Electronic – Arduino: Only reads serial input when Serial Monitor is open. Fails when it’s closed

While I haven't use a hardware simulator for a while, it think what s happen is something like this.

Each clock cycle of the AVR is simulated which takes multiple clock cycles on the host PC. Hence time on the simulated AVR is running slower than real time. This could explain why the delay loop would appear to hang the AVR simulator.

A similar scenario could be the case with the serial port.

Personally, I'm not sure if the simulating modern processors with flash memory is productive as it's quicker to just load the program into the system and "let it run" to see what happens.

Strings need to be terminated in a '\0' character (typically 0x00) to indicate where the end of the string is.

e.g if you initialise a string like this:

char message[] = "hello";

It will store 'h' 'e' 'l' 'l' 'o' '\0' in the memory.

Then in a routine like 'print', the code loops through the string printing each character until it reaches a '\0' character. If that character is not present it will keep going.

I think what is happening in your code is that the 'rd' byte array does not have a terminating '\0' character. To allow for different length strings try the following:

void loop() {
    lcd.setCursor(5, 1);
    //6 bytes long to include '\0'
    char rd[6];
    //Read up to 5 bytes
    byte bytesReceived = Serial.readBytesUntil('.', rd, 5);
    //Set the next byte to '\0' to terminate the string
    rd[bytesReceived] = '\0';
    Serial.flush();
    Serial.println();
    lcd.print(rd);
}