How to implement Serial port RS-232 from 2.5V IO signal?
Look up the voltages for RS-232, your 2.5v logic is nowhere close.  5v and sometimes 3.3v logic may work to a degree, but is out of spec.
RS232 serial is inverted (traditionally in the level translator chip) from the sense of logic level serial.
if you connected an RS 232 output to your fpga without a level translator you have likely damaged it.
it is unlikely that you are getting -2.5v out of your fpga, rather this sounds like a measurement or description error.


In summary you need a level translator.