Connectivity of OPC server and client

If the OPC server and OPC client reside on different computers, check connectivity before configuring your OPC server and OPC client computers for DCOM:

• Verify that the server and client can connect to each other on the network and that port 135 is open (use telnet).

• If port 135 is not open, check for issues related to a firewall or other network restrictions.

• After that initial connection, the Service Control Manager will inform the client what port should be used for further communication. The chosen port could be any port within the ephemeral port range:

  • XP/Win2K3: 1024-4999

  • Vista and later: 49152-65535

• You should also open a range of ports above port 5000. Port numbers below 5000 may already be in use by other applications and could cause conflicts with your DCOM application(s). Furthermore, previous experience shows that a minimum of 100 ports should be opened, because several system services rely on these RPC ports to communicate with each other

  Note: OPC operations use asynchronous callbacks. During callbacks, the OPC client becomes a DCOM server, and the OPC server becomes a DCOM client. When a server makes a callback to a client, it creates a new connection to the client and sends method calls over a separate TCP channel. The same dynamic port allocation, as described above, takes place in the OPC client side. This dynamic port allocation, in this ephemeral port range, makes DCOM a "firewall unfriendly" protocol.

For more information, see the OSIsoft Knowledge Base topic Configuring ports for DCOM for use with the OPC Interface. NAT and Firewall considerations.