Remote access of IO using DP/DP coupler
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You need to go read the manual for the DP/DP coupler.
You cannot read the IO modules directly. Each IO Module can only have **ONE** Master and you can only have **ONE** master on the profibus. So if the cards belong to the PM851A, then you need to read / write the IO channels in the PM851A then copy the values to and from the DP/DP coupler.
The PM866 cannot read and write to the IO modules directly - only to the PM851A via the DP/DP coupler. So whatever bytes you choose to configure in the DP/DP coupler is up to you.