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I have a got a question about my code. It's about communication between Arduino and Raspberry with serial communication and Json format.

Here my python script for Raspberry :

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import serial
import time
import simplejson as json

ser = serial.Serial('/dev/ttyACM0',9600)

buffer = ""

while True:
    try:
            buffer = ser.readline()
            print buffer
            data = json.loads(buffer)
            print data["three"]
            three = data["three"]
            print three
            #rcv = dummy.split()
            #print(rcv)
            buffer = ""
            print " "
    except json.JSONDecodeError:
            print "Error : try to parse an incomplete message"

Here my Arduino code :

int one=1;
int two=2;
int three = 3;
int four = 4;

void setup() {
  Serial.begin(9600);
}

void loop() {

    Serial.println("'{\"one\":\"" + String(one) +
    "\", \"two\":\"" + String(two) +
    "\", \"three\":\"" + String(three) +
    "\", \"four\":\"" + String(four) +
    "\"}'");

}

In the python code, buffer returns :

'{"one":"1", "two":"2", "three":"3", "four":"4"}'

In theory three returns :

3

I test it in python console and it works (without serial link) : 

buffer = '{"one":"1", "two":"2", "three":"3", "four":"4"}'
data = json.loads(buffer)
three=data["three"]
print three 3

And it return as expected :

3

But in the real time code, I cannot get the value of three and json.loads(buffer) raised an error all the time :

Error : try to parse an incomplete message

'{"one":"1", "two":"2","three":"3", "four":"4"}'

Error : try to parse an incomplete message

'{"one":"1", "two":"2","three":"3", "four":"4"}'

In that case, my code doesn't work. The aim is to obtain all the values send by Arduino in different variables in python.

Thank you.

Arnould
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  • Try to print the actual error and `buffer` when an error occurs: `except json.JSONDecodeError as ex: print ex ; print buffer` I suspect that you will have to introduce some "hangs" (ie `sleep`) to make sure the Arduino code writes everything before the Python code tries to read it. – DeepSpace Nov 27 '16 at 11:57
  • It returns > Expecting value: line 1 column 1 (char 0) > Error : try to parse an incomplete message > '{"one":"1", "two":"2", "three":"3", "four":"4"}' The buffer is normally correct and the print of buffer works. – Arnould Nov 27 '16 at 11:59
  • I quess the Arduino is running when you start the Python program. The python program will therefore open the serial port at some random point in time, maybe in the middle of a sentence being sent to the Raspberry. This way errors might occur when parsing the incomplete JSON strings. – J. P. Petersen Nov 28 '16 at 08:46

1 Answers1

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Correct JSON for your Arduino code: 

Serial.println("{\"one\":\"" + String(one) +
              "\", \"two\":\"" + String(two) +
              "\", \"three\":\"" + String(three) +
              "\", \"four\":\"" + String(four) +
              "\"}");
Arun Vinoth-Precog Tech - MVP
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Hao Le
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