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RE:Module Communication Error Codes
You failed to specify which module you are asking about; but, from the context of your message, I assume you are asking about the MVI56-MCM. In this module, the Module Status/Error Data and Command Error Lists for each port (when used as a Modbus Masters) reside in the upper region of module memory, above the 5000 registers of user data.
MCM.InStat tags are used to bring general module status data into controller tags, where it may be viewed. Likewise, the Error Status Pointer can be set to put this same information in an area of the ReadData array. However, this is unnecessary, as the data is already being brought into readable tags and the Error/Status Pointer should normally be set to -1 to disable the transfer to User Memory. This Error/Status data is the data described in the User's Manual, Appendix B, MVI56-MCM Status Data Definition.
If one or both of the module ports are configured to be Modbus Master(s), then the results of each Modbus poll command are stored in a Command Error List, a 100-word table for each port, since each port may use up to 100 commands. This table also resides in the upper region of module memory. To view this data, it is necessary to use the Command Error Pointer to have the module store this data inside the ReadData address range, normally at the top of this range, 100 words per Master Port. Then, the results of your Modbus commands can be monitored by logic.
If you do not intend to use these two types of status and error data tables as part of some alarm logic, you may set the Error/Status pointer and the Command Error Pointer(s) to -1 to prevent the module from writing them to User memory. However, even if the Error/Status pointer is set to -1, the MCM.InStat tags will still contain the values in Appendix B.
The Command Error List is the most useful data for detecting and diagnosing communications problems and only contains useful data when ports are used as Modbus Masters. The Error/Status Data is not very useful for detecting or diagnosing communications problem in for either Masters or Slaves.
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