Send JSON via POST on SIM900 shield with AT commands

Question

I'm trying to send a JSON POST in SIM900 shield with AT commands. After I call the AT+HTTPACTION=1 command, it returns a 601 error: +HTTPACTION:1,601,0.

I saw here that the error was 601 Network Error. Could someone help me? Following is my code.

void connectInternet() {
  if(digitalRead(9 != LOW)) {
    Serial.print("\r\n");
    Serial.print("------------------------------------------------- \r\n");
    Serial.print(" CONNECT ON INTERNET \r\n");
    Serial.print("------------------------------------------------- \r\n");
    Serial.print("\r\n");
    Serial.println("Connecting...\r");
    cmdAT("AT");
    cmdAT("AT+CIPSHUT");
    cmdAT("AT+CIPMUX=1");
    cmdAT("AT+CSTT=\"zap.vivo.com.br\",\"vivo\",\"vivo\"");
    cmdAT("AT+CIICR");
    cmdAT("AT+CDNSCFG=\"8.8.8.8\",\"8.8.4.4\"");
    Serial.print("IP Device: ");
    cmdAT("AT+CIFSR");
  }
}

void send2Api() {
  Serial.print("\r\n");
  Serial.print("------------------------------------------------- \r\n");
  Serial.print(" SEND TO API\r\n");
  Serial.print("------------------------------------------------- \r\n");
  Serial.print("\r\n");
  cmdAT("AT+CSQ");
  cmdAT("AT+CGATT?");
  cmdAT("AT+SAPBR=2,1");
  // Query if the connection is setup properly, if we get back a IP address then we can proceed
  cmdAT("AT+HTTPPARA=\"CID\",1");
  // Set the HTTP session.
  cmdAT("AT+HTTPPARA=\"URL\",\"http://api.com.br/endpoint\"");
  // set endPoint
  cmdAT("AT+HTTPPARA=\"CONTENT\",\"application/json\"");
  // set content json
  cmdAT("AT+HTTPDATA=100,5000");
  // POST session start
  // JSON payload
  cmdAT("{\"timestamp\":\"2016-03-08T19:36:45.000Z\",\"latitude\":\"-24.9458581\",\"longitude\":\"-53.4968357\"}");
  delay(100);
  cmdAT("AT+HTTPACTION=1");
  // POST session start
  cmdAT("AT+HTTPREAD");
  // Read the data of HTTP server
  cmdAT("AT+HTTPTERM");
  // terminate http
  ShowSerialData();

Answer 1

The PDP context setup is missing from your code. Many years ago I figured out that this initialization sequence works (i.e. extend your connectToInternet):

        //--if autobauding was enabled, send a blank AT
        command:='AT';
        send_gprs;

        //--switch to minimal mode - this is required to reset the SIM card
        command:='AT+CFUN=0';
        send_gprs;

        //--set dce speed
        command:='AT+IPR=57600';
        send_gprs;

        //--turn off command echo
        command:='ATE0';
        send_gprs;

        //--turn off command echo again - sometimes the first command is ignored
        command:='ATE0';
        send_gprs;

        //--turn on extended error messages     
        command:='AT+CMEE=1';
        send_gprs;

        //--set module to full function 
        command:='AT+CFUN=1';
        send_gprs;

Then it is very important to unlock your SIM card. The simplest solution is to disable PIN request (put your SIM to a real phone, and use the phone to set up this.) Verify if the module can proceed on SIM authorization:

        //--check pin 
        command:='AT+CPIN?';
        send_gprs;
        //--right answer is +CPIN: READY
        //--wrong answer is +CME ERROR: 772

Then set up the APN. This will actually initiate an asynchronous APN request, and you shall not proceed until the AT+CSTT returns OK:

    repeat
        //--reset connection
        command:='AT+CIPSHUT';
        send_gprs;

        //--set GPRS APN
        command:='AT+CSTT="internet.vodafone.net"';
        send_gprs;
    until is_ok=1;

And finally, open a GPRS connection:

    repeat
        //--Open gprs connection
        command:='AT+CIICR';
        send_gprs; 

        //--Get local address - for some reason this is required
        command:='AT+CIFSR';
        send_gprs;

        //--Suppress "SEND OK" after at+cipsend
        command:='AT+CIPSPRT=2';
        send_gprs;

    until is_ok=1;

(Ok sorry, this code is pascal and not especially golden, but you got the point about the sequence of the commands.)

Two very important things:

• any command can fail - it is a good idea to send back the response to an AT command to your console window
• AT+CIFSR is evil. If you don't use AT+CIFSR at the right sequence, the communication simply does not work. I was using SIM900D modules. Although you might feel it is optional to get your IP, actually it is not optional at all for some reason.

I was using AT+CIPSEND for the actual data communication. HTTP is so easy, and it is better to be in control.