Serial Port not receiving data. PLEASE HELP!!

hi all,
i having bit of problem receiving data serial port (pin 0&1). trying communicate external modem (telit). did connect arduino (rx) modem (tx) , arduino (tx) modem (rx).  when connect modem pc, works fine modem responding ok , able send sms. can connect modem pin 0&1 , still use usb write debug messages? , can please point me in correct direction?
thanks in advance!
pieter
some code tried not working.  

code: [select]

#include <liquidcrystal.h>

liquidcrystal lcd(12, 11, 5, 4, 3, 2);

int incomingbyte = 0;
#define inlength 16
#define interminator 13
char instring[inlength+1];
int incount;
int = 0;

void setup(){
 // initialize lcd
 lcd.begin(16, 2);
 lcd.clear();
 lcd.setcursor(0, 0);
 lcd.print("lcd startup");
 // initialize serial port
 serial.begin(115200);
 delay(5000);
 serial.println("at+cmgf=1");
 lcd.setcursor(0, 1);
 lcd.print("initialize done.");
 delay(1000);
}

void readserial(){
     if (serial.available() > 0) {
           instring[incount] = serial.read();
               incount++;
                   }
   if (instring [incount] == interminator) {incount = 0; lcd.clear(); = 0;}

   while (i <= incount){
     lcd.setcursor(i, 0);
     lcd.print(instring[i]);
     i++;
   }
   delay(300);
}

void loop()
{
 serial.println("at");
 delay(300);
 readserial();
}


you can not use serial on pin 0 , 1 same time usb.

did connect arduinos ground modems ground ?

you should!!

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