Below is a diagram of the Handshake Process between the two services and the PLC.

Explanation:

The XML Service writes a value of 1 to Connection Handshake in the ServiceData table in the LAP Database every second. The PLC Service reads the Connection Handshake value and if that value is 1 it writes a 0. If the value is still a 1 it starts a timer that runs for 10 seconds. If the value does not change to 0 within the 10 seconds a value of -2 will be written the LifeCounterIN tag in the PLC, otherwise it writes a 1 to the LifeCounterIN tag in the PLC.

The service takes XML data that is retrieved through stored Procedures in the LAP Database from the respective tables. This data is then stored inside of a predefined folder structure. Please note that changing the structure of the folders may cause the PLC Service to be disrupted. The service is triggered by an update occurring in the Database. Once this is triggered the Service begins calling the stored procedures to retrieve XML data about each table. This data is then stored into the XML Folders as well as turned into a call list for each tag necessary to transfer to the PLC per configuration.

The service is triggered through the Next Configuration in the “PLCName”.Overhead.NC being changed to something other than the running configuration. So for example if I have a Running Configuration, configuration 2, and then the next configuration to be loaded is configuration 3 the service will be triggered and the transfer to the PLC will begin. The way this works is by taking tags that have already been made in the LAP to XML Service and then updating the values inside of the PLC. Note, if this file, calls.xml, is misplaced or deleted the service will be unable to properly update the values to the PLC. If this becomes the case, try to rerun the LAP to XML Service to rebuild the folder tree.