Extending Communication Modules¶

Custom communication modules can be built with the CommModule class defined in comm_module.h.

An object of the CommModule class can get pointers to Input/Output memory of any managed CPU or the PLC’s RAM memory using its methods:

uint8_t * GetPtrToInputMemory(string ResourceName)
uint8_t * GetPtrToOutputMemory(string ResourceName)
uint8_t * GetPtrToRAMMemory()

Additional methods also return the size of each memory component:

int GetRAMMemSize();
int GetInputMemSize(string ResourceName)
int GetOutputMemSize(string ResourceName);

The CommModule class also allows controlled access to ACCESS variables which are defined in the system specification:
```
std::unique_ptr<PCVariableContainer>
        GetVariableContainer(string NestedAccessPath)
```

An access variable name can be specified as an input and it would return an object of type PCVariableContainer which allows controlled access to the access variable based on its specified qualifier (READ/READ_WRITE).

An example is illustrated in examples/idle_plc/comm_module.cc. A new communication module object is defined over the system_specification present in the example. The file describes in detail how the communication_module object can read/write to access variables and memory locations. Interested reader is encouraged to refer to the file for further clarifications.

The CommModule class thus provides a useful interface which can be used to build custom communication modules which can use a variety of SCADA protocols. The CommModule interface is used by the sample modus implementation (defined here) which is shipped with OpenSCADA.