Arnould Arnould - 14 days ago 10
Python Question

Send multiple values to Raspberry with Arduino using serial

I have got a question about serial communication between Arduino and Raspberry Pi. The fact is I want to send 2 variables to Raspberry Pi with Arduino and use them in different ways.

Here my sketch for Arduino :

int one = 1;
int two = 2;

void setup()
{

Serial.begin(9600);
}

void loop()
{
Serial.print(one);
Serial.print(two);
delay(3000);


}

Here my python script for Raspberry:

import serial
import time

ser = serial.Serial('/dev/ttyACM0', 9600)
time.sleep(4)

while True:
data=ser.read()
print data


The problem is that 'data' in the python code takes the two values send by Arduino (because of two print in the Arduino loop() ). But I want to receive the datas in Raspberry Pi with two different variables (to record them after). I try many technics to receive these datas in two different ways, but it not work.

Thank you for your help.

Answer

Arduino's Serial.print sends data as ASCII. So your commands from the Arduino actually send the string 12. There is no way for Python do see where the first value ends and where the second one begins.

One solution is to use Serial.println instead of Serial.print. This will add a carriage return and a newline after each call. So the string will become 1\r\n2\r\n.

On the Python side you can then use the split method. An example in IPython:

In [1]: recv = "1\r\n2\r\n"

In [2]: recv.split()
Out[2]: ['1', '2']

And you can then easily convert the values to integers.

In [3]: [int(j) for j in recv.split()]
Out[3]: [1, 2]

Note: it is possible for Python to get an incomplete message! So you just might receive the string 1 the first time you read and 2 during a second read! So you might want to consider formatting your data so you know you have received a complete message.

One option to do that is to format your data as JSON. In your example this would be {"one": 1, "two": 2}. In this simple calse you can check that you have received a complete message because it starts with a { and ends with a }. But you could also use Python's built-in JSON parser.

In [4]: data = {'one': 1, 'two': 2}

In [5]: import json

In [6]: json.dumps(data)
Out[6]: '{"two": 2, "one": 1}'

In [7]: recv2 = json.dumps(data)

In [8]: json.loads(recv2)
Out[8]: {'one': 1, 'two': 2}

Using the JSON parser has an advantage because it raises an exception when you try to parse an incomplete message:

In [10]: recv2
Out[10]: '{"two": 2, "one": 1}'

In [11]: recv3 = '{"two": 2, "on'

In [12]: json.loads(recv3)
---------------------------------------------------------------------------
JSONDecodeError                           Traceback (most recent call last)
<ipython-input-12-c1774b41dafa> in <module>()
----> 1 json.loads(recv3)

/usr/local/lib/python3.5/json/__init__.py in loads(s, encoding, cls, object_hook, parse_float, parse_int, parse_constant, object_pairs_hook, **kw)
    317             parse_int is None and parse_float is None and
    318             parse_constant is None and object_pairs_hook is None and not kw):
--> 319         return _default_decoder.decode(s)
    320     if cls is None:
    321         cls = JSONDecoder

/usr/local/lib/python3.5/json/decoder.py in decode(self, s, _w)
    337 
    338         """
--> 339         obj, end = self.raw_decode(s, idx=_w(s, 0).end())
    340         end = _w(s, end).end()
    341         if end != len(s):

/usr/local/lib/python3.5/json/decoder.py in raw_decode(self, s, idx)
    353         """
    354         try:
--> 355             obj, end = self.scan_once(s, idx)
    356         except StopIteration as err:
    357             raise JSONDecodeError("Expecting value", s, err.value) from None

JSONDecodeError: Unterminated string starting at: line 1 column 12 (char 11)

The correct way to deal with this is to keep reading data from the serial port and appending it to a string until parsing the data doesn't fail;

import serial
import json
import time

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

buffer = ''
while True:
    buffer += ser.read()
    try:
        data = json.loads(buffer)
        print(data)
        buffer = ''
    except json.JSONDecodeError:
        time.sleep(1)