I experienced the same issue when using data to server. I believe there is a bug in the User interface.
Modbus Data to Server service has a setting called "period" the mouse over help suggests that this is set in seconds and that the minimum setting is 1 (second).
I suggest that in reality this "period" setting is minutes.
If Data to server "period" set at 1 and Modbus Master TCP set at 10 seconds then my server gets 6 Messages every minute. If the modus master tcp is set at 2 seconds, then my server gets 30messages every minute.
To get 1 message at a time at my server, then the modus master tcp "period" needs to be set at 60 (ie seconds), while the data to server "period" needs to be set at 1. With these settings it is possible for my server to receive 1 message, containing one value from a register, but at a frequency no faster than one message per minute.
These issues have meant that we can't use the RUT955 for our project roll out.