How to send data from arduino to Temp Controller for setting temp? I send data from arduino by method following.
Configure the value : byte message[ ];

#include &lt;SoftwareSerial.h&gt;
#define RX 2
#define TX 3
int incomingByte = 0;

SoftwareSerial mySerial(RX, TX);

void setup()
{
  pinMode(5, OUTPUT);

  Serial.begin(9600);
  mySerial.begin(9600);
}

void loop()
{
  incomingByte = Serial.read();

  if (incomingByte == 's')
  {
    digitalWrite(5, HIGH);
    byte message[] = {0x01, 0x06, 0x00, 0x01, 0x02, 0x58, 0xD8, 0x90} ;
    mySerial.write(message1, sizeof(message1));
  }
}


It works. But I want to input from "Serial monitor" to set the temperature.
It does not work when input from "Serial monitor".
You're not writing any data to incomingByte and you're not detecting if data is sent towards the Arduino over serial.

You should change your void loop() into this:

void loop(){

  //Detects if something is sent over serial.
  if(Serial.available() &gt; 0){

    //reads the byte and puts it into the incomingByte variable
    incomingByte = Serial.read();

    //below is your old code
    if (incomingByte == 's') {
      digitalWrite(5, HIGH);
      byte message[] = {0x01, 0x06, 0x00, 0x01, 0x02, 0x58, 0xD8, 0x90} ;
      mySerial.write(message1, sizeof(message1));
    }
  }
}


When you're using the serial monitor to send data towards the Arduino, also make sure the baudrate you use is the same as the Arduino's. In this case you set your baudrate to 9600 Serial.begin(9600).