That message indicates that a read timeout occured. This usually suggests either a cabling problem (the RS-485 pin-out is in the NI-Serial manual pdf), a port settings problem (baud rate, etc.), a COM port number problem (using COM3 instead of COM4, etc.), or a communication protocol problem (incorrect command sent to the device, etc.). I suggest double checking your cabling and port settings, then run your vi with NI-SPY open to see what type of port activity is occuring.
