arduino rs232 problems with tc35 gsm modem!!

Forum 2005-2010 (read only) Hardware Interfacing
1

hello,
i have some problems to comunicate arduino with gsm tc35 module...
i use max232 and a serial cable like show the example in:

When i try to comunicate to hyperterminal or another terminal software, the commands are ok!!!!
arduino -> terminal = OK
terminal -> arduino = OK
terminal -> modem = OK
modem -> terminal = OK
arduino -> modem = NOT WORKS (it does not happen anything)

i try with the simple AT command "AT"
the correct string is:

"at\r\n" or in decimal: 65,84,13,10
if i type this into the terminal and send to modem,
the modem respond with a "ok" message

but if
i send this with arduino, the modem not respond...!!!!

what do you think about this????

this is my code to test the connection!!!

char serInString[100];  

int led1 = 2;
int led2 = 13;

void readSerialString (char *strArray) {
    int i = 0;
    if(Serial.available()) {    
       //Serial.print("reading Serial String: ");  //optional: for confirmation
       while (serialAvailable()){            
          strArray[i] = Serial.read();
          
          //if( (strArray[i] == 'K') && (strArray[i-1] == 'O') )
            digitalWrite(led1, HIGH);
          
          i++;
          //Serial.print(strArray[(i-1)]);         //optional: for confirmation
       }
       
       if (strArray[i] != 0) {     
         while(strArray[i] != 0) {
            //Serial.print( strArray[i] );
            strArray[i] = 0;                  // optional: flush the content
            i++;          
         }
       }

       //Serial.println();                          //optional: for confirmation
    }      
}


void printSerialString(char *strArray) {
     int i=0;
     if (strArray[i] != 0) {     
         while(strArray[i] != 0) {
            Serial.print( strArray[i] );
            strArray[i] = 0;                  // optional: flush the content
            i++;          
         }
     }
} 


void setup() {
  
  Serial.begin(9600);


  pinMode(led1, OUTPUT);  // declare LED as output
  pinMode(led2, OUTPUT);  // declare LED as output


}

void loop () {

  readSerialString(serInString);
  
  digitalWrite(led2, HIGH);
  

  Serial.print(65, BYTE); // 'A'  // first way
  Serial.print(84, BYTE); // 'T'
  Serial.print(13, BYTE);
  Serial.print(10, BYTE);
  
  delay(1000);
  Serial.print("at\r\n");  // second way
 
  
  delay(1000);
  digitalWrite(led2, LOW);
  digitalWrite(led1, LOW);
  delay(1000);
  
}

and this is my schematic of the shield!!!

1161×835 164 KB

thanks,

Alex

2

That readSerialString() function looks a bit wrong to me. For example, this loop might never end:

       if (strArray[i] != 0) {      
         while(strArray[i] != 0) {
            //Serial.print( strArray[i] );
            strArray[i] = 0;                  // optional: flush the content
            i++;          
         }
       }

I would put a similar check write in your loop, say after the Serial.print("at\r\n"). Add:

delay(1000);
if (Serial.available())
  digitalWrite(led2, HIGH);

followed by the delay(1000) and then the switching off of the two LEDs. If that doesn't work, maybe the Arduino serial communication is not configured properly, what parity, # of stop bits, etc. does the modem expect?

3

Or, just trying giving the modem a command that will make it do something visible, since it can be hard to debug the responses it gives you without the serial port free to talk to the computer.

4

hello,
the modem works at 9600 or 19200 !!!
not parity and

data: 8
parity: No
stop : 1
debug hardware: No

this is the configuration in the terminal that works...

i see the preference.txt and this is the standard setup of arduino.
This is not the problem...

more sugestions???

thanks

alex

5

Did you try the suggestions I made for changing the code or sending different commands to the modem?

6

thanks,
now it's working!!!
there was a problem with the cable and internal conections...

the modem is DCE and the arduino is DCE !!!!! :slight_smile:
for this reason the two DCE units works ok with computer, because this is a DTE !!!
i needed a null modem converter to cross all the cables!!

alex