Issue
Micronet BACnet (MNB) AV and BV objects in G3 are purple indicating they are overridden. The AV and BV objects are read only, how can they be overridden?
Product Line
TAC IA Series
Environment
- Niagara G3
- Micronet BACnet
- MNB-300
- MNB-VAV
- MNB-1000
Cause
Purple BACnet values (I/A Series R2 and G3 graphic override color) are internal BACnet points that can not be modified (display only). This behavior is normal and began to appear in MNB controller firmware versions 1.2 and higher.
Per the BACnet specification, read-only type points must be set to indicate an override condition. The root cause is a design change required by the BACnet specification to bring the controllers into compliance for BTL certification. The ASHRAE BACnet Specification 135-2004 specifies that the "read-only" type objects (e.g. Analog Value, Binary Value, etc.) OVERRIDDEN flag is:
Logical TRUE (1) if the point has been overridden by some mechanism local to the BACnet device.
In this context "overridden" is taken to mean that the Present Value property is not changeable through BACnet services.In this case, the override is meant to indicate to Niagara that the point is locally overridden (by the controller) and cannot be changed by the Niagara BACnet service (or user).When the ENC learns the device it reads what "Protocol Services" it supports. If it supports COV then the "use COV" flag is set to true in the device object and when the proxy points are brought in they are defined as COV and once subscribed they will send their present value and status flag values to the ENC. In this case the AVM & BVM (being read only) objects will show overridden along with their value and be purple by default.
Resolution
All MNB devices support COV so the use COV property is set to true when they are discovered in the ENC.If the use COV property of the Device object is changed to false the ENC will use readPropertyMultiple (if supported in the device) to poll for data. If readPropertyMultiple is not supported in the device the ENC will read one property at a time.
Now when the points are learned they will be defined as polled and when polled only the present value (PV) is requested (by default). Hence the AVM & BVM objects will show their value but not show overridden because their status flags are not known at the ENC.
If the use COV device property is set to true after the AVM and BVM proxy points are defined in the ENC they will show overridden and purple after they are successfully subscribed. Turning the use COV property in the device to false at this point will cause these objects to be polled but their status is already known so they will continue to show overridden along with their present value. The only way to change that is to delete and re-learn them while the device use COV property is sent to false.
