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A6ModemNew

I have named this library A6ModemNew to differentiate it from my previous offering A6General which is now obsolete.

The difficulty of any modem handling library is how to handle unsolicited messages from the modem and how to correctly define buffer sizes for data and avoid buffer overflow situations.

This library does that. The A6 modem theoretically can work at any baud rate but I have never managed to get it to change from the default of 115200bps. Accordingly I work only via a hardware serial port. SoftwareSerial does not work with this library.

There are 4 main classes on offer:

  • A6GPRS This is the class that handles all UART traffic to and from the modem. This is the only class where you will see AT commands and their responses.
    This class is based on another class, A6GPRSDevice, which performs the low level work of writing to the selected UART and waiting for the desired response. Most users will not need to concern themselves with the A6GPRSDevice class.
  • A6CALL This class offers phone services and SMS services. Calls may be sent or received. SMS messages (text mode only) may be sent or received.
  • A6HTTP This class offers HTTP GET and HTTP POST services
  • A6MQTT This class implements the MQTT protocol (including QOS 0, 1 and 2)
The Users application create one instance of the A6GPRS class and one of the other classes.

Callbacks

Extensive use is made of callback mechanisms whick make handling asynchronous events much simpler. The User does not have to implement all callbacks but may miss important imformation if not. For example the A6Call class has an OnPhoneEvent callback. If you have no need to answer incoming calls or read incoming SMSs the callback need not be implemented.

SerialEvent

The User must implement the appropriate serialEvent function as this is the only way that incoming data from the modem is passed to the library for processing. Example code shows how this is done.

Constructors

A6GPRS

A6GPRS(Stream,CircularBufferSize,LargestItemSize)
Stream may be any hardware serial port, Serial, Serial1, Serial2, Serial3.
Incoming data is saved in a circular buffer of size CircularBufferSize bytes.
Incoming data is parsed into individual items, usually strings ending in CR/LF or blocks of data coming from a TCP server. LargestItemSize as its name implies is the largest item you expect to receive.

A6CALL

A6CALL(A6GPRS instance)

A6HTTP

A6HTTP(A6GPRS instance) The LargestItemSize parameter of the A6GPRS constructor should reflect the largest response you will get from an HTTP GET or POST.

A6MQTT

A6MQTT(A6GPRS instance,KeepAliveTime,maxMessageLength) KeepAliveTime is the value (in seconds) passed to the MQTT broker when connecting.
maxMessageLength is the maximum length of an outgoing message. The buffersize of incoming messages is defined by the LargestItemSize parameter of the A6GPRS constructor.

Exceptions

Buffer overflow conditions e.g. receiving a packet from a TCP server that is larger than LargestItemSize, result in an Exception Callback which informs what went wrong and the size of the buffer needed. That actual data received will be truncated so no actual buffers are overflowed.

Porting to other modems

  • The A6HTTP and A6MQTT classes are pretty much modem agnostic, dealing only with a TCP connection created by the A6GPRS class.
  • The A6CALL class may need to be tailored for the differences in unsolicited messages for Ringing, CallerID and SMS arrivals.
  • The A6GPRS is the only module that uses the modem AT command set so may easily be modified for the differences between modems.

Helper Code

  • The User may choose to implement the debugWrite method of the A6GPRSDevice class. This prints debug messages to whatever device the user may choose.
  • The onException method of the A6GPRSDevice class provides useful information on buffer overflows.
  • The User should implement the HWReset method of the A6GPRSDevice class, to perform hardware reset of the modem.
  • Callback events of the A6CALL class (onDialin, OnPhoneEvent) provide indication of asynchronous events such as SMS arrival, someone dialling in, call answwer and call disconnect.
  • The A6MQTT class has callback methods (onConnect,OnSubscribe,OnMessage,OnPubAck,onPubCompletion) to help manage the progress of connecting to the broker. following the success of QOS2 and QOS3 publication and the arrival of new messages

Open Issues

  • Incoming block from MQTT Broker may contain more than 1 command. At the moment only the first command is processed. This typically happens when starting a new session releases of backlog of retained or uncompleted QOS-1/QOS-2 messages.
  • The standard allows for subscribing to more than 1 topic per packet identifier. Currently I only support 1 topic per packet ID
  • The MQTT example for QOS-1/QOS-2 does not have a strategy for repeating messages that were not acknowledged/completed.
  • Phone support needs phonebook control
  • Autoanswer does not work