# Servers

> The Connector's Servers folder: the embedded OPC UA server, the Modbus server and the MQTT broker, the three ways other systems and devices reach this station, each with its own row, its own switch and a dot that rolls up into nothing.

The second root of the Communication tree, beside the Drivers folder, is the **Servers** folder, which holds three rows: **OPC UA server**, **Modbus server** and **MQTT broker**. A driver reads equipment; a server is read by it. Each of the three waits for clients to connect and never connects to anything. The OPC UA server publishes the tags, Logic variables, alarms and device commands of its list, and the Modbus server the tags, Logic variables and alarm states its register map serves. The MQTT broker takes the messages the devices of the network publish, and the station reads them through the MQTT driver. Remote access, which serves the station's own dashboards to a browser on another computer or a tablet, is not here: it has a page of its own on the rail, [Remote access](remote-access).

## The three rows {#the-three-rows}

| Row | What it reads beside its name | Its dot |
| --- | --- | --- |
| OPC UA server | Its state: "Turned off", "Listening on port 4840" followed by the clients connected ("1 client connected"), "Starting…", "Could not start" or "Connector stopped". | Grey while it is off or while the Connector is stopped, green while it listens, amber while it comes up, red when it is on and could not start. |
| Modbus server | Its state, described on [Modbus server](connector-modbus-server#the-row-in-the-column). | The same colors for the same situations. |
| MQTT broker | Its state, described on [MQTT broker](connector-mqtt-broker#the-row-in-the-column). | The same colors for the same situations. |

All three start turned off on a new station, so no port opens until somebody chooses to open it. Each dot is the server's own: none rolls up into the Drivers folder, into the status bar's Connector health or into the tray icon, because a server that is off or refused costs no reading. The search box and the state chips reach the three rows as they reach every other: a search matches a row by its name, and "Servers" matches all three; **Off** holds a server turned off, **Issues** one that is on but could not start, and **OK** one that is on otherwise. Selecting one opens its panel, and the command bar above carries **Enable** and **Disable**, each greyed with the reason while the server is already in that state or while the station refuses configuration. The Servers folder's own row carries no count and no dot, and selecting it opens no panel, only the line "Select a server to see its state and settings."

## Who may change them {#who-may-change-them}

The rows follow the Connector's [permissions](users-permissions): **View connector** reads the three panels, and **Configure connector** turns the servers on and off and changes their configuration; a role without it is not drawn those verbs. While a unit runs or the runtime is stopped the verbs stay drawn and greyed with the reason, as everywhere on the [Connector](connector#engineering-boundaries).

## In this group {#in-this-group}

| Page | What it covers |
| --- | --- |
| [OPC UA server](connector-opcua-server) | The row and its panel: what the server publishes, Enable, Disable, Restart and Test connection, the Status card with the endpoint and six meters, the two checks a session passes, and what a remote browser may not do. |
| [Configuration](connector-opcua-configuration) | Application name, Listen port, Network name, Client authentication (Anonymous, Username & password), the user name and password, and Apply. |
| [Client access](connector-opcua-client-access) | The certificates of the applications that dial this station: Add client, Pending approval, Connection addresses, Windows Firewall, Connected now, Trusted clients and Blocked clients. |
| [Served nodes](connector-opcua-served-nodes) | The list of what the server serves, and all it serves: tags, Logic variables, alarms and device commands, read only until you allow writing, and where a client finds each. |
| [Modbus server](connector-modbus-server) | The station's own Modbus TCP server: turning it on and off, its port and clients, the register map, and what a client reads and writes. |
| [MQTT broker](connector-mqtt-broker) | The station's own MQTT broker: turning it on and off, its port and clients, client authentication, and the entry This station it gives the MQTT driver. |

The other direction, the certificates of the OPC UA equipment the Connector dials, is decided on the [OPC UA driver's page](driver-opc-ua#equipment-access).