Arduino to Arduino Serial Communication Issues

[willj224]

Hello, I'm working on a project with two Arduino UNOs connected via the RX and TX (pins 1 and 0). The sender Arduino is supposed to send a message to the receiver. When the receiver Arduino reads the message, it should turn on an LED. However, the LED isn’t turning on. I’m certain it’s not a hardware issue; the LED and wiring are fine, and I know the receiver is getting the correct message because I connected an I2C OLED and printed the message to it. Below is the code for both the sender and receiver—just wondering if I'm missing something or incorrectly reading the message in the reciever.

Sender:

#include "I2Cdev.h"
#include "MPU6050.h"
#include "RTClib.h"
#include <SPI.h>
#include <SD.h>
#include <Wire.h>

#if I2CDEV_IMPLEMENTATION == I2CDEV_ARDUINO_WIRE
    #include "Wire.h"
#endif

File myFile;
RTC_DS3231 rtc;
MPU6050 accelgyro(0x69); //AD0 Pin gapped
int16_t ax, ay, az;
int16_t gx, gy, gz;
#define OUTPUT_READABLE_ACCELGYRO

char daysOfTheWeek[7][12] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

unsigned long previousMillis = 0;
int previousSecond = -1;

void setup () {
  #if I2CDEV_IMPLEMENTATION == I2CDEV_ARDUINO_WIRE
        Wire.begin();
    #elif I2CDEV_IMPLEMENTATION == I2CDEV_BUILTIN_FASTWIRE
        Fastwire::setup(400, true);
    #endif
  Serial.begin(38400);

  accelgyro.initialize();

#ifndef ESP8266
  while (!Serial); 
#endif

  if (!rtc.begin()) {
    Serial.println("Couldn't find RTC");
    Serial.flush();
    while (1) delay(10);
  }

  if (rtc.lostPower()) {
    Serial.println("RTC lost power, let's set the time!");
    //rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
  }

  SD.begin(10);
  delay(2000);
  myFile = SD.open("data.txt", FILE_WRITE);
  if (!myFile) {
    Serial.println("Error opening file!");
    return;
  }

  myFile.println("Time(ms), Acceleration X, Y, Z, Gyro X, Y, Z");
  myFile.close();
}

void loop () {
  bool mpuStatus = accelgyro.testConnection();
  bool sdStatus = SD.exists("data.txt");
 
  Serial.println("MPU OK");
 

  DateTime now = rtc.now();
  accelgyro.getMotion6(&ax, &ay, &az, &gx, &gy, &gz);


  unsigned long currentMillis = millis();

  
  if (now.second() != previousSecond) {
    previousMillis = currentMillis; 
    previousSecond = now.second(); 
  }

  unsigned long ms = currentMillis - previousMillis; 
  


  // Log data to the SD card
  myFile = SD.open("data.txt", FILE_WRITE);
  if (myFile) {
    myFile.print(now.year(), DEC);
    myFile.print('/');
    myFile.print(now.month(), DEC);
    myFile.print('/');
    myFile.print(now.day(), DEC);
    myFile.print(" ");
    myFile.print(now.hour(), DEC);
    myFile.print(':');
    myFile.print(now.minute(), DEC);
    myFile.print(':');
    myFile.print(now.second(), DEC);
    myFile.print('.');
    myFile.print(ms); 
    myFile.print(", ax:"); 
    myFile.print(ax); 
    myFile.print(", ay:"); 
    myFile.print(ay);
    myFile.print(", az:");  
    myFile.print(az); 
    myFile.print(", gx:"); 
    myFile.print(gx); 
    myFile.print(", gy:"); 
    myFile.print(gy);
    myFile.print(", gz:");  
    myFile.println(gz); 
    myFile.close();
  } else {
    //Serial.println("Error opening file!");
  }

  delay(10); 
}

Reciever:

int mpuLedPin = 5; // LED for MPU6050 status
int sdLedPin = 7;  // LED for SD card status

void setup() {
    Serial.begin(38400);
    pinMode(mpuLedPin, OUTPUT);
    pinMode(sdLedPin, OUTPUT);
    digitalWrite(mpuLedPin, HIGH);
    digitalWrite(sdLedPin, HIGH);   
    delay(1000);
    digitalWrite(mpuLedPin, LOW);
    digitalWrite(sdLedPin, LOW); 
}

void loop() {
    if (Serial.available() > 0) {
        // Light up SD LED briefly whenever data is received
        digitalWrite(sdLedPin, HIGH); 
        delay(100);  
        digitalWrite(sdLedPin, LOW); 
        
        String message = Serial.readStringUntil('\n'); 
        
        if (message == "MPU OK") {
            digitalWrite(mpuLedPin, HIGH); // Turn on LED if MPU is OK
        } else if (message == "MPU FAIL") {
            digitalWrite(mpuLedPin, LOW); // Turn off LED if MPU fails
        }

        //if (message == "SD OK") {
          //  digitalWrite(sdLedPin, HIGH); // Turn on LED if SD is OK
        //} else if (message == "SD FAIL") {
          //  digitalWrite(sdLedPin, LOW); // Turn off LED if SD fails
        //}
    }

[van_der_decken]

From your transmitter:

Serial.println("MPU OK");

From Print.cpp

size_t Print::println(void)
{
  return write("\r\n");
}

From your receiver:

        String message = Serial.readStringUntil('\n'); 
        
        if (message == "MPU OK") {

If you think about what println is appending to your string, and what readStringUntil is stripping from your String, I think the problem will become apparent to you.

[gcjr]

@willj224

I know the receiver is getting the correct message because I connected an I2C OLED and printed the message to it.

there just 1 serial interface on an UNO, primarily used for communication via USB to the PC to download programs and support the serial monitor

how can the Uno support 2 connections to it's RX pins: one from the USB inteface and the other from another UNO

[GolamMostafa]

Try using Software UART Port (Fig-1) insted of (Hardware) UART Port which usually remains engaed with PC for sketch uploading and debugging.

Figure-1: