Any signal input to and FPGA that goes below 0V, like true RS-232 is incompatible with FPGA pin DC specifications. If you add an interface that converts these to on the the 3.3V single-ended IOSTANDARD logic compatible logic then you are good to go. Don't drive FPGA pins below ground. There protections diodes in the FPGA device but these are not meant to counter widely out of specification input voltage levels. Read the datasheet for the Artix family to see what the DC specifications are. I haven't looked at the Digital Discovery so I don't know what design information is available. 0-3.3V would seem to be a safe bet for inputs to the product as overshoot is likely for some signal sources.
I'm not sure that the Digital Discovery product was design to compete with the other products that you mention. None of these product are meant to replace those clunky expensive logic analyzers from the big instrument vendors.
If you really want something to analyze RS-232 or RS-485 traffic there are likely cheap products to do that. Really, you could turn any cheap FPGA board with external memory into such an analyzer. If the board has an FT232H or similar device and supports synchronous 245 FIFO mode then even better. You'd still have to make an adapter to convert those signals into FPGA friendly logic levels. You then have an instrument that you can tweak to provide just about any analysis you can ever want. I did this a long time ago so I know that it's possible and doesn't require any exotic programmable logic design expertise. Also a good educational project.
If you just want a ready made inexpensive instrument the AD1 or AD2 is hard to beat. You still need to condition signal inputs to appropriate levels.
