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Wallboard Installation and Maintenance Wallboard 22
Compact Contact Center Installation & Maintenance
40DHB0002USBG Issue 1 (11/14/2001) Wallboard Installation and Maintenance
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Page 39
RS485 Communication Systems
This standard officially allows 30 wallboards to be connected via a RS485
transmission cable of up to 1200 meters to a PC RS485 port.
The PC TX+ is connected to the wallboard RX+; similarly, the PC TX- is
connected to the wallboard RX-. The recommended technique is to run a
cable from the PC to the nearest wallboard then from this wallboard to the
next nearest wallboard etc.
At the last wallboard on the bus it is necessary to connect a 120Ω
terminating resistor between the wallboard RX+ and RX- connections using
the supplied terminated RJ45 Plug and the RJ45 doubler.
The cable specification must include a twisted pair with an overall screen
and characteristic impedance of 100 ohms.
Ground Loops
Proper operation of RS485 requires a signal return path. A third wire
should connect all the system signal grounds together and is most often an
additional shield conductor.
Distributed power systems in industrial applications may have differences in
the AC power line grounds of several volts as measured from location to
location. Ground differences that are greater than 5V RMS can destroy the
RS485 transceivers. Loop currents can exist within the shield conductor
because of these voltage potentials, causing data errors. Loop currents
can be minimized by installing 100 ohm resistor in series with the cable
shield and the RS485 Line Driver ground connection or by ensuring that the
screen of the cable does not make contact with the wallboard frame earth.
Spurs, which lead off from the main RS485 cable to a wallboard, should be
kept to less than one meter long.