Issue
What is the limit of how many devices a BACnet controller can know about (Known devices) and what does that mean.
Product Line
Andover Continuum
Environment
- BACnet bCX
- bCX4040
Cause
More details required about Known devices
Resolution
The known device limit of a Continuum Ethernet level BACnet controller is 255 and for a B3 level MSTP controller it is 32.
This limit means is if a controller needs to either subscribe to a COV from another controller or deliver alarms (Event notifications) to a workstation, that each one of these target devices is a known device.
So, for example, if a b3 controller is subscribing to COV's from 30 other controllers and needs to deliver alarms to 3 WorkStations, it would require 33 known devices and the last request for a target device would fail. If a Controller is making 30 COV Subscriptions from one a single controller that would only count as 1 known device.
This limit does not apply to the number of other controllers which a COV exporter can deliver COV data to. For example, if a controller had a global point (OAT) that 50 other controllers needed to subscribe to, this is allowed because each COV subscription has within it the target controller's address information and does not use the known device limit. There are two cautions that must be considered.
- Refer to What are the differences between Standard Continuum Import/Export and Continuum BACnet COV?.
- Each COV subscription in an exporting controller takes away user memory. Each subscription uses approximately 56 Bytes of user memory. So the limit of how many other devices can subscribe COV's in an individual controller is limited by free memory in the exporting controller.
NOTE: To reset the known devices following a change in the Controllers that will effect the number of known devices, the controller needs to be reloaded.
