BACnet Basics
Downloads:
Powerpoint Presentation BACnet Basics
René Quirighetti, Siemens Schweiz AG, Building Technologies GroupPowerpoint Presentation: BACnet – Global standard for open building management solutions
Christian Mueller, Honeywell Building Solutions Germany
BA-System Levels
Positioning of BACnet in the CEN Level Scheme
Protocol Layers and their Meaning
| Data Transport | Data Interpretation |
|
|
Data Transport: The Bus
- Flexibility by different media
- Media request for distinct link layers
- Ethernet / TCP/IP: TCP/IP provides access to company networks
- LonTalk: including all media defined there
- Point to Point (PTP): mainly used for modem connections
- Arcnet
- MS/TP
Data Transport: The Network Layer
- BACnet provides a homogeneous network layer
- Routing through different busses is possible, eg. from a modem link (PTP) through Ethernet to all LonTalk segments
- Annex J of the BACnet Standard defines the routing through a TCP/IP network. This ensures the integration of a BACnet network into a company network
- The homogeneous network layer is important for the flexibility of BACnet
internetworking
Example: Networks
Routers:
- Are working on network layer, i.e. they are totally independent from the application layer
- Standard routers in IP-networks, i.e. BACnet can be integrated in any
given company network
Application: Objects
Datapoint objects
- Analogue in / out / value
- Binary in / out / value
- Multistep in / out / value
- Accumulatior / Pulse Converter
Alarm handling objects
- Notification class (distribution of alarm messages)
- Event enrollment (defining the alarm conditions)
Miscellaneous objects
- Device object (provides device informations)
- Schedule object / calendar object
- Trenddata object
- Loop object
- Program / file object
- Virtual terminal object
Application:
Object Properties
- Properties are parameters of objects
Examples: present value, alarm limits, name, status - Bacnet distinguishes between mandatory and optional properties
- Properties may either be read only or also writable, i.e. modifyable by BACnet services)
Application: Services
BACnet offers 38 services on application layer
The services are partitioned in these 6 classes:
- Alarm handling
- Object access
- Device management
- Network security
- File access
- Virtual terminal
Examples are: read, write, change of value notification, time synchronisation
alarm messaging
Models: Real Device and BACnet Objects
BACnet objects are modelling the view onto a device through the network
- BACnet objects don’t define internal functionality of devices (algorithm)
- BACnet objects give the outside view onto device functions
- Example: The BACnet loop object is defined in a way, that different loop algorithm e.g. PI, PID, sequence, predictive control.. can be mapped
Models:
Client - Server Relations
The client is claiming services of the server
- The client
- subscribes for changes of values
- gives order for trend data registration
- defines alarm limits
- The server maintains an image of the device functionality and executes the services
Model: Peer to Peer Communication
Example: COV-Handling
1. Client subscribes for a value (data point) of the server
2. Server returns the value together with the acknowledgement
3. Server returns the value whenever it changes
4. Client renews or cancels subscription
Extensability of the BACnet Protocol
The BACnet protocol is designed in a way, that extensions are easily possible
- Extensions by the BACnet Standard Committee (SSPC-135)
- Proprietary extensions by manufacturers
- Objects: new objects or new properties
- New services
Therefore BACnet is future proof
Compatibility and Conformity
Compatibility of BA-systems - Interoperability
- BACnet interoperability Building Blocks BIBBs and device profiles provide an overview
- PICS give the details: client- or server-role, object types, bus types....
Conformity to the standard - a premise
- Only with conformity to the standard interoperability becomes possible
- ASHRAE is defining test procedures
- BACnet Interest Group is about to define testbeds and a certification process