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One of the main requirements of professional users is the ability to access applications in individual buildings or their complexes, as well as the availability of visualization tools for control devices. This is precisely what the BACnet standard was developed for.
At the field level of the network, open communication is required between actuators and sensors, the choice of which is determined primarily by the amount of financial costs and technical features.
In addition, such devices must be easily configurable.
These are the starting points for the development of KNX, adopted as standard EN50090 by CENELEC and CEN.
The focus of KNX is precisely on these functions, as well as on data transfer protocols.
The main goal of the KNX standard is to cover all applications at the field level of building automation, ensuring open and transparent information exchange.
It is KNX and BACnet that can ensure the transmission of alarm and other messages to a higher level of the automation system.
The main advantages of the KNX standard at the field level:
– interoperability;
– a wide selection of high-quality components from various manufacturers;
– ease of adaptation during installation and compliance with changing customer requirements
Each manufacturer can choose one of three KNX configuration options:
1) S+mode (system) – design and configuration using a computer with ETS software installed;
2) E+mode (simple) configuration without a computer, using a central controller and buttons;
3) A+mode (automatic) – automatic configuration when connecting devices. In addition to the three configuration modes, the KNX standard supports three data transmission media. Each of them can be used with any of the three configuration modes.
Thus, the manufacturer selects the most suitable combination for a specific market segment:
– twisted pair;
– power line (230V wiring);
– wireless communication (radio).
When several contractors work on one project, the overall KNX network is often divided into several independent subsystems, where the contractor can install, configure and present his own subsystem, independent of the subsystems of other participants.
The field level is the basis of the building automation system. The advantages of the BACnet standard are most obvious when moving from the field level to the management level.
BACnet is designed to manage and process data coming from engineering systems, minimize energy costs and ensure a high level of comfort for residents even with 100%+ occupancy of the building.
However, data on the state of all building functions is available only to professional users.
Unified data mapping Representatives of the BACnet and Konnex Association (Belgium) have decided to create a common standard data mapping system for both standards.
This unified system strengthens the position of both open standards and encourages installers to use them in combination: BACnet for the top and automation levels, and KNX for the system level.
The results of this collaboration are shown in Appendix H.5 of the BACnet standard, «Using BACnet and KNX», which describes not only the semantics of the KNX function blocks, but also how to define access to the services associated with them.
To achieve this, each function block has at least one so-called data point, and the function blocks are located inside physical devices, at least one in each.
Object types in the BACnet standard define the functions within the semantics and the services required to access these functions.
Object types in BACnet have corresponding properties. One object type in BACnet contains at least one mandatory property,
and in some cases other optional properties.
The BACnet standard also defines device objects. For example, a BACnet device consists of a set of object types.
However, each BACnet device contains only one object (device object).
KNX function blocks can be compared to object types in BACnet. At the same time, access points in KNX correspond to the properties of objects in BACnet.
Data conversion between KNX and BACnet is described in Appendix H.5 of the BACnet standard «Using BACnet and KNX».
Data conversion of six KNX functional blocks into BACnet objects is described as follows:
1. Binary input;
2. Binary output;
3. Binary value/quantity;
4. Analog input;
5. Analog output;
6. Analog value/quantity.
Functional Blocks in KNX and object types in BACnet have similar semantics in the data structure, so these data can be transformed.
In accordance with Appendix H.5 to the BACnet standard, it is possible to select unified low-cost interfaces that allow the open standards KNX and BACnet to be linked with each other.
This will help to offer an extremely promising option for a common building management system.
Based on materials from the KNX magazine, No. 1, 2006