Point Attribute Mapping

If you want to export more then the essential C-Bus attributes to BACnet, you must explicitly enable this option. First, you have to globaly activate the mapping feature in the menu Preferences -> C-Bus -> Map C-Bus Attributes (see chapter Configuration). In Point Manager, you must select the points for which you want to map additional attributes. Afterwards, click on the Options button and in the pop-up window, select the attributes you wish to apply.

Save the changes with the Save button.

To be able to see the attributes in the Options popup window, all selected data points must be of the same type (e.g. Analog Input).

If you decide to export additional data point attributes to BACnet, then the C-Bus traffic considerably increases, which affects the performance of the entire gateway. Because there is no Refresh / Change of Value Push mechanism for the C-Bus attributes, the attribute values must be polled and response time to BACnet requests increase. Even C-Bus problems may occur.

The behaviour of the gateway when exporting additional attributes is mainly affected by the behaviour of the BACnet client and is strongly dependent on the building automation system provider. Some implementations send the requests very selectively and require only the data processed by the system. Other systems boost demand for all objects with all properties and eventually even several times per second, which exceeds the capabilities of the C-Bus. To avoid the C-Bus overloading the gateway does not support BACnet feature ReadPropertyMultiple when the option Map C-Bus Attributes is active.

Attributes Mapping

The following table shows the mapping between the C-Bus attributes and BACnet properties.

C-Bus Attribute BACnet Property Writable
SUPPRESS_ALARM limit-enable true
ALARM_DELAY time-delay true
TREND_HYSTERESIS cov-increment true
LOW_ALARM_LIMIT fault-low-limit true
LOW_WARNING_LIMIT low-limit true
HIGH_WARNING_LIMIT high-limit true
HIGH_ALARM_LIMIT fault-high-limit true
TIME_TO_CLOSE motor-runtime-close true
TIME_TO_OPEN motor-runtime-open true
ALARM_HYSTERESIS alarm-hysteresis true
ALARM_SEVERITY critical-alarm true
OPERATING_MODE operating-mode false
SENSOR_OFFSET sensor-offset true
  • Note: BACnet knows two different limit-enable properties. To change the C-Bus attribute SUPPRESS_ALARM, both BACnet limit-enable properties (hi/lo) must be set identically, e.g either both are true or all are false.

  • If the attribute mapping is generally disabled or if the point attribute TREND_HYSTERESIS is not enabled for export into the BACnet, the value of the property cov-increment is set to 0.0. A not null value of cov-increment is the prerequisite for the activation of the COV subscribe service.

Some of the BACnet properties have gateway specific proprietary ID:

BACnet Property Property ID
operating-mode 2048
motor-runtime-open 2049
motor-runtime-close 2050
alarm-hysteresis 2051
critical-alarm 2052
sensor-offset 2053