Serial.available() doesn't work but Serial.print does

Projects Interfacing w/ Software on the Computer
1

Hi, I have a terribly aggravating problem. When I use this code...

void setup() {
  Serial.begin(115200);
  Serial.println("System Started.");
}

void loop() {
  while (Serial.available() > 0) {
    char a = Serial.read();
    Serial.print(a);
  }
}

while having my Arduino plugged into the PC and open the serial monitor it works fine. It prints back everything that I sent over the serial connection. If I power it externally 5 volts and connect with a USB to serial converter to the hardware serial (pins 0, 1) it does not talk back. It prints the "System Started." but the Serial.available() never goes true if I transmit to it. I have tried it on Mega 2560, Uno, and some China clone Mega board. I have used a USB to serial converter, tried using a UNO board with processor removed as a serial converter, so that part works fine. I have used Arduino IDE on Windows 10 and on Raspian same results.

Thanks in advance.

2 
void setup() {
  Serial.begin(115200);
  Serial.println("System Started.");
}

void loop() {
  if (Serial.available() > 0) {
    char a = Serial.read();
    Serial.print(a);
  }
}
3

I would toggle a LED when you receive data so you can see if you actually receive something. Something in the line of (base on IEEE488's code):

const byte ledPin = LED_BUILTIN;
byte ledState = HIGH;

void setup()
{
  Serial.begin(115200);
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, ledState);
  Serial.println("System Started.");
}

void loop()
{
  if (Serial.available() > 0)
  {
    ledState = !ledState;
    digitalWrite(ledPin, ledState);

    // optional delay for testing
    delay(1000);

    char a = Serial.read();
    Serial.print(a);
  }
}
4

Thanks for the replies.
But has none of you experienced that problem? There is no more explaining needed because my first post said it all.

I would toggle a LED when you receive data so you can see if you actually receive something.

Yes, I have done that and it never turned on.

I will take another shot at it later this week. See what results I get this time.

5

edgar_wideman:
But has none of you experienced that problem?

Probably not. Are you trying to talk to Arduino with the serial monitor connected to hardware serial pins 0,1?