Solution
Yes, you can implement RS-232, RS-422, and RS-485 protocols on NI FPGA cards. The main consideration is how to deal with the voltage levels of the signal when implementing RS-422 or RS-485. The DIO lines on most CompactRIO modules and R Series devices are single-ended 3.3V TTL, while RS-422 and RS-485 are differential voltage signals. To have a true RS-422/RS-485 interface you will need to add a signal translator between the FPGA card and your RS-422/RS-485 device(s).
An implementation of the RS-232 Interface using LabVIEW FPGA can be found in the following example.
Developer Zone Example: RS-232 Interface Using the LabVIEW FPGA module