arduino data stream format

Question

I wounder what is the format of the serial data stream that usually is sent from computer to arduino through serial connection . my project use serial connection with arduino to send data from python(pyserial) to arduino , and when I send a string and I try to print it on LCD it appears but the first character changes to a chinese or japanese character .

so can any body explane the steam format to understand what is the wrong or what happens .

this is the arduino code and python code I used : arduino :

#include <LiquidCrystal.h>
LiquidCrystal lcd(7, 6, 5, 4, 3, 2);

void setup() {
    lcd.begin(16, 2);
    lcd.print("hello, world!");
    Serial.begin(9600);
}

void loop() {
    lcd.setCursor(5, 1);
    char rd[5] ;
    Serial.readBytesUntil('.', rd, 5);
    Serial.flush();
    Serial.println();
    lcd.print(rd);
}

python

import serial
s = serial.Serial('/dev/ttyACM0',buadrate = 9600)
s.write('est.')

update I found through serial monitor that the text is recived correctly but next loop it gives © as a value of rd .

Answer 1

Strings need to be terminated in a '\0' character (typically 0x00) to indicate where the end of the string is.

e.g if you initialise a string like this:

char message[] = "hello";

It will store 'h' 'e' 'l' 'l' 'o' '\0' in the memory.

Then in a routine like 'print', the code loops through the string printing each character until it reaches a '\0' character. If that character is not present it will keep going.

I think what is happening in your code is that the 'rd' byte array does not have a terminating '\0' character. To allow for different length strings try the following:

void loop() {
    lcd.setCursor(5, 1);
    //6 bytes long to include '\0'
    char rd[6];
    //Read up to 5 bytes
    byte bytesReceived = Serial.readBytesUntil('.', rd, 5);
    //Set the next byte to '\0' to terminate the string
    rd[bytesReceived] = '\0';
    Serial.flush();
    Serial.println();
    lcd.print(rd);
}

Answer 2

This is the Python function I use for configuring Arduino serial ports. Give it a try:

def configure_port(port_id):
    ser = serial.Serial()
    ser.port = port_id
    ser.baudrate = 9600
    ser.rtscts = True
    ser.dsrdtr = True
    return ser

Usage:

port = configure_port("/dev/ttyACM0")
port.open()

I think setting rtscts and dsrdtr to true is what worked for me. The above code at least works for Pro Micros; you didn't specify which Arduino you are using, so this is my best guess.

Answer 3

thanks to geometrikal, the problem was : arduino recives a not ended string with \0 , and when the next loop comes it goes to the position (5,1) which is the first character of the printed string on the screen and arduino prints over it the default value of the variable rd if nothing is recived .