Serial Communication stop when Arduino Due powered by battery.

Hello

I have a problem about serial communication between Arduino Dues.

Serial communication is run well when Dues were powered by USB cable from computer.

but when Dues powered by battery or power supply, serial communication does not run. I think it has noise but i don’t know solution.

I used arduino due and Serial1 communication.
Please help me.

You can refer to following link that shows signal from RX1.
http://blog.naver.com/wkdxoghdi/60213366247

and Code is below.
(If you know other good way to communicate float type serial, recommend for me please)
//master code
float A,B,C;
const byte dataCount = 12; // [bytes] (max:32)

union T {byte b[4]; float d;} T;
union X {byte b[4]; float d;} X;
union Y {byte b[4]; float d;} Y;

void setup() {
// put your setup code here, to run once:
Serial.begin(115200);
Serial1.begin(115200); // for TX1, RX1 wire communication
}
void loop() {
// put your main code here, to run repeatedly:
A = 5.5;
B = 5.5;
C = 5.5;
//{ added on 20140409 for TX1, RX1 wire communication
T.d = A;
X.d = B;
Y.d = C;

byte data[dataCount];
byte j;

for (byte i = 0; i < 4; i++)
{
j = i * 3; // dataCount / 4 = 3
data[j] = T.b*;*
_ data[j + 1] = X.b*;_
_ data[j + 2] = Y.b;
}*_

* Serial1.write(data, dataCount);*
* delay(100);*
}

//slave code
* float A,B,C;*
* const byte dataCount = 12; // [bytes] (max:32)*

* union T {byte b[4]; float d;} T;*
* union X {byte b[4]; float d;} X;*
* union Y {byte b[4]; float d;} Y;*
void setup() {
* // put your setup code here, to run once:*
* Serial.begin(115200);*
* Serial1.begin(115200);*
}
void loop() {
* // put your main code here, to run repeatedly:*
if (Serial1.available() >= dataCount)
* {*

* for (byte i = 0; i < 4; i++)*
* {*
_ T.b = Serial1.read();
X.b = Serial1.read();
Y.b = Serial1.read();
* }
//
A = T.d;
B = X.d;
C = Y.d;*_

* Serial.print(A); Serial.print(" , ");//8*
* Serial.print(B); Serial.print(" , ");//8*
* Serial.print(C); Serial.println(" , ");//8*
* delay(50);*
}
}

but when Dues powered by battery or power supply, serial communication does not run

How is the serial data supposed to get TO the PC when there is no USB cable being used?

Use code tags!!

What's connected to Serial1? And how (wiring diagram or photo)?

What kind of battery and where did you connect it to?