What you are going to need is a max485 converter with your PIC... the way it works is the same as the RS232, the difference being is that they use different voltages to communicate.... RS232 reference to ground, RS485 reference between the two lines....
So to be able to hook up to the PC more than 1 PIC then you are going to have to make the PC the master, while every PIC listens to the line, but only the PIC being address responds..... so unless the PC polls the PIC, the PIC don't generate any comunication.
You will find this article very informative **broken link removed**
Ivancho