Result from Coolterm VS Arduino Echo

Hi I am a newbie of Arduino. I’ve got a really noob question.
I am trying to use Arduino to get serial data from my device.
From my serial monitor (CoolTerm) I got the following byte array:
15 02 00 12 10 03

By when I used Arduino to echo the serial data, I got this:

Which differs from the result I have from Coolterm

This is the code on Arduino

int byteRead;

void setup() {                
// Turn the Serial Protocol ON
  Serial.begin(115200);
}

void loop() {
   /*  check if data has been sent from the computer: */
  if (Serial.available()) {
    /* read the most recent byte */
    byteRead = Serial.read();
    /*ECHO the value that was read, back to the serial port. */
    Serial.println(byteRead);
    delay(100);
  }
}

Thanks for your help!

byteRead is an integer, it is printed as an integer. The values read by the Arduino are not very stable:

0 0 18 16   6 21
0 0 18 16   2 21
2 0 18 16 102  5
2 0 18 16   6 21

Do you use pin 0 RX for serial input ? That is also the serial input from the computer (on a Arduino Uno). It is possible to override that signal with a stronger signal. Perhaps the serial output of the device is not very strong.

The Arduino Leonardo has a spare hardware serial port. The Arduino Mega 2560 has 3 spare hardware serial ports.