Reassembling Serial data sent from Arduino to Python

Question

I am trying to send six integers at a time through Serial.write() on arduino and send them to python. so far, I have managed to turn the Integers into byte pointers (I think thats the name) and send them over serial. however, when I try to reassemble them on the python side, I get some strange numbers that I did not intend to see.

Arduino Code:

int myInts[6]= {1455, 1446, 6766, 974, 365, 455};

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

  byte *p = (byte)myInts;
  for(byte i = 0; i < sizeof(myInts); i++){
    Serial.write(p[i]);
  }
}

Python Code

import argparse
import numpy
import time
import serial 
import struct

ser = serial.Serial('/dev/ttyACM0', 9600)
while True:
    b = ser.read(4)

    i = struct.unpack('i' ,b)
    print(i)

Output:

(-1090650113,) (285341695,) (10683141,) (-1090650177,) (285341695,) (10683141,)

Thanks!

Vik

TomServo · Accepted Answer · 2021-06-22 21:13:55Z

1

I will offer several alternatives. Easiest one is listed last.

First thing you should realize is that int is a different size on the two platforms. Arduino int is a signed 16-bit int, -32768 -> 32767. Your python int is a 32-bit int. Certainly the numbers being returned are far outside the range of any 16-bit number.

I would recommend changing your int in the Arduino to unsigned int and then change your python unpack format string to H (unsigned short). Or, leave the Arduino as int and change your python unpack format string to 'h' (short) and see if that makes a difference for you. Oh and of course you should change to b = ser.read(2) because these are now all 2-byte integers.

Next, Arduino's Serial.write() method supports a single byte, a string, or a buffer with a length. Not a byte pointer. So while I don't have an Arduino here in front of me to test, I think you should be using the Serial.write(buf,len) form of the command and get rid of the "byte pointers", which I've never seen mentioned in any Arduino documentation:

Serial.write(myInts, 12);

Finally, the easiest thing to do is simply to use Serial.print(str) or Serial.println(str) and send a string across. Then on the python side simply convert the string to an integer type of your choosing.

edited Jun 22, 2021 at 21:13

answered Aug 2, 2017 at 14:51

TomServo

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4 Comments

VikramB Over a year ago

Thank you very much! This was a perfect reply! In the past, I was using the Serial.println() from arduino, but I was getting some weird strings on the python side, such as "/xff" or "/xfe". I also read that Serial.write() is better for sending data, which is why I wanted to use bytes, since I feared that using Strings would cause the problem, like it did with Serial.print() . What do you think?

TomServo Over a year ago

@VikramB I think strings are easiest on Arduino. I'm glade to have helped, and hope you'll click the check mark to "Accept" my answer if I feel my answer is best.

VikramB Over a year ago

I have tried sending a string through the Serial.write by casting the integer I want to send to a string, but I get:"no matching function for call to 'HardwareSerial::write(String&)' "

TomServo Over a year ago

@VikramB Indeed. I misread the documentation: "write() Description Writes binary data to the serial port. This data is sent as a byte or series of bytes; to send the characters representing the digits of a number use the print() function instead. So use print() or println(). Sorry for the confusion.

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