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Unable to access only Read Write addresses of AB Micrologix 1400 PLC using NI DSC and NI OPC server


I have been using NI DSC and OPC servers (NI and Kepware) to communicate with Allen Bradley Micrologix 1400 PLC (1766-L32BXB). Recently at one  site I found that I could access Process values from the PLC correctly (Read Only tags) but could not access the Read Write Tags at all. The latter are addresses to which it should be possible to write Set (i.e., references) values from MMI or PC software. I am able to do so from the MMI not from the PC software. I get communication error message on the MMI and PC, mentioning the PLC address being accessed (e.g. N7:0).


Support from the supplier is not available. Hence can anyone let me know if ladder logic could have been written to prevent PC software based access of Read/write addresses (N7:0 to 9 in this case), while permitting access to Read only addresses? Is the problem at the PLC end or OPC server end? Is there a way to get around this?


Thanks in advance.

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The OPC Server cannot force Outputs so if th registers in question are the outputs of ladder rungs you cannot write to them via the server. The controller will accept the write from the server but will not execute the write. In some cases the server event log will post an error if it is the PLC. Do you get errors when you try to write and if so what are the posted error messages.

Fred Loveless
Kepware Technologies
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