Trying to do serial communication between 2 Arduino Unos.


#include <SoftwareSerial.h>

int ledPin = 13;
int board;   // 1 = Uno. 2 = Mega.
int on_off;  // 1 = On. 0 = Off.
int buzzerPin = 9;
int Ack_tx = 2; //Acknowledgement tx.
int rec;

void setup()
  Serial.begin(9600);      // start serial communication at 9600bps
  Serial1.begin(9600);     // start serial1 communication at 9600bps

  pinMode(ledPin, OUTPUT);
  pinMode(buzzerPin, OUTPUT);


} // End Setup

void loop()

  Serial.println("Enter Uno=1/Mega=0 ...");  //Select board
  //  if(!Serial){ board = Serial.read();}
  while(Serial.available()==0) { // Wait for User to Input Data
  board = Serial.parseInt();  //Read the data the user has input

  Serial.println("Enter LED On=1/Off=0 ...");  //Select On or Off
  while (Serial.available()==0) { // Wait for User to Input Data
  on_off = Serial.parseInt();


  switch (board) {

  case 0:  //Mega board
    if (on_off == 1) digitalWrite(ledPin, HIGH); //Select Uno LED On
    else digitalWrite(ledPin, LOW);              //Select Uno LED Off
    break;  //End Case

  case 1:  //Uno board
    //Serial.println("Off to Uno...");
    //Send on_off info to Uno
    Serial1.write(on_off/256); //Send the Quotient or "how many times" value
    Serial1.write(on_off%256); //Send the Modulo or Remainder.
    delay(50); //Wait for the serial port.

    while(Serial1.available()<2) //Wait for 2 bytes to arrive
      //do nothing
     } //End While

  byte b1=Serial1.read();  //Read Upper byte
  byte b2=Serial1.read();  //Read Lower byte

    if (rec == Ack_tx) tone(buzzerPin, 300, 300); //Ack_tx from Uno
  } //End Switch Case



Arduino: 1.8.5 (Windows 10), Board: "Arduino/Genuino Uno"

C:\Users\Akshit\Documents\Arduino\sketch_mar06a\sketch_mar06a.ino: In function 'void setup()':

sketch_mar06a:13: error: 'Serial1' was not declared in this scope

   Serial1.begin(9600);     // start serial1 communication at 9600bps


C:\Users\Akshit\Documents\Arduino\sketch_mar06a\sketch_mar06a.ino: In function 'void loop()':

sketch_mar06a:51: error: 'Serial1' was not declared in this scope

     Serial1.write(on_off/256); //Send the Quotient or "how many times" value


Multiple libraries were found for "SoftwareSerial.h"
 Used: C:\Program Files (x86)\Arduino\hardware\arduino\avr\libraries\SoftwareSerial
 Not used: C:\Program Files (x86)\Arduino\libraries\SoftwareSerial
exit status 1
'Serial1' was not declared in this scope

This report would have more information with
"Show verbose output during compilation"
option enabled in File -> Preferences.

This is a very basic problem I know, but nothing on the internet is helping.

  • 3
    Why do you think "Serial1" should exist? Commented Mar 6, 2018 at 16:55
  • I picked this code off of the internet. I feel it should exist because we would need one serial for communication with the serial monitor of computer and a second one for the actual serial communication between the two arduinos Pls correct me if I am wrong.
    – Aki
    Commented Mar 6, 2018 at 16:58
  • 1
    So... you saw it in random code and thought it would work everywhere? Commented Mar 6, 2018 at 16:58
  • The code made sense to me. Couldnt think of any changes to make.. Ill try the answer given by Majenko
    – Aki
    Commented Mar 6, 2018 at 17:00
  • @Aki An Uno has only one hardware serial interface (Serial). What do you expect Serial1 to be?
    – gre_gor
    Commented Mar 6, 2018 at 17:14

2 Answers 2


The Uno has no Serial1. It only has one serial port, called Serial. To use Serial1 you will need a bigger board such as a Mega2560, or define it as a SoftwareSerial port and use two other IO pins of your choice for it, though at lower baud rates than Serial can run at.

  • I have arduino micro, and I can use serial (form comunication pc )and serial1 (form com. wifi module). Commented Jan 28, 2019 at 15:22

You import the SoftwareSerial library but you however never instance an object of the SoftwareSerial class.

You just need to add the line

 SoftwareSerial Serial1(10, 11); // RX, TX

after including the library header (taken from here)

This now creates an object Serial1 on which the operations down there should work the same as with a real hardware serial (API-wise).

Side note: Careful with the wiring. You are using this Serial1 emulated UART to communicate between the boards, so if you're using above pin numbers (D10 = RX, D11 =TX) then you must connected D10 (Board1) -> D11 (Board2), D11 (Board1) -> D10 (Board2), i.e. RX->TX, TX->RX.

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