I think rs232 was meant to go to a computer to a printer and that was about it. I think it is meant to go 15 feet. It does not even use differential signalling. Anyway, capacitance rears it's ugly head over distances and impedance matching becomes all-important. The problem you bring up is not trivial and is actually a huge part of communications engineering. You need to look atconverting it to 485 as mentioned, and using twisted pair shielded wiring and test the signal. Once the voltage drops a certain amount you would need to amplify it back to original levels. This could be every 100 feet, or every 500 feet, I don't really know. Also, you would want to filter it. And if you are thinking of doing this in an industrial environment with motors and whatnot, forget it. Again, it was designed for quiet office environments. Take a look at a pic microcontrollers with CAN, it has registers for testing phase and noise jitter and all kinds of things to do with noise. And it still is not meant to go a mile. There are reasons why nobody looked at trying to transfer data over any distance with rs-232. They simply went to better technologies.