Modbus TCP Client Failover Score Calculation

The AVEVA Adapter for Modbus TCP calculates the client-level failover score as a percentage of the number of good scan-statuses received versus the total number of data selection items configured. When the Modbus device cannot be reached, all scan statuses for that device are considered to be bad. Adapters in the primary role calculate the failover score in the same way regardless of failover mode, but there are some differences between cold, warm, and hot secondary adapters to be aware of. For instance, the failover score for secondary adapters stays at 0 until the adapter becomes primary. This is due to adapters not maintaining a connection to the Modbus devices as a cold secondary adapter. In warm mode, the adapter maintains a connection to the Modbus devices, but does not receive scan statuses for data selection items. Therefore, warm mode secondary adapters calculate the failover score assuming all scan statuses are good unless the associated device is not connected. Hot mode secondaries function the same as primary adapters since the adapter is connected to the Modbus devices and actively collecting data.