Can a USB 485 be used to sniff or view data on a RS485 data highway ? Yes.

Install the USB 485 drivers on the computer

Connect the USB 485 pins to that of the data highway
70 = - = Yellow
71 = + = Blue
74 = COM = Green

May need to swap +/- to suit your data loop.

Use a serial program such as hyper terminal to view ascii data characters on the datahighway. The USB 485 will not transmit any data unless you type characters on the keyboard.

This has been tested to allow hyperterminal to view data on a microscan data loop.