Many Modbus TCP clients send polls with the Station / Device address (ID) set to zero.
Fieldserver’s cannot be configured as station zero so they cannot respond. There is a work around. Set the FieldServer Node_ID parameter to a value of 239. When Modbus TCP messages are received, the FieldServer looks at the one byte station number. If it is zero it changes the byte to 239 and then passes the message for processing by the driver. Thus if the server node is configured as 239 the FieldServer can respond correctly.
One example of a Modbus Client that sends requests to station zero is the EZ Touch Panel HMI from EZAutomation.net