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
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...