mqtt serial message without ethernet

Question

everywhere I look, if it comes to MQTT messaging, it is all over Ethernet. Unfortunately, I do not have Ethernet wires available.

I am automating my home, wanting to use mqtt as messaging service. My buttons give a signal (like light 'on') to an arduino board running a mqtt client. How do i put out the mqtt message on the serial lines so the transmitted message can be straight forwardly be picked up by an mqtt broker, without using the ethernet cables?

Answer 1

As I was looking for the same answer and didn't find anything to my liking, I decided to write an implementation that enables any Bluetooth, USB or pure serial port communication to send MQTT message. You can find the result here : https://github.com/vortex314/serial2mqtt It enables any linux machine ( Raspberry Pi 1, Raspi 3 , PC ) to act as a gateway. It's written in C++ so should be lightweight enough for small devices. Hope this helps.

Update 27/6 : build folder contains pre-build version for Linux Intel and Raspberry.

Answer 2

I am trying to achieve similar goals for my house automation, although I do have Cat5e in the walls, so could use Ethernet.

Here are some possibilities for using MQTT over serial communication:

• Use SLIP between the Arduino and the gateway. The SerialIP client library which implements TCP/IP over serial: http://playground.arduino.cc/Code/SerialIP . Although I have not tested if this works with a MQTT client library. Nick O'Leary's MQTT Client theoretically works with any implementation of the Arduino 'Client' class.
• It may be possible to implement MQTT directly over serial, without TCP/IP, although I am not aware of any gateways to do this.
• MQTT-SN (a slightly different protocol to MQTT) was designed for sending messages over unreliable transport, and may be better suited to a serial line. If you are interested in publish-only, I wrote a very basic library to send MQTT-SN packets over serial: https://github.com/njh/DangerMinusOne
• You could use something like Node-RED on the Linux gateway, talking some other serial protocol (Firmata?) to the Arduino and then converting to MQTT using Node-RED.

I did start looking at MQTT over serial implementations but one of the problems I didn't resolve was how to deal with multiple devices:

• Adding lots of serial ports to Linux is complex/expensive (well sort of https://twitter.com/njh/status/570298977310150656)
• Reliable serial over longer distances requires extra components for RS-485
• MQTT doesn't work well over half-duplex, without an extra layer of software to deal with a master polling slaves for messages

So I am current looking at implementing MQTT-SN for the ENC28j60 ethernet controller...