13 Replies Latest reply on Feb 15, 2019 2:16 PM by gregor

    Same value in the tag for two different time stamp

    PrasadChaudhari01

      Hello,

       

      There are some PI tags for which we are receiving the same value for two different time stamp.

      The tag configuration is as per the requirement, there is no issue in tag configuration.

      could anyone please help to understand what will be the reason and resolution for this?

       

       

      Regards,

      Prasad

        • Re: Same value in the tag for two different time stamp
          Lal_Babu_Shaik

          Hi Prasad

           

          Please let us know the interface you are collecting data from. Also interested to know whether it is an input tag or output for any calculation.

          • Re: Same value in the tag for two different time stamp
            Rhys Kirk

            Looks like you have a 10 minute exception window on the PI Tag configuration (in the PI Server).

            1 of 1 people found this helpful
              • Re: Same value in the tag for two different time stamp
                PrasadChaudhari01

                Hi Rhys,

                 

                Yes exactly, but it should not give the same value at two different time stamp.

                  • Re: Same value in the tag for two different time stamp
                    gregor

                    Hello Prasad,

                     

                    Rhys is right. With PI Interface for OPC DA the excmax behavior with advise and event points differs from what you may be used to. Please refer to the PI Interface for OPC DA User Guide - Appendix A. Set ExcMax on advise and event points

                    2 of 2 people found this helpful
                    • Re: Same value in the tag for two different time stamp
                      Lal_Babu_Shaik

                      Hi Prasad

                       

                      Couple of  settings that play a pivotal role in the amount of data getting sent across and being evaluated. The exmax and compmax tag attributes. The exmax tag attribute specifies an amount of time (in seconds) to wait before sending a value across even if it does not vary by more than the exdev amount. That is why you see data coming in about every 10 minutes to the PI server with exception on and compression off. Exmax is set at 600 in your tags (10 minutes – the default setting).

                      Compmax specifies an amount of time to wait before sending a value to the archive EVEN if that values does not vary by more than compdev. Tag compmax attribute is set to 28800 (8 hours – the default setting). This means that after waiting 8 hours with no values breaking the compression setting, the current snapshot value will be archived. This current snapshot value could have a timestamp anything up to 10 minutes prior to the 8 hour mark due to the exception settings you have.

                      If you look at the time difference between the values for that tag, there are in pairs where there is 10 minutes difference. This pattern continues through the series of data. This is because when a value passes the exception test and is sent to the PI server (in your case due to exmax rather than the value differing by more than 0.1), the value AND the previous value are sent to the PI server

                       

                      2 of 2 people found this helpful
                          • Re: Same value in the tag for two different time stamp
                            gregor

                            Hi Prasad,

                             

                            Which one is it?

                            Please mark it as the [Correct Answer]

                              • Re: Same value in the tag for two different time stamp
                                PrasadChaudhari01

                                Both ans are correct.

                                In addition to that how should i set my EXmax and CompMAx.

                                Currently i have set it as :

                                Exmax : 10min and CompDev : 1 day

                                will this issue be fix at tag configuration level?

                                  • Re: Same value in the tag for two different time stamp
                                    gregor

                                    Hello Prasad,

                                     

                                    Your concern are the values repeated every 10 minutes. Hence your issue is likely caused by excmax = 600 (10 minutes) and yes, the excmax value need to be changed for each tag you consider those duplicates to be problematic. I believe to disable the behavior, you would have to set excmax = 0. Likely a better option is to think about what setting would be appropriate by taking the update rate (scan class) into account and until when you indeed expect an update. Could you think of a time you would want the snapshot to be repeated in case there is no update from the OPC Server?

                                     

                                    Sorry, but you need to pick a [Correct Answer]. Because Rhys was the first one pointing out the issue could be due to excmax, I believe he also deserves the credits.

                                      • Re: Same value in the tag for two different time stamp
                                        PrasadChaudhari01

                                        Hi Gregor thank you for all your comments.

                                         

                                        But i am little confused of this Exmax theory.

                                        if it is problem with EXmax then how i am getting normal values for some duration and repetitive for some.

                                        In below screen shot you can see that the value which are in blue box are single values and having time difference is more than 10 min.

                                        It means in 10 mins values has not been changed and when next value occurs then only it has been archives.

                                        so why this has not happen at yellow highlighted time?

                                        On other note the tag configuration is through out the same.

                                         

                                        regards,

                                        Prasad

                                          • Re: Same value in the tag for two different time stamp
                                            gregor

                                            Hello Prasad,

                                             

                                            When you retrieve recorded values using option "Replace Duplicates" this means you are filtering the recorded values which makes it even more difficult to understand why you sometimes see values repeated each 10 minutes and sometimes not. Explaining the behavior just based on recorded values is also pretty hard with compression enabled. I don't like to suggest disabling compression either.

                                             

                                            The PI Interface for OPC DA allows to log information about what the interface receives from the OPC Server and when (/DB=4096 and /DT=<tagname>). With /UITRACE=<tagname> the interface will log information about the writes to PI. Options referring a tag by its name can only be used once per interface instance and the PI Interface for OPC DA User Guide advises to use these debugging options with care as they can easily make your logs grow huge. So, please don't forget to disable the logging options again when you are finished with your investigations. Also worth to mention is that those logging options represent configuration changes of that kind which require a restart of the interface service.

                                            You will find information about when, what value with what timestamp and what quality is received from the OPC Server. More important for your case is the /UITRACE=<tagname> option as it will make the interface log information about what and when the PI Point defined with <tagname> receives an update.

                                             

                                            You also need a good understanding of how Compression works to be able to evaluate if the value pushed off the snapshot, by a new arriving snapshot value, will make it to the archive as recorded value or not.