You will need RS-485 hardware to create the proper signals. This can either be a dedicated RS-485 port via PCI, PXI, USB, Ethernet, etc. or an external converter box which connects to a RS-232 port.
Once you have the RS-485 hardware, communication is performed via serial reads and writes (using VISA for example). RS-485 may also require transceiver control, which usually is performed either automatically by the transceivers, or via DTR or RTS.
