data link failure between DCS OVATION and SIEMENS PLC


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We have DCS OVATION which is connected to siemens PLC to monitor fuel treatment plant(Westfalia). Ovation controller communicate to PLC via MODBUS protocol by means of serial link controller RS485, 4 wires. Recently, we have recived DATA LINK FAILURE alarm and all field data lost. The serial link status has APPLICATION TIMED OUT alarm. What could be the reason of this problem ?
Please can you give me the wiring diagram for the connection or overview diagram for the system?
Please send it to this email:
aboforat_2005 @ yahoo. com