I am having communication issues with my BACnet network. Thermostats are going offline and back online, and some wrong values are being displayed at the front end.
[Answer]
When having BACnet communication issues, some basic settings and installations should be verified before going into further troubleshooting. The first step is to verify the blinking of the green status LED on the BACnet board inside the thermostat. Refer to the BACnet integration manual for the signification of each blinking type (VT7200 integration manual, VT7300 integration manual, VT7600 integration manual).
These are the main reasons that could cause communication issues (making the verification in this same order can accelerate the solving process):
Duplicate MAC addresses: Having duplicate MAC addresses on the same network would cause improper token pass on the trunk. If using more than 127 thermostats, you should restart from MAC 0 (MAC addresses above 127 are not supported by our thermostats), so you will have duplicate MAC addresses. In this case, make sure to change the Device Instances of thermostats to not have any duplicate device instances. By default, the Device Instance (Device ID) is generated using the first two digits of the model of the thermostat + the MAC address. Example: For a VT7200C5000 thermostats with MAC address 115, the default Device Instance would be 72115.
Daisy chain wiring: Make sure that the whole trunk is daisy chained.
Polarity: A reversed polarity would cause the thermostat to lose its communication to the network, and hence go offline.
End of Line resistances: Make sure you have EOL resistances at each end of each trunk. Refer to the BACnet integration manual for more details about EOL resistances.
Reference terminal: Make sure nothing is connected to reference terminal of the thermostat
Baud rate: Using the 76,800 baud rate with some automation systems can be problematic. Reduce the speed of the system to 36,800, power cycle the thermostats and verify if you still have erratic values.