Advantech ADAM-5511 Manual page 195

else
printf("\nRS232 port install failed!");
}
RS232CallBackRoutine(unsigned char rddata)
{
//your interrupt program
}
Remarks:
This is a very important library. As you using RS-232 port, please
remember to add it in the end of your program.
modem_command
Syntax:
void modem_command(char *cmdstr)
Description:
Sends an AT command string to the modem. For details, refer to the
AT command document provided by the manufacturer.