title: how S7-1500 OPC UA server communicates with custom client
body: <p>I'm little bit confused regarding how S7-1500 embeded OPC UA server communication works in case of a custom client installed on PC since all examples I've seen so far include a Simatic PC Station as OPC server!!!</p>
  
  <p>On PLC side, I have S7-1511-1PN with OPC UA server activated. This PLC is connected over PROFINET to ET200SP+several AI modules.</p>
  
  <p>On PC side (not Simatic PC Station), I have an application in which there is some kind of OPC UA client functionnality that reads inputs from AI modules, writes some tags on PLC and if necessary sends reconfiguration records of AI modules to PLC (reconfiguration via user program).</p>
  
  <p>What I can't understand, is:</p>
  
  <p>Why the Simatic PC station is always added as OPC server if the PLC already has this feature?</p>
  
  <p>Does/How PLC OPC server communicates directly to my custom client? (Any programming needed on PLC)</p>
  
  <p>For the certificates limitations as defined in "Function Manual, 10/2018, A5E03735815-AG (page 222)", what means "Max number of implementable server methods", "Max number of arguments", "Max number of server interfaces", "Max number of nodes in user-defined server interface", "Max size of loadable server interfaces"?</p>
  
  <p>How can I estimated these numbers from my application?</p>
  
  <p>(Yes/No) Do I need a second communication (TCP for example) between my PC application and PLC to send/receive reconfiguration data?</p>