This is not a direct answer but I doubt if you can get 250 feet out of it. The low level (5 volt) signal from the PIC or most serial ports these days are not powerful enough.

The best I have ever been able to do was around 85 feet using shielded cable in metal conduit.

To get more distance I went with a 422 using full voltage.

I built two converter boxes to add to the system. This was going from a PC to a PIC. PC to box 1 with short cable and PIC to box 2 with short cable. Ended up with about 300 feet between the two converter boxes.