PLC 5 Control Net Media Adapter fault


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We have received one alarm in our AB PLC (PLC-5 Processor 1785-L40C15) 'Adaptor # 3 Fault' and persisting in alarm page. In panel Adapter # 3 (1794-ACNR15)all the LEDs are healthy. (COM A&B, Status LED are green and steady) Our system is redundant system, processor and all the IOs, Node 3 is main and Node 4 is redundant.

After receiving this alarm some of the solenoid-operated valves are not operating properly because the valve feed backs are not changing in the data table. Toggle the signal from Ladder logic, valve is operating, input module status is matching with field but in the data file it is not changing.

Is it Adapter fault or any other fault? Becoz the LEDs confusing
Is it safe to change the Control net redundant adapter on line ?
Rest of the process is running well except the NODE 3 & 4

Kindly help us to solve this problem.

Thanks in advance.


We solved the problem.

First we did the power cycling, then the status LED became RED, so we confirm the hardware fault in the Adapter. So we decided to replace the adapter in the Node#3.

Replaced adpter with the new one, again we faced some software revision problem. the existing Redundant Adapter version was 4.1, but we replaced the Adpter with 3.6, so the system was not communicating with Node3, then we decided to match the node 4 adapter. We have taken shutdown the PLC and change Node4 also with the same version ie. 3.6.

After powerup both adapter was showing heathy but not communicating with the processor.

We changed both processor pimary and secondary in the programme mode and control net reconfigured.

Both primary and secondary taken back to run mode.
Reset all alarms, found all are working OK

If anybody want further details contact me in the below ID

joyvjoy [at]