In general each client is free to decide if it reads information from an OPC UA server. There is no mechanism for a server to enforce that a specific client gets a message. But OPC UA offers different levels of client / user interaction that may be selected for information being published.

The message is published at the interface and no feedback is expected to inform the server if the information has been read by any client.

After it is published on the interface the server waits for one client to acknowledge the reception of the message. Usually such an acknowledgement is automatically generated by a client PLC reading the information.

If a message requiring an acknowledgement has been published it will stay active till an acknowledgement has been received, even if the cause triggering this message has already been cleared.

Application Note:

As a Reset method call tries to bring the system back to an initial state it clears all active messages. If an error state could not be cleared the message will show up again after performing the reset.

After the message is published one of the receiving clients has to acknowledge the reception and afterward has to confirm that the server may clear the information. This mechanism is usually used to call for human assistance. The client PLC acknowledges the reception and shows a message on its HMI. After the operator gives the command to continue the PLC sends the confirmation to inform the server.

If the vision System has its own user console such a user confirmation may be entered there. Therefor it is allowed that the vision system may clear the message even without the confirmation received from a client. But it has to wait for the reception of an acknowledgement before doing so.

As a number of messages may be published at the same time the vision system may optionally offer a ConfirmAll method that sets the confirmation for all active messages.