Rs485 Communication Systems - Avaya Compact Contact Center Installation Manual

Page 56 - Ferrograph Wallboard 22

RS485 Communication Systems

This standard officially allows 30 wallboards to be connected via a RS485 transmission
cable of up to 1200 metres 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 metre long.
Page 56 - Ferrograph Wallboard 22
Wallboard Installation & Maintenance
Wallboard Installation & Maintenance
Compact Contact Center Installation Manual
40DHB0002USBG Issue 4a (05/29/2003)