Representation of the Item Value with PyOPC Data Types

OPC items have a value, containing the actual information, which corresponds to the value of a fieldbus data point. This value will be of a certain OPC data type, such as string or integer but also more complex types, such as an array. In order to access this information, PyOPC has to map the OPC data type to a corresponding Python data type, resulting in a possible data conversion. Table 1 describes all possible data conversions.

Table 1: Data Conversion Between OPC XML-DA and PyOPC

Currently, the following data types are not supported by the PyOPC framework¹⁰:

• The decimal data type is currently not supported at all and cannot be used.
• The time based data types are not fully supported. Instead of using Pythons datetime module, all data types are converted to the Python time type. The duration type cannot be used. During conversion, fractions of seconds and the time zone is lost.
• The base64Binary type is mapped to a Python string. It may be reasonable to use a corresponding Python type instead, however this is currently not supported.

[OPCXMLDA] further defines arrays, which may contain OPC data types from the above. These arrays are directly mapped to Python lists¹¹ and its elements are converted as shown in Table 1.