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:

  1. deviceAddress (e.g. 192.168.2.160:43712)

  2. deviceInstance (e.g. 27335)

• At the Cybus::Endpoint:

  1. objectType (e.g. analog-input)

  2. objectInstance (e.g. 2)

  3. 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.

Connection Properties

Endpoint Properties

Example Configuration

The following example demonstrates how to configure a simple BACnet connection and endpoint that subscribes to an analog-input BACnet object.

Download:

3KB

bacnet-example.yml

# ----------------------------------------------------------------------------#
# BACnet Commissioning File Example
# ----------------------------------------------------------------------------#
# Copyright: Cybus GmbH (2021)
# Contact: [e-mail]
# ----------------------------------------------------------------------------#
# Source Interface Definition - BACnet
# ----------------------------------------------------------------------------#

description: |
    Sample commissioning file for Bacnet protocol connectivity and data mapping

metadata:
    name: Bacnet Protocol Connectivity
    icon: https://www.cybus.io/wp-content/uploads/2019/03/Cybus-logo-Claim-lang.svg
    provider: cybus
    homepage: https://www.cybus.io
    version: 0.0.1

parameters:
    IP_Address:
        type: string
        default: 192.168.10.30

    Port_Number:
        type: number
        default: 47808

    Device_Instance:
        type: number
        default: 2000

    initialReconnectDelay:
        type: integer
        default: 1000

    maxReconnectDelay:
        type: integer
        default: 30000

    factorReconnectDelay:
        type: integer
        default: 2

resources:
    # Lets define the connection to the device.
    bacnetConnection:
        type: Cybus::Connection
        properties:
            protocol: Bacnet
            targetState: connected
            connection:
                deviceInstance: !ref Device_Instance
                deviceAddress: !sub '${IP_Address}:${Port_Number}'
                connectionStrategy:
                    initialDelay: !ref initialReconnectDelay
                    maxDelay: !ref maxReconnectDelay
                    incrementFactor: !ref factorReconnectDelay

    # The Bacnet protocol supports endpoints of type read, write and subscribe.
    # Below are example on how to configure such operations.
    # Read endpoint
    bacnetReadBinaryOutput:
        type: Cybus::Endpoint
        properties:
            protocol: Bacnet
            connection: !ref bacnetConnection
            # Specifying the topic here instead of using a Mapping block allow us to remove an extra hop.
            read:
                objectType: binary-output
                objectInstance: 303
                property: present-value

    # Write endpoint
    bacnetWriteBinaryOutput:
        type: Cybus::Endpoint
        properties:
            protocol: Bacnet
            connection: !ref bacnetConnection
            write:
                objectType: binary-output
                objectInstance: 303
                property: present-value
                priority: 8
                propertyTag: 7

    # Subscription endpoint
    bacnetSubscribeBinaryOutput:
        type: Cybus::Endpoint
        properties:
            protocol: Bacnet
            connection: !ref bacnetConnection
            subscribe:
                objectType: binary-output
                objectInstance: 303
                property: present-value
                interval: 1000

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:

{ "value": "<value>" }

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:

{
  "value": "<value>",
  "timestamp": "<msSinceEpoch>"
  "id": "<correlation id>"
}

Further reading:

• BACnet FAQ

• The language of BACnet

• BACnet ASHRAE Standard