Update process values - Basic Documentation - RMK770-1

Communication via the KNX bus

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Air conditioning controller
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Basic Documentation
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outside air temperature

Process values such as the outside air temperature can be transmitted to other bus devices via distribution zones. As the process values rarely are constant, transmitting every minimal change in value would result in heavy traffic on the bus.

COV = Change of value

The Synco controller updates the process values (in our example the outside air temperature) when it changes by a fixed, predefined differential ("delta value"). Thus, the fluctuations in the outside air temperature are added up until the delta value is reached, and only then is the new value transmitted or updated. The term COV ("change of value") is applied to process values updated in this way.

Minimum repetition time

If a process value fluctuates widely and the delta value is reached before the "minimum repetition time" of 10 seconds has expired, there is a delay of 10 seconds before the process value is updated.


With a constant outside air temperature, it may take a long time until the delta value is reached. For this reason, the outside air temperature is updated every 15 minutes. This is referred to as a "heartbeat".

Fault status message

If the outside air temperature is still not transmitted after the first 15-minute delay, the controller waits another 15 minutes plus 1 minute (i.e. a total of 31 minutes since the last update) to transmit a fault status message.

Depending on the application, the controller now continues to operate with the preset values, e.g. if the outside air temperature sensor is faulty, the flow temperature is controlled as if the outside air temperature were 0 °C. Certain functions are also disabled, e.g. there is no summer/winter compensation.

Important notes

  • Process values are not updated if the bus device is set to device address 255.

  • Process values are not updated when the controller is in commissioning mode, i.e. if the "Commissioning" operating page is selected directly or as part of a sequence.