User's Manual

Chapter Two Installation
Man_905UD_2.0.doc Page 15
As the RS485 communication medium is shared, only one of the units in the system may
send data at any one time. Thus communication protocols based on the RS-485 standard
require some type of arbitration. The 905U-D “holds off” for three character times after
receiving data from the RS-485 port before transmitting on the RS-485 port.
RS485 is a balanced, differential standard but it is recommended that shielded, twisted pair
cable be used to interconnect modules to reduce potential RFI. An RS485 network should be
wired as indicated in the diagram below and terminated at each end of the network with a 120
ohm resistor. It is important to maintain the polarity of the two RS485 wires.
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