The SNMP driver implemented in the server works only via UDP protocol. This causes a number of considerations as compared to devices operating over the TCP protocol. For example, if the Modbus TCP driver does not receive a response from the slave node during the Request Timeout, this means that communication with the slave node is lost and all tags associated with the driver must go to the BadNoCommunication state. This happens because TCP provides reliable data transfer. UDP does not guarantee data delivery, therefore, if the response from the target device is not received during the Request Timeout, only the tags associated with this request will go to the BadNoCommunication state.
Use of UDP as transport protocol brings its own aspects to the use of diagnostic variables described in the “Devices” section. The CommunicationEstablished and CommunicationLost variables should not be used to diagnose driver operation, as they do not reflect the state of the communication channel between the driver and the target device.