Rule #1 of all PanelView questions is "what is the full part number" ?
My guess is that you have a DH-485 PanelView terminal, like the 2711-B5A2, which has two plugs for the DH-485 port. One plug connects to the SLC-500 controller's DH485 network port, and the other one accepts a connection from a 1747-PIC or 1747-UIC.
Explain more about what precisely you're trying to do with a "passthrough port". Is your PC connected to the SLC-5/03 serial port and you're trying to 'pass through' to the DH485 network port, or is the PC connected to the PanelView's network port ? Or something else ?