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BACnet Basics

Downloads:

Powerpoint Presentation BACnet Basics
René Quirighetti, Siemens Schweiz AG, Building Technologies Group

Powerpoint 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
Network Layer Link Layer Physical Layer	Application Layer Services Object

 

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

Extensible are

• 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