Unlike some protocols, Modbus has no ability to receive multi-packet messages—the information has to arrive in one packet and include the complete message within that packet.
This may mean that ClearSCADA needs to make more requests than for some protocols, in order to receive data updates. For example, if a controller has 200 points, but the message limit is reached once the message includes the data for only 125 of those points, the data for the other 75 points will be requested in the next message from ClearSCADA.
You specify the Maximum Packet Size, using the field on the Modbus tab of the Channel Form (see Specify the Required Modbus Protocol and Packet Size).