C-Bus/BACnet Gateway
C-Bus in the BACnet
The whole C-Bus is treated as a one BACnet device. From the client side you can update the BACnet device properties Name and Description. All other device properties are currently read-only. This can be changed in future releases.
C-Bus data points are exported as BACnet value objects. Analog C-Bus points are mapped as BACnet Analog Value (AV) objects, digital C-Bus points are mapped as Digital Value (DV) objects. There is no special mapping for Input and Output points. All BACnet points are commandable. However, only the priority 8 (manual-operator) is allowed to write a new present-value. BACnet points support COV subscription.
C-Bus Attributes and BAcnet Object Properties
While C-Bus data points are comparable to BACnet object types, C-Bus point attributes correlate to BACnet properties. The following table shows the mapping of essential attributes between C-Bus and BACnet. These attributes are discovered while engineering the gateway and periodically refreshed in runtime.
| C-Bus point attribute | BACnet point property | Writable |
|---|---|---|
| NAME | object-name | true |
| VALUE | present-value | true |
| DESCRIPTOR | description | true |
| ALARM_STATUS | status-flags.in-alarm | false |
| ALARM_STATUS | event-state | false |
| OPERATING_MODE | proprietary-2048 | false |
| UNIT (analog point) | engineering-unit | true |
| UNIT (digital point) | inactive-text, active-text | true |
| - | reliability, status.fault | false |
Writing Point Value
To override the point value, you need to write the new value to the commanding priority 8. When written, the gateway switches the C-Bus point mode into Manual and writes the new C-Bus point value. For C-Bus points in Manual mode the BACnet property out-of-service changes to true.
To return to Auto mode, you need to write a NULL value to the commanding priority 8. When written, the gateway switches the C-Bus point mode into Auto. For C-Bus points in Auto mode the BACnet property out-of-service changes to false.
Time Synchronization
The gateway uses BACnet protocol to synchronize the date/time between the central building automation system and the C-Bus controllers. The message UTCTimeSynchronization sets the time in all controllers where the BACnet mapping is enabled. Controllers not enabled for BACnet export are not synchronized.