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Serial2Mqtt.md

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serial2mqtt

WORK IN PROGRESS !! Major rework

<script type="text/javascript" src="https://unpkg.com/mermaid@8.0.0-rc.8/src/mermaid.js"></script> <script type="text/javascript" src="https://unpkg.com/mermaid@7.1.0/dist/"></script> 
sequenceDiagram
µC->> serial2mqtt: M,PUB,0,0,src/drive/motor/rpm,1234
Note right of serial2mqtt: connects to mqtt broker at first message.
serial2mqtt-->>MQTT Broker: CONNECT(broker,port)
MQTT Broker --x serial2mqtt: CONNACK
serial2mqtt-->>MQTT Broker: SUBSCRIBE("dst/drive/#")
serial2mqtt-->>MQTT Broker: PUBLISH("src/drive/motor/rpm",1000,0,false)
µC->> serial2mqtt: M,PUB,0,0,src/drive/motor/temp,20
serial2mqtt-->>MQTT Broker:  PUBLISH("src/drive/motor/temp",20,0,false)
Note right of µC: no more messages after 5 sec, serial2mqtt disconnects.
serial2mqtt-->>MQTT Broker: DISCONNECT
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MQTT for all micro-controllers ! The purpose is to offer MQTT publisher/subscriber functionality to all small micro controllers. Those with just a UART or USB interface. Example : a cheap STM32 board on ebay. Afbeeldingsresultaat voor ebay stm32 This program will act as a full MQTT Client gateway and make integration as simple as possible. This was created because Ethernet or WiFi is still absent in most ( cheap ) controllers . Also the concept behind is that a central PC or Raspberry PI can act as the intelligent mind behind commodity components.

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Arduino Sample program to communicate with the serial2mqtt gateway

class Mqtt {
  public:
    static void publish(int qos, bool retain, String topic, String message ) {
      Serial.printf("M,%d,%d,%s,%s\n", qos, retain, topic.c_str(), message.c_str());
    }
};
int i = 0;
void loop() {
  delay(100);                       // wait for a 0.1 second
  String  message=String(i++);
  Mqtt::publish(0, false, "src/arduino1/system/upTime", message);
  Mqtt::publish(0,false,"src/arduino1/system/host","arduino1");
}

Working assumptions

  • Topic Names --The design will take into account some assumptions about topic names and tree-structure to make it simple to use. Structure topic to and from device : -- dst/DEVICE/SERVICE/PROPERTY -- src/DEVICE/SERVICE/PROPERTY -- if DEVICE is not known yet the serial2mqtt will subscribe to the dst/PORT/prog , where PORT is for example ttyUSB0
  • Serial messages will be BINARY or ASCII -- BINARY format will be CBOR encoded in a SLIP envelope -- ASCII will be text delimited by newlines
  • The serial2mqtt should be able to reset the device ( hard reset )
  • The serial2mqtt should be able to program new code into the device
  • Through the same communication, debugging logs can be handled without disturbing the mqtt flow
  • the serial2mqtt establishes the client MQTT link and subscribes to dst/DEVICE/#
  • when there is a big delay on the serial2mqtt serial input, it will stop the TCP MQTT connection
  • serial2mqtt is event driven and not blocking
  • one instance of serial2mqtt should be able to handle different serial ports
  • USB devices coming and going should be tracked by serial2mqtt
  • Configuration can be command line and config file driven ( JSON )
  • serial2mqtt should be able to program the device through the serial interface, for this purpose a third party app will be launched with the concerned serial port as argument.

Protocol

ASCII TEXT

  • M,PUB,<qos>,<retain>,<topic>,<message>

BINARY CBOR SLIP

<END><SLIP ENCODED MESSAGE><END>
<SLIP ENCODED MESSAGE> == <'M'><"PUB">,<qos Integer><retain boolean><topic string><message binary><CRC integer>

CONNECTION SETUP

sequenceDiagram
µC->> serial2mqtt: M,PUB,0,0,src/drive/motor/rpm,1234
Note right of serial2mqtt: connects to mqtt broker at first message.
serial2mqtt-->>MQTT Broker: CONNECT(broker,port)
MQTT Broker --x serial2mqtt: CONNACK
serial2mqtt-->>MQTT Broker: SUBSCRIBE("dst/drive/#")
serial2mqtt-->>MQTT Broker: PUBLISH("src/drive/motor/rpm",1000,0,false)
µC->> serial2mqtt: M,PUB,0,0,src/drive/motor/temp,20
serial2mqtt-->>MQTT Broker:  PUBLISH("src/drive/motor/temp",20,0,false)
Note right of µC: no more messages after 5 sec, serial2mqtt disconnects.
serial2mqtt-->>MQTT Broker: DISCONNECT
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Programming through serial2mqtt

A command line utility will send a single mqtt request to the serial2mqtt gateway to program the microcontroller.

sequenceDiagram
µC->> serial2mqtt: M,PUB,0,0,src/drive/motor/rpm,1234
serial2mqtt->>MQTT Broker:  PUBLISH("src/drive/motor/temp",20,0,false)
programmer CLI -x MQTT Broker: PUBLISH("dst/drive/prog/flash",flash image binary)
MQTT Broker -x serial2mqtt : PUBLISH
activate serial2mqtt
serial2mqtt -x µC : program flash image
deactivate serial2mqtt
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Logging through serial2mqtt

The micrcontroller will also log to the central logging system