sim900 | Golang package for SIM900 GSM Modem | SMS library

I have figured out it finally, It happens because this sim800 only supports TLS 1.0. But it's a deprecated version and many websites and host services have disabled it. You can check your server SSL version from here cdn77.com/tls-test

Just as an introduction, we may say that AT+SAPBR command, as described in SIM900 AT Commands guide, is used to configure and to activate the PDP context (data traffic).

In particular, the meaning of AT+SAPBR=1,1 is

= 1 - Open bearer
= 1 - Bearer profile identifier

From you code

Configure the accepted content type with

Use this command

If you read the SoftwareSerial Library documentation, you will noticed it said:

The library has the following known limitations:

Not all pins on the Mega and Mega 2560 support change interrupts, so only the following can be used for RX: 10, 11, 12, 13, 14, 15, 50, 51, 52, 53, A8 (62), A9 (63), A10 (64), A11 (65), A12 (66), A13 (67), A14 (68), A15 (69).

So change your line on SoftwareSerial SIM900A(6,7); to something else than pin 6 and 7.

It sets the bearer profile ID of the connection.

With these commands

I'm not sure about the underlying implementation of Serial however there may be some kind of an infinite wait for the Serial.println() that is happening.

And reading the documentation on Serial.flush() it appears that it may also be the cause of an infinite wait for the serial output to finish before it returns.

Since Serial.println() seems to use the Serial.write() functionality to do its thing I would suppose that if you have no device reading from the serial port at some point the write buffer is getting full and causing the Serial.println() to block. See https://www.arduino.cc/reference/en/language/functions/communication/serial/write/

Notes and Warnings

As of Arduino IDE 1.0, serial transmission is asynchronous. If there is enough empty space in the transmit buffer, Serial.write() will return before any characters are transmitted over serial. If the transmit buffer is full then Serial.write() will block until there is enough space in the buffer. To avoid blocking calls to Serial.write(), you can first check the amount of free space in the transmit buffer using availableForWrite().

See this explanation of the Serial.flush() function https://www.arduino.cc/reference/en/language/functions/communication/serial/flush/ which notes:

Serial.flush()

Description

Waits for the transmission of outgoing serial data to complete. (Prior to Arduino 1.0, this instead removed any buffered incoming serial data.)

flush() inherits from the Stream utility class.

And see this article, https://www.baldengineer.com/when-do-you-use-the-arduinos-to-use-serial-flush.html which says"

What does Serial.flush() do?

From the Arduino reference for Serial.flush (found on this page):

Waits for the transmission of outgoing serial data to complete.

The key to that statement is “outgoing”. Serial.flush() doesn’t empty the “incoming” buffer as many people think. It pauses your program while the transmit buffer is flushed.

I would use the Serial.availableForWrite() function before doing any output using Serial.println() and if the number of bytes available indicates that the write buffer is getting full then skip the output.

Probably the best approach would be as part of the Setup() function to check the write buffer size after doing the Serial.begin() and store that in a global variable which you then use to check to see if the write buffer is getting emptied or not.

See https://www.instructables.com/id/Arduino-Serial/ which has this to say:

Step 3: Command : ​ AvailableForWrite()

Description

Get the number of bytes (characters) available for writing in the serial buffer without blocking the write operation.

Syntax

Serial.availableForWrite()

See also https://www.arduino.cc/reference/en/language/functions/communication/serial/availableforwrite/

There is also if (Serial) which can be used to check if the port is available. https://www.arduino.cc/reference/en/language/functions/communication/serial/ifserial/ however I suspect that it is more of a check that the requested port is available rather than whether the port is actually functioning with a device on the other end of the link.

And there is also the Serial.available() https://www.arduino.cc/reference/en/language/functions/communication/serial/available/

Serial.available()

Description

Get the number of bytes (characters) available for reading from the serial port. This is data that’s already arrived and stored in the serial receive buffer (which holds 64 bytes).

Serial.available() inherits from the Stream utility class.

Syntax

Serial.available()

Parameters

Serial: serial port object. See the list of available serial ports for each board on the Serial main page.

Returns

The number of bytes available to read.

Suggested course of action

First of all, I don't see that the Serial.flush() in the Setup() function is necessary and it could be safely removed. While this makes the console output immediate during the Setup() it does introduce a wait for the remote case in which there is no device reading from the serial line to empty the output buffer.

I also suggest that for each line you do Serial.println() you do a check on the number of bytes of buffer available with Serial.availableForWrite() first as in:

So it was not working when trying to connect to the cloudMQTT server, but when I connected to my MQTT server or the test MQTT server, it worked. There must be an issue dealing with the protocol that the cloud mqttServer uses (MQIsdp) as apposed to the standard MQTT protocol that a normal MQTT server utilizes. I made a change to the connect packet so that it specified the MQTT protocol and pointed the TCP connection to the test server ( test.mosquitto.org ) and I can now Connect and Publish successfully.

For anyone who is just starting out with trying to interface an Arduino with a module like a SIM900 ( GPRS/GSM module ) to a MQTT server, I suggest you grasp a full understanding of standard AT Commands ( To establish a solid TCP connection to server ), how to compile the packets that will be sent through the GPRS module from the Arduino and lastly I suggest not using any code you have not written to achieve this. Learn how to establish the TCP connection with AT commands, then learn how to compile and send a connect packet by compiling the byte array yourself and sending it. Once you understand the packets and the communication between the client and the server, the MQTT world is your oyster.

You can use this code as a guideline but please try write as much of your own as possible: