Serial communication between two arduino unos

Dear members,

I am trying to make two arduino unos (A1 & A2) talk to each other using a simple 5 mtr three core cable. I connect the following:
TxD (A1) → RxD (A2)
RxD (A1) → TxD (A2)
Gnd (A1) → Gnd (A2)

The transmitter sends the pressure data from two pressure sensors after converting each reading to chr array and then concatenated. The code is given as:


// These constants won't change.  They're used to give names
// to the pins used:
int analogInPin0 = A0;  // Analog input pins that the Refernece sensors are attached to
int analogInPin1 = A1;

int sensorValue0 = 0;        // value read from the pot
int sensorValue1 = 0;
int sensorMin = 1023;        // minimum sensor value
int sensorMax = 0;           // maximum sensor value
float pressure0, pressure1;
char outstr1[10];
char outstr2[10];
void setup() {
 pinMode(A0, INPUT);
 pinMode(A1, INPUT);

  // initialize serial communications at 9600 bps:

void loop() {

  sensorValue0 = analogRead(analogInPin0); 
  sensorValue1 = analogRead(analogInPin1);  
  float voltage0 = sensorValue0 * (5.000 / 1023.000);
  float voltage1 = sensorValue1 * (5.000 / 1023.000);
  pressure0 = ((voltage0*12.5)-28.125);
  pressure1 = ((voltage1*12.5)-28.125);
  strcat(outstr1," ");
  strcat(outstr1," ");
  //strcat(outstr1," ");

The receiver is just reading this char array from serial port and converting it into a String object.
The code is:

#include <stdlib.h>

char instr[18] = {'\0'};
void setup()
    Serial.begin(9600);  // Debugging only


void loop()
   int i=0;
   if (Serial.available())
    while(Serial.available()) {
      instr[i++] =;
      String str(instr);
      String s1 = str.substring(0,8);
      String s2 = str.substring(9,18);

My sensor data is getting displayed if I try to print ‘str’ or ‘instr’. But I want to split my incoming string into two to separate between data from different pressure sensors. That’s why the ‘substring’ function is used. But somehow it doesn’t work. If I try to split and print s1’ and/or ‘s2’ individually, then whole incoming string is copied into ‘s1’ and nothing appears on the serial monitor for ‘s2’.

Can somebody indicate to me why ‘substring’ is not working? It would be greatly helpful!! And any additional suggestions to improve this code for serial communication are welcome with a big smile and a thank you!! :slight_smile: :slight_smile:



In the sender, outstr1 is 10 characters long (9 + nul character). Using your strcat, you try to squezze more in there.

And get rid of String (capital S) in the receiver; you can use strchr to split when you e.g add a comma between the two values (in sender).

Below the receiver code that can handle it

char *ptr;

// split on comma
ptr = strchr(intr,','); 
if(ptr == NULL)
  // no comma found;
  // replace space by nul character
  *ptr = '\0';
  // increment pointer to point to next char

Have a look at the examples in Serial Input Basics - simple reliable ways to receive data.

Don't use the String (capital S) class in an Arduino as it can cause problems in the small memory. Use cstrings which are char arrays terminated with a 0.



Both of your replies were really helpful. After some studying and thinking I was able to adapt my code and make it work and much more simple!! :slight_smile: :slight_smile:

Though I just had one question…

Can I also create a virtual serial ports in my ‘Sender’ and receiver sides? If I try the command for e.g.:
mySerial.write (‘outputstring’) - at sender end (rather than Serial.print) (‘incoming string’) - receiver end (rather than

Would this work?

Thank you again for your support!!



No. read and write communicate one character at a time.

may be you could try some thing like below.

void loop(){
   if (mySerial.available()){
     String serialData = mySerial.readString();

//use this function to send some string via SoftwareSerial
//String may not be a good idea.
void send()
   String result="my string";
   char payload[result.length()+1);
   result.toCharArray(payload, sizeof(payload));
   mySerial.write((uint8_t *)payload,sizeof(payload))