the corner. The industry is no longer playing a wait-and-see game to assess whether the Building Automation and Control networks protocol delivers as promised.

BACnet stands for Building Automation and Control Network. BACnet is the standard that was developed by ASHRAE — in conjunction with building management organizations, system users and building system manufacturers — specifically for building automation and control equipment. In 1995, after years of development and revision, the ASHRAE Board of Directors ratified and published the standard as ASHRAE 135-1995. The standard was submitted to ANSI and, in 1995, it also became an American National Standard, ANSI/ASHRAE 135-1995.

Over the next six years, the standard underwent a number revisions and upgrades. In 2001, ASHRAE released an updated standard, ASHRAE/ANSI 135-2001. In 2003, BACnet became an international standard, ISO-16484-5.

Essentially, BACnet is a set of rules governing the exchange of data over a computer network to facilitate
interoperability of various building systems. The five interoperability areas are:

  • Data sharing
  • Alarm and event management
  • Scheduling
  • Trending
  • Device and network management

As a common communication language, BACnet makes it possible for systems from different manufacturers and/or systems designed for different building automation and control functions to work together. BACnet equipment extends to controllers, gateways, routers and diagnostic tools, and provides a means to send data to a workstation.

Main Benefits for Building Owners
The benefits of interoperability within a BACnet-based facility are obvious. In theory, virtually any automated building control function can be monitored from a single operator workstation, regardless of the control system manufacturer and without the need for gateways that translate data between different systems. This simple, seamless interface levels the playing field between manufacturers and building owners, resulting in more competitive procurement of control systems.

Empowering end users through standardization was the primary motivator for ASHRAE to form a committee that led to BACnet’s creation. The standard is especially beneficial for large facilities and campus environments, where building control and automation is extensive.

BACnet devices are limited in their effectiveness unless they can carry messages over a data network. There are a number of ways that BACnet allows this: Ethernet, Arcnet,
LON,® MS/TP, and RS-232. The BACnet standard was amended in 1997 to include BACnet/IP. To date, BACnet/IP over Ethernet has been the most common choice of BACnet networking between systems from different vendors.

BACnet is a nonproprietary, open protocol communications standard. It can be applied to practically any type of system found in buildings today, including HVAC, lighting, life safety, access control, transportation and maintenance. By design, it can use a wide range of network technologies for communications. It is a written specification that includes everything from what type of cable must be used to how to initiate a particular information request or command. Its rules are specifically designed for building automation and control equipment, covering such tasks as how to request a temperature reading, send a status alarm or establish a fan schedule.

The approach that BACnet developers took when developing the standard was that for a system to be truly interoperable, there must be some standardized agreement covering two major areas: overall system operation and individual system components. They accomplished this by using an object-oriented approach in examining, controlling, modifying and interoperating with information in different devices. The BACnet object-oriented model has two major components: objects and services.

In BACnet, objects are collections of properties, each representing some bit of information. In addition to standard defined properties, objects may include vendor-defined properties as long as they function in accordance with the standard. BACnet also defines the expected behaviour from each property for that object. What makes the object-oriented approach work is that every object and every property as defined by the system is accessible in exactly the same manner.

The process of reading or writing to a property is what BACnet calls a service. Services are the methods used by any BACnet device when it communicates with another BACnet device, including retrieving information, transmitting information or communicating an action. The standard defines a wide range of services for accessing objects and their properties.

To help ensure that products developed by different manufacturers conform to the BACnet standard, a testing laboratory was established. The laboratory tests and certifies any device for conformance with the standard. The laboratory has also developed a complete set of testing procedures that are to be used by manufacturers.

BACnet is “a data communication protocol for Building Automation and Control networks.” This data communication protocol is a set of rules developed by the BACnet committee at ASHRAE governing the exchange of data over a computer network. The rules take the form of a written specification that spells out what is required to conform to the protocol.

There are 5 different options for BACnet, each of which fills a particular niche in terms of the price/performance tradeoff. The first is Ethernet, the fastest at 10 Mbps with 100 Mbps also available. (“Mbps” stands for “millions of bits per second.”) Ethernet is also likely to be the most expensive in terms of cost per device.

There are two forms of BACnet for Ethernet. One is called BACnet Ethernet for dedicated BACnet lines and the other is BACnet for TCP called BACnet/IP which can run on a non-dedicated Ethernet line. Next comes BACnet ARCNET at 2.5 Mbps. Both Ethernet and ARCNET can use a variety of physical media including coaxial cable, twisted pairs, even fibre optic cable.

For devices with lower requirements in terms of speed, BACnet defines the BACnet MS/TP (master-slave/token-passing) network designed to run at speeds of 1 Mbps or less over twisted pair wiring (RS-485). All of these networks are examples of “local area networks” or LANs. BACnet also defines a dial-up or “point-to-point” protocol called BACnet PTP for use over phone lines or hardwired RS-232 connections. A key point is that BACnet messages can, in principle, be transported by any network technology, if and when it becomes cost-effective to do so.

BACnet is Global
BACnet is not just an American phenomenon. It has steadily gained acceptance throughout the world. Recent surveys have revealed that BACnet installations are in nearly 100 countries and on every continent, including Antarctica. That’s because the inherent interoperability of BACnet and the many enhancements over the years have given end users unprecedented freedom to mix and match building control equipment from a growing array of manufacturers.

All recommended modifications to the ASHRAE standard are published openly and anyone can comment. Suggestions for improving BACnet come from professionals throughout the world. BACnet is truly living up to its original promise of making it easier to integrate building systems with diverse functions from different manufacturers. A global network of designers, manufacturers, installers, building owners and operators are more frequently turning to BACnet as a way to make all the unique systems within a building perform as a more cohesive entity. And it is rapidly becoming an international standard that is supported by technical experts from around the world. BACnet has been approved as ISO Standard 16484-5, it is a Korean national standard, and is a European pre-standard. BACnet Interest Groups have formed in Europe, the Middle East, AustralAsia and Russia to market the protocol and resolve
technical issues.