MQTT Client

MQTT is specified in ISO/IEC 20922 and offers a simple protocol for remote data sources. Thanks to its easy and flexible use, it is frequently selected for IoT applications. Data packets with any content and a size of up to 64 kB can be exchanged together with a readable designator (topic) between distributed devices and a central broker. This enables the distributed devices to be fully independent and only require a communication relation to the broker, so that very flexible networks can be created easily.

The MQTT client is a software module for the M200 controller that is immediately available and does not require any additional hardware. The network connection to the external MQTT broker is created and monitored automatically. The login at the broker can be protected if required with a name and password, and the user data can be encrypted if required with TLS certificates. This also enables sensitive production data to be transferred via the Internet.

The MQTT Client can be used as publisher or subscriber. Publishing is the typical application for a controller, in which the controller software collects and aggregates the already available process data, and sends it to the broker at defined times (e.g. for each finished workpiece). In order to receive data, the client can subscribe selected topics from the broker so that it then only receives the new data from the other stations in which it is interested. These are stored in a receive queue and can be fetched and processed by the controller software.

The structure and content of the data can be selected as required by the application software. Both binary as well as text-based data such as XML or JSON can be transferred up to a maximum size of 64 kB, the content must only be understandable for publishers and subscribers. The broker itself only observes the topics for management purposes. The content of the message is not important for the broker. To operate an external MQTT broker is required. Commercial cloud providers are possible here, solutions for Windows or Linux are available for the test or local operation.

• Automatic connection and monitoring with automatic re-connect and re-subscribe
• Publishing of messages up to 64 kB
• Parallel subscription to different topics
• Can be used in parallel by several software modules on the controller
• Application through the application software via PLC module library or C/C++ header file
• Event-driven processing of incoming messages possible in C/C++
• Recognition of new messages via Boolean status display in PLC
• Optional communication to broker via web sockets for asynchronous transmission in both directions
• Simple commissioning without programming via variables interface
• Last will and testament
• Persistence: Data is stored over the reboot until it is acknowledged