void serial_input()

This custom function is where I am actually going to read the data from the Nextion HMI into the cmd string until there is no more data to be read.

Then I am going to parse the string data into sections using String() functions to deciphor the commands that where sent. Once it has determined which command has been sent I will turn off or on the requested relay.

void serial_input() {
  while (Serial1.available()) {
    cmd += char(Serial1.read()); // read avaialbe bytes into dfd string
  }
  if (cmd.substring(0, 3) == "R1:") { // does the substring = R1:
    String relay1_cmd = cmd.substring(3, cmd.length()); // read the cmd into relay1_cmd string variable
    if (relay1_cmd == "ON") { // if true then turn on relay 1
      digitalWrite(relay1, HIGH); // set the digital pin high
    } else { // if false then run this statement and set the digital pin low
      digitalWrite(relay1, LOW);
    }
  }
  if (cmd.substring(0, 3) == "R2:") { // does the substring = R2:
    String relay2_cmd = cmd.substring(3, cmd.length()); // read the cmd into relay2_cmd string variable
    if (relay2_cmd == "ON") { // if true then turn on relay 2
      digitalWrite(relay2, HIGH); // set the digital pin high
    } else { // if false then run this statement and set the digital pin low
      digitalWrite(relay2, LOW);
    }
  }
}