RMS Technologies USB485 CONVERTER CARD - Manual (Page 14)

RMS Technologies 14 Version 1.00
USB485 Converter Card September 28, 2004
5. RS485 Communication
The Interface
The EIA specification RS485 defines an integrated circuit that is to be used to connect up to 32 nodes
to a two-wire party line bus that does not exceed 4,000 ft. in length, and for use with data rates up to
10M Baud.
The two-wire bus must be terminated at one-end for short wire runs and at both ends if the runs
exceed 20 ft. One of the two wires must be biased positive with respect to the other by approximately
700 millivolts.
For wire runs over 20 ft, twisted pair cable with a characteristic impedance of approximately 100 to
200 ohms, and the far end of the run should be terminated by a 150 ohm resistor across the line pair.
For runs under 20ft almost any wire can be used.