Avaya 38HBK00001SCM manual RS485 Communication Systems, Ground Loops

Wallboard Installation and Maintenance INDeX 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 minimised 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.

INDeX Contact Centre Modules	Page 101
Installation & Maintenance	38HBK00001SCM - Issue 11 (05/01)