How BACnet delivers the right integration at the right level

BACnet’s design allows multiple BACnet networks to be connected to create a BACnet internetwork.
STEVE KEAR explains why BACnet/IP provides a solution for the complete integration of building systems.Supported by ASHRAE, the American Society of Heating, Refrigerating & Air Conditioning Engineers, BACnet was adopted as an ANSI standard in 1995. It has also been adopted as ISO 16484-5 and as European standard EN/ISO 16484-5. BACnet is a software-based protocol so it can run on current and future hardware platforms. BACnet/IP has been developed to allow the protocol to use TCP/IP networks, and this is now the standard integration platform for BACnet. Using BACnet/IP decouples the logical network from the physical network and provides maximum flexibility of physical network choice — now and in the future. Developed specifically for building services, BACnet defines how all the elements of the building-management system interoperate. In BACnet terms, interoperate means that two or more BACnet devices may share the same communications networks and ask each other to perform various functions on a peer-to-peer basis. Although BACnet does not require every system to have equal capabilities, it is possible for designers of system components at every level of complexity to have access to functions of other automation system peers. There are two key concepts in BACnet that are critical to understand. First is the idea that BACnet is applicable to all types of building systems: HVAC, security, access control, fire, maintenance, lighting etc. The same mechanism that gives BACnet this flexibility has other important benefits such as vendor-independence and forward-compatibility with future generations of systems. This is accomplished using an object-oriented approach for representing all information within each controller. The second key idea is that BACnet/IP can communicate with different local-area-network (LAN) technology for transporting BACnet application messages via BACnet routers. These types of supplier and LAN choices give the system designer or owner significant flexibility in choosing the best fit among price/performance options that suits each situation. In Cylon’s experience, BACnet as an integration platform delivers important benefits to owners and specifiers systems. Focus on the management and automation level Field-level integration at floor or unitary-controller level is possible to other manufacturers’ controllers but is often not practical because of different I/O configurations, physical sizes and power requirements. BACnet focuses on integration at the management and automation levels which delivers the real benefit. Ease of integration Integrating external systems is achieved by allowing additional third-party building-automation networks to connect to the BACnet network. Owners and specifiers need only ensure that additional vendors provide a BACnet/IP interface to their distributed control equipment. Flexibility in field bus technologies Manufacturers can use optimised protocols at the field bus level providing enhanced system communication performance and reliability. Integration with other BACnet network technologies (e.g. MS/TP, Ethernet, and Arcnet) or proprietary networks is provided using appropriate BACnet/IP routers or gateways. Standard support and openness The BACnet/IP protocol has already been adopted as the new CEN (European) and ISO (worldwide) building-management standard. It uses Ethernet and TCP/IP and will run on all current and future hardware platforms. Future proof BACnet is an evolving standard which allows users to shape its future direction through a structured system of public review of draft standards. An example of this openness and flexibility is seen with the adoption of support for ‘life-critical’ systems. As a result we increasingly seeing BACnet/IP support as mandatory with European tenders. BACnet’s design allows multiple BACnet networks to be connected to create a BACnet internetwork. This capability is most often used to allow networks employing different data link layer technologies to be linked together through routing devices. For example, a large building could have multiple programmable controllers and operator workstations connected by an Ethernet network, and each programmable controller could be connected to low-cost application-specific controllers using a master-slave RS-485 bus or other low-cost bus. The combination of BACnet and TCP/IP allows distributed control and monitoring to occur over the Internet and intranets. In any implementation of BACnet, owners and specifiers need to ensure that they can achieve the following. • specify BACnet objects available for all field level controllers. • Specify BACnet objects available for all automation level controllers. • That BACnet routing functionality is provided on robust low controllers and not reliant on PC gateways. • There is one integration level in the building. A practical example with Cylon Controls of an integrated BACnet solution for cooling is as follows. A Cylon UnitronUC32 BMS monitors outside air temperature and inside air temperature. Based on these inputs, setpoint and time schedules, the strategy will change a status point to request cooling for fan-coil units. The OPC client monitors the points across the entire integrated building system. Based on the change in status of the BMS cooling request point the OPC client will enable the chiller and cooling will be provided to the fan coils. There is now an increasing focus on managing the entire building as an integrated system. Fortunately all major vendors have now embraced TCP/IP as the networking solution of choice. This presents an opportunity for closer integration based on a common building automation standard. The value to owners and specifiers of the BACnet suite of standards is at the management and automation layers. TCP/IP is now pervasive, and integration at this level removes the dependency on physical networks which evolve over time. BACnet/IP is the recognised BACnet standard to integrate an overall building-automation system. BACnet/IP provides the right integration at the right level. Steve Kear is with Cylon Controls UK Ltd, 50 Centenary Business Centre, Hammond Close, Attleborough Fields, Nuneaton CV11 6RY.
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