Im working on a project to build an autonomous lego car. Im running into some problems here, and i could use some help. Right now, im struggling to send and receive data from RPI (4) to Arduino (Nano). In my case, the Nano is the slave and RPI the master. I want to simulate the PiCamera until i receive mine, so i tought to use the keyboard like that: If i press "W" key GoForward() If i press "S" key GoBackwards()

The problem is that im not sure how to use bus.write_byte to send data to my arduino, which then set my two motors in any direction

This is my Arduino code #slave:

Some of the comments are in Romanian, ignore them. I translated the useful ones

#include <Wire.h>

//pinii motoarelor atasati la L298N
//motors pins attached to L298N
#define M1_vit 6 //ENA
#define M1_d1  7 //in1
#define M1_d2  8 //in2

#define M2_vit 9 //ENB
#define M2_d2 10  //in4
#define M2_d1 11  //in3

void setup() {
  //intram pe i2c pe adresa 8
  //using the addres 8

  //setam pinii atasati L298N ca OUTPUT
  //motors pins attached to L298N as OUTPUTS
  pinMode(M1_vit, OUTPUT);
  pinMode(M1_d1, OUTPUT);
  pinMode(M1_d2, OUTPUT);
  pinMode(M2_vit, OUTPUT);
  pinMode(M2_d1, OUTPUT);
  pinMode(M2_d2, OUTPUT); 
  //oprim motoarele ca sa evitam orice problema
  //turn off the engines
  digitalWrite(M1_vit, LOW);
  digitalWrite(M2_vit, LOW);
  digitalWrite(M1_d1, LOW);
  digitalWrite(M1_d2, LOW);
  digitalWrite(M2_d1, LOW);
  digitalWrite(M2_d2, LOW);


//functia folosita pentru primirea comenzilor de la RPI
//function used to receive data from RPI
void SemnaleMotoare(int cat)
  while(Wire.available() )//wait for commands
    char c = Wire.read();//catch the command as a char
    if(c == "W")
    else if(c == "S")

void GoForward ()
  digitalWrite(M1_d1, HIGH);
  digitalWrite(M1_d2, LOW);
  digitalWrite(M2_d1, HIGH); 
  digitalWrite(M2_d2, LOW);
  analogWrite(M1_vit, 255);
  analogWrite(M2_vit, 255); 

void GoBackwards()
  digitalWrite(M1_d1, LOW); 
  digitalWrite(M1_d2, HIGH);
  digitalWrite(M2_d1, LOW); 
  digitalWrite(M2_d2, HIGH);
  analogWrite(M1_vit, 255);
  analogWrite(M2_vit, 255);  

void loop() {
 delay(100); //nu face nmc asta, e pus la shto


And the RPI code is here as #master:

addr = 0X8 #adresa busului/bus addres
bus = SMBus(1) #obiect numit bus de tip SMBus indicand /dev/i2c-1

numb = 1

print("Apasa W pentru inainte sau S pentru inapoi")#press W for forward and S for backwards
while numb == 1:
    motoare = input("---- ")#store the user input in motoare

    if (motoare == "W" ):
    elif (motoare == "S"):

        numb = 0;

I would appreciate any help!

  • What is not working as expected?
    – joan
    Commented Feb 26, 2021 at 10:49
  • The motors dont spin. My guess is that i didnt send the command correctly. I dont understand how to send it from RPI to NANO, so it will execute either GoForward(), or GoBackwards() functions
    – Andy
    Commented Feb 26, 2021 at 11:50
  • You need to edit any useful information into your question. You need to print out what is being received by the Arduino.
    – joan
    Commented Feb 26, 2021 at 12:06
  • You long question can be shortened to just one sentence: "What is the I2C command to send one byte?"
    – tlfong01
    Commented Feb 26, 2021 at 12:37
  • Yes, i guess it can be. I wanted to give it some context so everyone who reads it can understand what im trying to do. My problem is that when i want to send data from rpi i dont understand how to use bus.write_byte command
    – Andy
    Commented Feb 26, 2021 at 12:46

1 Answer 1


import smbus2


smbus2.SMBus(1).i2c_rdwr(smbus2.i2c_msg.write([ADDRESS],[[ARRAY OF BYTES]]))

Smbus2 has an i2c read and write feature. For read you would do this:

smbus2.SMBus(1).i2c_rdwr(smbus2.i2c_msg.read([ADDRESS],[NUMBER OF BYTES]))

(smbus2 is a rewrite of smbus, and includes this feature which makes it easier to use with i2c than smbus)

If you don't have the smbus2 module installed, you can install it with

pip install smbus2

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