Unable to access only Read write addresses of AB Micrologix 1400 from OPC server.

I have been using dotnet and OPC servers (NI and Kepware) to communicate with Allen Bradley Micrologix 1400 PLC on different environmental chambers. 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 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).

Hence can anyone let me know if ladder logic can be written to prevent PC software based access of Read/write addresses (N7:0 to 5 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?

Support from the supplier is not available. Hence this request.

Thanks in advance.

paul, not sure if this helps but try opening the program in your lader editor[rs logix micro/logix 500] scroll down the left menu find n7 under data files right click on it select properties look at protection., Steve

How often we overlook the nose! I just realised that whenever I accessed the PLC via the MMI I entered a password there. Maybe I need to pass that password from the OPC server or dotnet code to the PLC in order to access those tags? Not too sure, but I shall check. This may also tie up with your suggestion, Steve. Got to see how to do this though.