MQTT is a publish-subscribe messaging protocol built on top of TCP/IP. Applications and devices connect to an MQTT Broker and do one or both of the following:
A Topic is a string that identifies, or labels, a data value.
The data value is a string, which could be:
Fernhill SCADA supports topic values using these formats:
Fernhill SCADA interfaces to an MQTT Broker using the Message Services Driver and the MQTT Driver:
The Message Services Driver can publish reports to MQTT topics.
The MQTT Driver maps I/O Data Point Tags to topics in an MQTT Broker:
Note: Fernhill SCADA does not include an MQTT Broker. There are a number of open source brokers you can use:
The MQTT Driver in Fernhill SCADA uses these MQTT commands:
| Command | Use in Fernhill SCADA |
|---|---|
| CONNECT | Connects Fernhill SCADA to an MQTT Broker. |
| DISCONNECT | Sent by Fernhill SCADA to all connected MQTT Broker when Fernhill SCADA is shutdown. |
| SUBSCRIBE | Used to subscribe to the topics required by the I/O Data Point Tags associated with the MQTT Connection Tag. |
| UNSUBSCRIBE |
Used when an I/O Data Tag associated with an MQTT Connection is edited.
Edit operations that can cause an UNSUBSCRIBE command include:
|
| PING | Periodically the Fernhill SCADA driver sends a PING request to test the availability of the MQTT Broker. |
| PUBLISH |
Publish messages are both sent and received by Fernhill SCADA. Messages published by the MQTT Broker update the value of tag(s) in the Fernhill SCADA Server. Fernhill SCADA sends a Publish request to the MQTT Broker in the following circumstances:
|
The MQTT driver provides this tag type to communicate with an MQTT Broker:
| Icon | Tag Type | Version | Description |
|---|---|---|---|
| MQTT Connection Tag | 3.22 | A connection between Fernhill SCADA and an MQTT Broker. |
You can use these I/O Data Point Tags to map to topics in an MQTT Broker:
| Icon | Tag Type | Version | Description |
|---|---|---|---|
 | I/O Analog Data Point Tag | 3.12 | When associated with an MQTT Connection Tag, maps an Analog Data Point Tag to a topic in the MQTT Broker. The IOItemName field sets how the Analog Data Point Tag> is mapped to a topic in the MQTT Broker. See MQTT IOItemName Format to learn how this field is formatted. |
 | I/O Digital Data Point Tag | 3.12 | When associated with an MQTT Connection Tag, maps a Digital Data Point Tag to a topic in the MQTT Broker. The IOItemName field sets how the Digital Data Point Tag is mapped to a topic in the MQTT Broker. See MQTT IOItemName Format to learn how this field is formatted. |
 | I/O String Data Point Tag | 3.12 | When associated with an MQTT Connection Tag, maps a String Data Point Tag to a topic in the MQTT Broker. The IOItemName field sets how the String Data Point Tag is mapped to a topic in the MQTT Broker. See MQTT IOItemName Format to learn how this field is formatted. |
 | I/O Word Data Point Tag | 3.12 | When associated with an MQTT Connection Tag, maps a 16-bit Word Data Point Tag to a topic in the MQTT Broker. The IOItemName field sets how the Word Data Point Tag is mapped to a topic in the MQTT Broker. See MQTT IOItemName Format to learn how this field is formatted. |
 | I/O Double Word Data Point Tag | 3.12 | When associated with an MQTT Connection Tag, maps a 32-bit Double Word Data Point Tag to a topic in the MQTT Broker. The IOItemName field sets how the Double Word Data Point Tag is mapped to a topic in the MQTT Broker. See MQTT IOItemName Format to learn how this field is formatted. |
 | I/O Long Word Data Point Tag | 3.48 | When associated with an MQTT Connection Tag, maps a 64-bit Long Word Data Point Tag to a topic in the MQTT Broker. The IOItemName field sets how the Long Word Data Point Tag is mapped to a topic in the MQTT Broker. See MQTT IOItemName Format to learn how this field is formatted. |
To learn how to choose topic values and optional read filters.
For information about other drivers available in Fernhill SCADA.
For the meaning of terms used in Fernhill SCADA.
To learn about the MQTT standard.