Override behavior to function block command

Normal operation

For normal operation (output BACnet object is not overridden), output [PrVal] of the function block supplies the value for property 'Present value'. The function block monitors bit 'Override' in the 'Status flag' of the BACnet object to detect the overridden state.

The property 'Present value' is replaced with the value (12.34) commanded on the BACnet object via input [ValIn1]. The controller takes over this value from the BACnet object and supplies it on the I/O module. The I/O module converts the value to its physical representation and supplies it on the field device.

If the value on the I/O module is not overridden, the module returns this operating state to the controller and the state is represented on the BACnet object in property 'Status flag' via bit 'Override'. A value of 'False' means the output is not overridden.

The I/O module also returns the output value to the controller. The returned value is saved to the property 'Tracking value' of the BACnet object.

Override on I/O module

The module supplies the operating state to the controller if an output is overridden on the I/O module. The overridden state is represented by the value 'True' on bit 'Override' in the property 'Status flag' of the output BACnet object.

The overridden value of the output is also returned by the I/O module to the controller and saved to property 'Tracking value' of the BACnet object.

If the value 'True' on bit 'Override' of property 'Status flag' is detected, the function block takes the value from property 'Tracking value' and supplies it on output [PrVal], which synchronizes the value at [PrVal] to the value on the I/O module.

In addition to displaying the overridden state of the output, the function block sets the value 0 (overridden) on the output [PrPrio].

Override - online mode

In the online mode, the ‘PrVal’ of the CFC block shows the present value of the BACnet object. The pin reads the value from the BACnet object under Properties > BACnet properties, but does not override it. You can temporarily override the ‘PrVal’ of the CFC block to test the program logic, but:

• this updated value does not update the BACnet object, and therefore not visible on a BACnet browser / management station.
• this updated value is temporary; as soon as the BACnet object gets updated, the ‘PrVal’ pin gets refreshed.

To update the 'PrVal' logically, it is recommended to update the related BACnet object, i.e function blocks and its BACnet objects.