Hi,
Confusing...I think you mix all: Physical cables, interfaces, protocols, software....
First you should decide
* what data you want to transmit (it's a different if you want to send an audio stream, or just some bytes coming from a keyboard)
* data rate (the ammount of bytes and/or frames per second)
* then decide which distance you want to transmit (both PCs may be placed next to each other or kilometers apart)
* .....
and after this all you may decide which interface is suitable
and what interfaces your PCs have built in.
Mind: "DSub 9" just describes a connector, but not an interface. It may be RS232, but it may also be RS485 (maybe Profibus protocol)
It may be any other interface with any other protocol...
Klaus