User's Manual

Chapter Two Installation
Man_905UD_2.0.doc Page 17
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.
2.5 Communications OK (DCD) Output
The 905U-D provides a digital output signal to indicate “communications OK” in controlled
mode. The DCD (data carrier detect) output is “on” or active when a radio link has been
established with the destination module. The output will reset (switch “off”) if a
communications failure occurs. If the 905U-D unit does not receive an acknowledgment
message after attempting to transmit a data packet five times, it will reset the DCD output.
The output is a FET output to common, rated at 30VDC 500 mA.
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