Access Variables¶

Access variables are defined in IEC 61131-3 specification as a means to allow access to PLC program variables. Variables which are declared under this scope can be accessed within the program as well as by external modules (e.g communication modules). In OpenSCADA, Access variables can be defined in the system specification. The may be referenced inside any POU by declaring the same variable as VAR_EXTERNAL in the PoU definition. They are not statically allocated but they may point to memory locations or to other global variables or to input and output variables of PROGRAMS defined in any CPU.

For illustration purposes we use the idle_plc example. The system_specification.prototxt file included with the example declares many access variables in the var_access block:

var_access {
        name: "__CONFIG_ACCESS__"
        datatype_field {
                field_name: "GLOBAL_BOOL_VAR"
                field_datatype_name: "BOOL"
                field_storage_spec {
                    full_storage_spec: "global_bool_var"
                }
                field_qualifier: READ_ONLY
        }
        datatype_field {
                field_name: "GLOBAL_INT_VAR"
                field_datatype_name: "INT"
                field_storage_spec {
                    full_storage_spec: "global_int_var"
                }
                field_qualifier: READ_WRITE
        }
        datatype_field {
                field_name: "GLOBAL_BOOL_ARR"
                field_datatype_name: "BOOL"
                field_storage_spec {
                    full_storage_spec: "global_bool_arr"
                }
        }
        datatype_field {
                field_name: "GLOBAL_INT_ARR"
                field_datatype_name: "INT"
                field_storage_spec {
                    full_storage_spec: "global_int_arr"
                }
        }
        datatype_field {
                field_name: "ACCESS_INT_ARR"
                field_datatype_name: "INT"
                dimension_1: 2
                dimension_2: 2
                field_storage_spec {
                    full_storage_spec: "%MW100"
                }
        }
        datatype_field {
                field_name: "GLOBAL_COMPLEX_VAR"
                field_datatype_name: "COMPLEX_STRUCT_3"
                field_storage_spec {
                    full_storage_spec: "complex_global_2"
                }
                field_qualifier: READ_WRITE
        }
        datatype_field {
                field_name: "SENSOR_IN_1"
                field_datatype_name: "INT_TYPE_DEF"
                field_storage_spec {
                    full_storage_spec: "CPU_001.PROGRAM_1.sensor_input_1"
                }
                field_qualifier: READ_WRITE
         }

         datatype_field {
                field_name: "SENSOR_IN_2"
                field_datatype_name: "BOOL_TYPE_DEF"
                field_storage_spec {
                    full_storage_spec: "CPU_001.PROGRAM_1.sensor_input_2"
                }
                field_qualifier: READ_WRITE
        }


        datatype_field {
                field_name: "MOTOR_OUT_1"
                field_datatype_name: "INT_TYPE_DEF"
                field_storage_spec {
                    full_storage_spec: "CPU_001.PROGRAM_1.motor_output_1"
                }
                # must be read when pointing to an output variable
                field_qualifier: READ
        }
        datatype_field {
                field_name: "MOTOR_OUT_2"
                field_datatype_name: "BOOL_TYPE_DEF"
                field_storage_spec {
                    full_storage_spec: "CPU_001.PROGRAM_1.motor_output_2"
                }
                # must be read when pointing to an output variable
                field_qualifier: READ
        }
        datatype_field {
                field_name: "SENSOR_IN_3"
                field_datatype_name: "COMPLEX_STRUCT_3"
                field_storage_spec {
                    full_storage_spec: "CPU_001.PROGRAM_1.sensor_input_3"
                }
                field_qualifier: READ_WRITE
        }
        datatype_field {
                field_name: "MOTOR_OUT_3"
                field_datatype_name: "COMPLEX_STRUCT_3"
                field_storage_spec {
                    full_storage_spec: "CPU_001.%QW2000"
                }
                field_qualifier: READ_WRITE
        }
}

In this example 12 Access variables are declared. All of these access variables point to other global variables or memory locations or input/output variables of PROGRAM POUs. Each access variable is also assigned a field qualifier which indicates permissions given to an external module trying to access it. If an access variable only has READ permissions, it cannot be written to by an external module.
The GLOBAL_BOOL_VAR access variable points to the memory location of the global_bool_var global variable (which is also declared in the var_global section in the specification file. Similarly GLOBAL_INT_VAR, GLOBAL_BOOL_ARR and GLOBAL_INT_ARR also point to other global variables.
ACCESS_INT_ARR is a 2 x 2 INT array pointing to RAM memory 100 while MOTOR_OUT_3 is of type COMPLEX_STRUCT_3 and points to output memory byte no 2000 of CPU_001
Access variables can also point to input and output memory locations of POUs of type PROGRAMS. These POUs could be defined in any of the managed CPUs. For instance, in this example, SENSOR_IN_1 and SENSOR_IN_2 point to sensor_input_1 and sensor_input_2 input variables of PROGRAM_1 defined in CPU_001. Similarly, MOTOR_OUT_1 and MOTOR_OUT_2 point to two output variables of the same program.

Access variables can be used to build custom communication modules as we show in next section.