Serial communication Arduino

The voltage for Logical "0 "is 0 volt or 5 volt?The voltage for logical "1"?I want make serial communication with mobile phone sony ericsson k700i.My phone Tx is always 2,7 volt and Rx is 0 volt with no command.Any idea?

The DC voltage for a high (1) is 5V.

However, if the pin is transmitting it means the signal is toggling 0 to 5 to 0. On average, this means you would measure a DC voltage of 2.5V. It is not surprising you see 2.7V.

You really an oscilloscope to see the full voltage.

I make serial comunication Arduino Mega328 with mobile phone Sony Ericsson K700i.I test it with serial monitor.Send "AT" and reply "O","K".How i read a sms for control a led on/off?Could reset the pdu mode and use text mode for this phone or support only PDU mode?

Hi i make the code for communication Arduino and Sony Ericsson K700i.....

char incomingByte;
void setup(){

void loop(){
  if (Serial.available()>0){,BYTE;

I have problem with reply sms display pdu only phone number and date ,time no text ....