Only two devices can use the RS232 link at a time. If you connect the Arduino to the PC via the serial terminal, then you cannot use the serial port on pins 0&1 for the GSM modem. You need to setup an soft-serial port for the modem.
//Per.
Only two devices can use the RS232 link at a time. If you connect the Arduino to the PC via the serial terminal, then you cannot use the serial port on pins 0&1 for the GSM modem. You need to setup an soft-serial port for the modem.
//Per.