Can't send serial data throught serial pins but it works with the usb port

Question

I am just starting out a project and I am having a little bit of trouble. I want to send data collected from an IR distance sensor, and send it with an XBee Pro S2B to my computer so I can do other stuff with it. The code I am using right now is pretty simple:

void setup()
{
  Serial.begin(9600);
}
void loop(){
 int distance = analogRead(0);
  Serial.println(distance);
  delay(1000);
}

The problem I am having is that when I turn on the arduino with batteries and disconnect the usb cable, the tx light stops turning on and no serial data is sended, AND if I connect it to the computer via USB again it won't work anyway, only works right when I upload the code. The XBees are configured correctly, I have done the chat example on the Building Wireless Sensor Networks by Robert Faludy.

Is it some code error? I must be really obvious. By the way, when I upload the code and the serial data is sended throught usb, the data displayed on Arduino IDE's console is correct, so the sensor is wired correctly.

I am using an Arduino UNO and I have the XBee's and arduino's drivers installed correctly, and for the XBee I am using the XBee Explorer from Sparkfun

Answer 1

I suggest you try to flash the pin 13 LED in your loop(). It seems like the Arduino is not booting correctly on the batteries?

The serial port on the uno is kinda messy, it shouldn't really be shared with the usb port like that. I prefer the Leonardo for these applications - where the USB serial and serial pins are totally independent.

Answer 2

You have an XBee shield on top of the Arduino UNO?

Do you have an XBee attached to the computer?

Can you describe your setup a little more.

The TX Led on the Arduino is only on the USB side, and doesn't signify TX on the XBee. The XBee itself should/could have a TX/RX leds itself. Check the Arduino schematic to confirm this.

How are you reading the Serial on the computer? Thru the Arduino serial monitor? That won't work for the XBee. You'll need another Serial monitor for the XBee attached to the computer. ie, Arduino with XBee shield. Then an XBee connected via some breakout board to the computer. The Arduino Serial monitor won't see the XBee (use hyperterm on windows or some serial port monitor on other systems).

Also, check your switch on the XBee (if you have one), most redirect the USB/Serial connection between computer/Arduino and Arduino/XBee. It's one or the other not both.

Answer 3

Try interchanging the Tx-Rx pins of your XBee-Arduino connection.

Since you are getting correct values on the Arduino IDE, this means that both sensor and serial is working, but it sends values only when connected via USB which means that problem lies in your link between Arduino-Serial Device (in this case XBee)

Answer 4

Ok, I have verified this behavior with a bare Uno on an external power supply driven from Vin.

The serial is outputting, but because of the way the TX and RX LEDs are wired in the Uno, they do not activate unless the USB processor is active.

This is super dumb and super confusing, but you can clearly see it's wired up this way:

I have confirmed that serial data is coming out of the Arduino with a logic analyzer, but the LEDs do not light up.

Answer 5

Finally found out how to get the Arduino Uno serial port to work without a USB cable! I used a scope to see what was going on and found that using an external power supply on the Arduino power port only showed +3.3 volts on the +5 line. I pulled the plug on the external power supply and put a regulated +5 VDC on the +5 VDC pin. Now, both software serial and the UART ports work fine - hope this helps!