This is Dominic L. from Applications Engineering at National Instruments. How would you like to communicate between the cFP-2020 and cFP-1808 module? Two methods of communication can be used: ethernet or serial. I would recommend using ethernet simply because it is faster. Once deciding to use ethernet, you can choose a protocol: OPC, FieldPoint API/driver, or Modbus. I believe Modbus is the fastest protocol. To help you get started you can search the discussion forum for more information on FieldPoint and Modbus.
The cFP-1808 module requires FieldPoint 4.1.2 or later. If possible, I would work with FieldPoint 5 because that would allow you to use simple FP-read and FP-write VIs rather than making your 1808 a Modbus slave (with older drivers).