Instruction manual

Page 26
RS-485 Simple Serial Mode
RS-485 communications use differential voltage signals to improve on the noise immunity and
distance limitations of RS-232. When fully implemented, RS-485 enables many devices to
share a single communications bus. The kV RSX implementation of RS485 is point to point, with
no multi-drop capability at this time. It is useful if the communications distance is greater than
the 50 feet provided by an RS-232 link. Also, RS-485 communications are more immune to the
disturbances that are experienced in an electrically noisy environment.
The kV RSX communications cable does not connect directly to a computer port, although some
embedded controllers have a dedicated RS-485 port. Typically, an RS-485 Serial Interface Card
is installed in the host computer to provide the correct RS-485 signals over a twisted pair of
wires. The connection to the kV RSX is half duplex, using two wires. For half-duplex, the signal
lines marked RX+ and TX+ must to be connected together and the signal lines RX- and TX- must
be connected together, as shown in Figure 20. Terminating resistors are not required for this
application and should not be used.
The kV RSX, RS-485 communications, have been tested using a host computer and the
RS422/485 Serial Interface Plus, Part # IC050C . This serial interface card is made by:
Black Box Corporation
P.O. Box 12800
Pittsburgh, PA 15241
Though not yet tested with the kV RSX, other serial interface cards and RS-232 to RS-485 signal
converters are available from:
B & B Electronics Manufacturing Company
P.O. Box 1040
Ottawa, IL 61350
http://www.bb-elec.com
Telebyte Technology, Incorporated
270 Pulaski Road
Greenlawn, NY 11740
http://www.telebyteusa.com
The EIA-485 standard does not specify a standard connector for RS-485 communications.
There are two styles of communications cables available for the kV RSX. One is terminated with
a DB-9 connector and the other with a “telephone type” four conductor RJ-11 connector. If the
environment is electrically noisy, use of the kV RSX DB-9 communications cable is
recommended. The shell of the DB-9 connector is connected to the shield which surrounds the
wire in this cable. Over the run of the cable, the shielding should be continuous to the end
device, where it should be grounded.