The serial ports an the Mega are TTL level, not RS232Mega schematic; found via the Mega product page
Hello, I want to transfer data from the Arduino to the computer.
So, you don't want to just transfer data from arduino to computer using direct USB connection between computer and arduino, right?Do you specifically want to use RS232 as the middle guy?
DB9 also has a Ground connection, on pin 5. Power is usually not supplied on any pin.If you connect the DBP and leave the power supply off, the Arduino should not have power. Try it.
The Arduino has a built in power switch.Your 7 volt to the power jack takes precedent over the USB 5v from the USB cable.
Maybe the OP is using Vin.Maybe they are not measuring the voltage to the decimal.