Served nodes
The list of what the embedded OPC UA server serves, and all it serves: tags, Logic variables, alarms and device commands, each read only until you allow writing, where a client finds each, and what an updated station needs.
Served nodes is the fourth card of the OPC UA server's panel on the Connector. The server serves this list and nothing else: a client that connects finds the tags, Logic variables, alarms and device commands listed here, and none other. The card says so under its title: "The server serves this list and nothing else: each row is a tag, a Logic variable, an alarm or a device command. A row stays read only until you allow writing. A client's write then takes the same path as every other write and is recorded in the journal with the client's session." View connector reads the card; Configure connector changes it.
An empty list
The list starts empty, on a new station and on a station updated from a version that served every device whole. While it is empty, a caution note on the card says: "The list is empty, so the server serves nothing of the station: an OPC UA client finds no tag, variable, alarm or command until you add rows." The server still listens and still accepts the sessions Client access allows; they browse an address space with nothing of the station in it. An agent reads the same thing on server_status, whose servesNothing is true while the list is empty.
The table
| Column | What it shows |
|---|---|
| Source | What the row serves, as the picker names it, or "No source yet" while its source is not chosen. |
| Access | Read only or Read and write, and Callable for a device command. |
| Node | The address a client is configured with, in the form nsu=<namespace>;s=<identifier>; empty while the row serves nothing. |
The rows are listed by kind (tags, variables, alarms, commands), then by name. With none, the table reads "No node is served yet. Add one with New row." Clicking a row selects it and opens its fields under the table; which row is selected belongs to the window you are in.
| Command | What it does | Greyed when (situation) | Not drawn when (role) |
|---|---|---|---|
| New row | Adds a read only row that serves nothing yet and selects it, so its source is picked next. The feed says "Row added to the OPC UA server's list. Pick its source." | A run holds the station, or the runtime is stopped. | Without Configure connector. |
| Delete row (danger ink) | Deletes the row the table has selected, at once and with no question; its tooltip names the source the row serves, and the feed confirms it. The source is not touched: a client no longer finds its node. | No row is selected ("Select a row of the list to delete it."), a run holds the station, or the runtime is stopped. | Without Configure connector. |
A row's fields
Each field is stored the moment it changes, and the server serves the new row a moment later, without dropping a session: it reads its list ten times a second.
| Field | What it does | Values and default |
|---|---|---|
| Source | What the row serves. The list opens with "(none)", then every enabled tag of an enabled device, named after its driver, its device and itself ("Tag · Simulated · Oven · Temperature"), then every Logic variable that holds a value, by its folders and its name ("Variable · Oven / Setpoint"), then every alarm, named the same way ("Alarm · Oven / High temperature"), and last every command of an enabled device: the commands declared on the device page and those a protocol fixes, such as a charge point's ("Command · OCPP · Charger · Reset"). A source is listed once. The row holds its source by identity, so a rename leaves the row on the same source, which the server then serves under its new name. A source the list no longer offers stays selected and named, a disabled tag or command with what keeps it out, and only a source that no longer exists reads as missing, with its identity. A tag's Used by card lists the rows that serve it. | "(none)" for a new row. |
| Access | Whether a client may write the row. Read and write stays listed and greyed where the source takes no write, its tooltip saying "This source takes no write: only a tag with write access or a memory variable does." Picking an alarm sets the row back to read only. For a command the field shows Callable as a fixed value, with no picker, because a command is called, never read or written. | Read only or Read and write; Callable, fixed, for a command. Read only for a new row. |
Under the fields a line says what the row does for a client:
| The card says | When |
|---|---|
| "This row serves nothing yet: pick its source." | No source is chosen. |
| "The source of this row no longer exists, so the server serves nothing for it." | The tag, the variable, the alarm or the command was deleted. |
| "A client may read this node and not write it." | A tag or a variable, read only. |
| "A client may read and write this node: each write is recorded in the journal with the client's session." | A tag or a variable marked read and write. |
| "A client reads the alarm's Active, Acked, State, Priority and Message, and never writes them." | An alarm. |
| "A client calls this command as a method of its device for as long as it is in the list." | A device command. |
Under that line, A client finds it at is followed by the node.
