Do I know why the computers work well? Is that the question you're asking?
If you meant to say that when you had a single computer with the serial port(s) looped back and Hyperterminal worked and now when you use two different computers with Hyperterminal it doesn't work, then you either have the cable incorrect or you have setup the serial ports differently. If you can send on pc1 and receive on pcs but can't send from pc2 and receive on pc1, then I strongly suspect it's the cable.
