just a quick note on the USB-RS485 converter :
RS485 networks require bias and termination (for more information check out here : http://digital.ni.com/public.nsf/websearch/12BB6CD900B2D0EF862564980050F5A7?OpenDocument
From the Serial Hardware and Software Help that you get when you've installed NI Serial (free download here :
On the USB-485 hardware, there are programmatically controlled onboard bias resistors. In addition, the USB-485 two and four-port hardware has user-configurable socketed bias resistors. By default, the USB-485 hardware uses the programmatically controlled bias resistors, which are connected to the receive signals of each port to maintain a known state when the bus is idle. The connections are made as follows:
So if you need to add your own bias resistors in, then you need to specifically disable the programatically controlled ones.
Now on the USB 485 1 port, it's a moulded plug, so you can't physically get to the internals to put a resistor in, so you have to do it on the 9 way D-type
pins 1 (GND) to 5 (RXD-) with a 620Ohm Bias resistor
pins 4 (RXD+) to 7 (RTS-) with a 620Ohm Bias resistor
pins 4 (RXD+) to 5 (RXD-) with a 120Ohm Termination resistor
pins 9 (TXD+) to 8 (TXD-) with a 120Ohm Termination resistor
but as I said, that will depend on your actual network and where (and whether) you need bias resistors since they only go on one node on the RS 485 network.
Hope that helps
National Instruments (UK)