you need a serial crossover (nullmodem) cable to connect the PPT computer to another computer over serial. usually the nullmodem cables are available with a maximum of 15feet (5meter) length.
you can extend the nullmodem cable with a STRAIGHT THRU serial cable (called serial extension cable). Those are available as long as 3meter, usually.
if you want to do really long distances, you should consider outputting as VRPN with the PPT and use ethernet. this gives you basically unlimited distance