OPC UA Source System: Subscription Tab

Area: Subscription Parameters

Display Name

Name of subscription list.

For OPC UA, the subscription list contains the tags to be monitored.

Event Interval (ms)

With this interval, you specify how often value changes are to be reported.

Keep-Alive Interval (sec)

If there is no value change, the UA server sends a message to PCo to this effect. Here you specify the interval at which a message is to be sent.

Lifetime Interval (sec)

Indicates the lifetime of a subscription when the session is interrupted so that the subscription can also be used by another client.

Maximum Notifications Per Event

Here you specify the maximum number of value changes per event that are to be passed on. In the case of OPC UA, an event can contain several value changes.

Acceptable Status Code

You can specify here that you want notification messages to be filtered using the status code. The status code defines the minimum quality of the notification messages sent from the OPC UA source system.

The following options are available:

• Any Status Code

  This is the default setting. In this case, all notification messages are processed.

• At Least Uncertain

  With this setting, only notification messages whose status codes are good or uncertain are forwarded.

• At Least Good

  With this setting, only notification messages whose status codes are good are forwarded.

If you set the log level to Information or Verbose on the Host tab of the agent instance, an entry is written to the log when a notification message has been filtered out due to its status code.

Enable Edit Mode for Subscription Items

If you select this indicator, you can enable editing for subscription items for the agent instance of your OPC UA source system.

It is not normally possible to manually enter or change subscription items in the case of OPC UA servers because the server is prone to error. Therefore, it is recommended that you use the Browse dialog to browse the address space of the server and select the tags that you want to subscribe. This is why the pushbuttons for manual editing of subscription items for OPC UA agent instances are not active as a rule.

However, individual OPC UA servers do not allow you to browse the address space completely and so it is the case that specific IDs represent an entire group of tags and that individual elements of the group need to be accessed using a special syntax. This is described in the server documentation. In such a case, this indicator lets you enable manual editing of the subscription items for the agent instance.

Area: Monitored Item Parameters

Queue Size

Here you specify how large the queue of value changes may be. The individual value changes are placed in the queue.

Discard Oldest

With this indicator, you specify that the oldest values in the queue are to be discarded. Since new value changes are continually placed in the queue, it makes sense to discard the oldest values.

Area: Retry Settings for Node Access After Server Restart

In this screen area, you define the settings for a new connection to the OPC UA server after it has been restarted. These settings allow a waiting period that enables the PCo system to go into a queue and to try to connect to the OPC UA server again for node access.

This is a setting that is considered a workaround and should not normally be required.

Max. No. of Retries

Here you specify the maximum number of connection attempts to the OPC UA server. The default value is 0.

Retry Interval

Here you specify the number of milliseconds before the next connection attempt is made.

The value 0 is set by default, which means that the agent switches to the Faulty status if it cannot read the node classes for its subscriptions.