Tuesday, 18 April 2017

Using a different tool for each job and linking them together with MQTT - part 2

Now we have a working MQTT server, it's time to start making use of it.

As I was planning to use MQTT to broker messages between Web server CGI scripts, Javascript in Chrome web browser and an Arduino the obvious way to do this is push data at the USB serial port of the Arduino.

Normally this port is used for uploading the Arduino sketch to the board and sometimes people end up filling it with debug/status messages once the sketch is running. However there's a long history of it being used as an actual way to control stuff.

So I threw together a piece of 'middleware' that shuffles data between some MQTT topics and the USB serial port.

#!/usr/bin/python
import time
import os
import mosquitto
import serial
# commands to the arduino are as follow...
# R red LEDs
# G green LEDs
# B blue LEDs
# N no LEDs
# O open/go
# P power off
arduinoBootupTime = 5
arduinoIsBooted = 0
debug = True
# Define the various MQTT callback functions as this is an event driven model
def on_connect(mosq, obj, rc):
if debug:
print("rc: "+str(rc))
return
def on_message(mosq, obj, msg):
if debug:
print(msg.topic+" "+str(msg.payload))
arduino.write(str(msg.payload))
return
def on_publish(mosq, obj, mid):
if debug:
print("mid: "+str(mid))
return
def on_subscribe(mosq, obj, mid, granted_qos):
if debug:
print("Subscribed: "+str(mid)+" "+str(granted_qos))
return
def on_log(mosq, obj, level, string):
if debug:
print(string)
return
#
time.sleep(arduinoBootupTime)
# Connect to the local MQTT server to pass stuff to/from a browser
mqttc = mosquitto.Mosquitto()
mqttc.connect("localhost", 1883, 60)
mqttc.on_message = on_message
mqttc.on_connect = on_connect
mqttc.on_publish = on_publish
mqttc.on_subscribe = on_subscribe
# Subscribe to the topic that sends commands TO the Arduino
mqttc.subscribe("arduino/in", 0)
while 1:
if debug:
print "Trying to connect to Arduino\n"
arduino = serial.Serial('/dev/ttyUSB0',115200,timeout=1)
while arduino.isOpen():
if arduinoIsBooted == 0:
if debug:
print(arduino.name) + " connected - Giving " + str(arduinoBootupTime) + "s for it to bootstrap"
time.sleep(arduinoBootupTime)
arduinoIsBooted = 1
mqttc.loop()
arduinoOut = arduino.readline()
if len(arduinoOut) > 0:
if debug:
print "Received " + arduinoOut.rstrip() + " from the Arduino"
if arduinoOut.rstrip() == 'powerDown':
powerOffFile = open('/tmp/poweroff', 'a')
powerOffFile.close()
else:
mqttc.publish("arduino/out", arduinoOut.rstrip())
arduino.close()
time.sleep(10)

This is about as simple as things can be and mostly cribbed from example scripts. It waits a while to give the Arduino time to boot as the USB port attaching to Raspberry Pi will cause it to reset. Then it tries to connect and loops round forever passing messages back and forth.

The meat of this is in two functions, the first of which gets triggered as a callback when an MQTT message arrives...

def on_message(mosq, obj, msg):
arduino.write(str(msg.payload))
return
This simply sends the message straight to the Arduino. Coming the other way is almost an exact reverse except we're using newlines to mark the end of messages so we can read them with 'readline'. This means we need to strip them off before publishing to MQTT. Python has a handy function for this in 'rstrip'.

arduinoOut = arduino.readline()
if len(arduinoOut) > 0:
if arduinoOut.rstrip() == 'powerDown':
powerOffFile = open('/tmp/poweroff', 'a')
powerOffFile.close()
else:
mqttc.publish("arduino/out", arduinoOut.rstrip())
There's another little if/else in here to handle the shutdown sequence. I used the VERY simple method of writing a file to /tmp/ if the Raspberry Pi needs to shut down, which is controlled by the Arduino. I could have had a second script subscribed to the topic for this, but this keeps all the MQTT code in one place. Because the Pi uses memory for /tmp/ instead of saving to its SD card, this safely disappears once the machine is shut down.

That's about it, things coming out of the Arduino USB serial port end up in the topic '/arduino/out/' and stuff in the topic '/arduino/in/' get sent to the serial port. The test for the existence of the Arduino serial port '/dev/ttyUSB0' means it loops round forever trying to reconnect if the Arduino get unplugged or resets.

To make this run at startup I put it into /etc/rc.local which is another quick and dirty 'make it work' solution. It works. This is not a server it's a prop, so I'm being cavalier with scalable/secure ways of doing things.

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