What a row refuses
A change the list's rules refuse is not stored: the reason is said in red above the row's fields, the change stays on the screen so you can correct it, and the row keeps serving what it served.
| The card says | When |
|---|---|
| "This source is already in the list: a client finds it at the node that row serves." | The source is listed in another row. |
| "The source cannot be written: pick a tag with write access or a memory variable, or keep the row read only." | Read and write on a read-only tag, a tag of a driver that never writes, an expression variable, an alarm, a command or a source that no longer exists. |
| "A row that takes writes needs the tag or the variable they go to." | Read and write on a row with no source, which only an agent can ask for. |
| "This row of the list no longer exists. Reload and retry." | The row was deleted elsewhere, in another window or by an agent, while you were editing it. |
While a run holds the station or the runtime is stopped, a change is refused the way every change of the page is, with the reason in the action feed, and nothing is stored.
Where a client finds each source
| Source | Where it is | What a client reads |
|---|---|---|
| Tag | Where it has always been: in its device's namespace, http://opcfoundation.org/UA/<driver>.<device>/, under the tag's own name. A client configured before the list existed finds it at the same address. A client that browses finds it in the Devices folder under Objects, inside the object named after its device. |
The value the device answered, with its quality and its stamp, as the Connector reads it. |
| Logic variable | In the Logic folder under Objects, inside the Logic's own folders, in the server's namespace http://opcfoundation.org/UA/EdgeTranslator/ under its Logic address (Variables.Oven.Setpoint). |
Its value, typed by the station's one conversion: Boolean, Int64 for an Integer, Double for a Decimal, String for a Text and DateTime for a Date and time. A variable with no good value reads BadDataUnavailable. |
| Alarm | In the Alarms folder under Objects, inside the Logic's own folders, as one object under its Logic address (Alarms.Oven.HighTemperature). |
Five read-only members: Active and Acked (Boolean), State (Int32: 0 normal, 1 active and unacknowledged, 2 active and acknowledged, 3 cleared and unacknowledged), Priority and Message (String). While the alarm holds its state through bad quality, Active, Acked and State read UncertainLastUsableValue. |
| Command | A method of its device, beside the device's tags. | Called with the values the command declares, it answers with what the equipment answered. |
Every device keeps its namespace whatever the list serves of it, so the index a client finds a namespace at does not move when another device's tags join or leave the list. A device the list names nothing of has no node.
Writing
A client's write reaches the station only on a row marked read and write, and the list is asked again the moment the write arrives, so a row made read only a moment ago is never written. A tag's write goes to its device's delivery queue as every write from outside does, and the answer waits for the equipment; a memory variable's write goes through the Logic's write funnel and is converted to the variable's type. The source locks apply: a source a running Process holds is refused. Every write is journaled with the client's application and session. An expression variable and an alarm never take a write, and the server answers BadNotWritable. A command is called rather than written, and a client calls only a command that is in the list.
An updated station
A station updated from a version without the list served every tag of every device. Its list starts empty, so after the update its OPC UA server serves nothing until the list is built: add a row for each tag a client reads, mark read and write the ones a client writes, and add the commands a client calls. A client configured before the update finds each tag at the address it had, so it needs no change of its own.
With an agent
server_list_upsert(source: "Connector.Simulated.Oven.Temperature")
server_list_upsert(source: "Variables.Oven.Setpoint", access: "ReadAndWrite")
server_list_upsert(source: "Alarms.Oven.HighTemperature")
server_status()
A row created by an agent is read only unless access says otherwise, and server_list_delete deletes one by its id. The tools are described on The tools, area by area.
What the list does not do
It does not turn the server on (the command bar's Enable does), it does not decide which client may connect (Client access does), and it does not acknowledge an alarm from outside: acknowledging stays an operator's command. It serves the values above and no OPC UA alarms and conditions events. A variable that holds an image, an icon or a colour is not offered, and a constant of a device's description is not served.