BACnet
BACnet is a data communication protocol for Building Automation and Control networks. A data communication protocol is a set of rules 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.
BACnet glossary (advanced reading)
Device
The device represents a server that is managing one or more BACnet objects. A device has a unique device instance number (which is sufficient for addressing if using the same network interface) and a unique UDP port as device address (in the format <ip-address>:<port-number>) which can be used for addressing throughout the entire LAN. Technically, the device is an object with the name device. The device instance in fact is its object instance with the additional requirement to be unique.
Object
An object reflects a physical hardware actor or sensor (i.e. an I/O device). BACnet defines a list of standardized object types (such as analog-input, analog-output, binary-input, binary-output, etc.). Object types are identified by a fixed number or by a fixed ASCII string identifier (s.a.). Depending on the installation, a device may serve an arbitrary number of object instances of arbitrary type. A device may for example run 3 analog-inputs, 1 analog-value and two binary-outputs. An object is addressed using its type and numeric instance ID which must only be unique within the device.
Property
Every object contains a type-dependent set of properties. The most important property that exists in every object is the present-value property. This is conceptually very similar to the process-variable (PV) of other industry protocols. Like object types, the properties are identified by a fixed number or by a fixed ASCII string (e.g. present-value). Examples of other properties are version or object-name. The property is the final data-endpoint where values can be read from or written to. The value type can be any scalar value, arrays of scalars or specific complex types (this depends on the object type).
For unambiguously addressing any data endpoint (in BACnet: property) the following information must be provided:
At the Cybus::Connection:
deviceAddress
(e.g. 192.168.2.160:43712)deviceInstance
(e.g. 27335)
At the Cybus::Endpoint:
objectType
(e.g. analog-input)objectInstance
(e.g. 2)property
(e.g. present-value)
If only one device is running on a given host (with fixed IP), the port can be omitted from the deviceAddress field.
Example Configuration
The following example demonstrates how to configure a simple BACnet connection and endpoint that subscribes to an analog-input BACnet object.
Download:
Input Format
To write data to BACnet a request must be sent to the MQTT endpoint of the service using a /set
suffix providing a JSON object with the following format:
No response message is written to the /res topic of the Endpoint for this protocol.
Output Format on Read
For a read endpoint, additionally a correlation id (id) can be set in the payload of the message request, to ensure the correct identification of responses to specific requests.
When data is read results are published to the /res topic of the Endpoint. The output message is an object with the following format:
Further reading:
Last updated