User manual MATLAB OPC TOOLBOX 2

[. . . ] OPC ToolboxTM 2 User's Guide How to Contact The MathWorks Web Newsgroup www. mathworks. com/contact_TS. html Technical Support www. mathworks. com comp. soft-sys. matlab suggest@mathworks. com bugs@mathworks. com doc@mathworks. com service@mathworks. com info@mathworks. com Product enhancement suggestions Bug reports Documentation error reports Order status, license renewals, passcodes Sales, pricing, and general information 508-647-7000 (Phone) 508-647-7001 (Fax) The MathWorks, Inc. 3 Apple Hill Drive Natick, MA 01760-2098 For contact information about worldwide offices, see the MathWorks Web site. OPC ToolboxTM User's Guide © COPYRIGHT 2004­2010 by The MathWorks, Inc. The software described in this document is furnished under a license agreement. The software may be used or copied only under the terms of the license agreement. [. . . ] OPC servers require all values to be written to server items in COM Variant format. The server also provides the toolbox with COM Variants when an item's Value property is read or returned by the server. The toolbox automatically converts between the COM Variant type and MATLAB data types according to the table shown below. Table 5-1 Conversion from MATLAB Data Type to COM Variant Data Type OPC Server Data Type (COM Variant Type) VT_R8 VT_R4 MATLAB Data Type double single Remarks 5-18 Working with Different Data Types Table 5-1 Conversion from MATLAB Data Type to COM Variant Data Type (Continued) OPC Server Data Type (COM Variant Type) VT_BSTR VT_BOOL VT_UI1 VT_UI2 VT_UI4 VT_UI8 VT_I1 VT_I2 VT_I4 VT_I8 N/A MATLAB Data Type char logical uint8 uint16 uint32 uint64 int8 int16 int32 int64 function_handle cell struct object Remarks Not allowed Not allowed Not allowed Not allowed Not allowed Not allowed Returns the empty matrix ([]) N/A N/A N/A VT_DISPATCH VT_BYREF VT_EMPTY N/A N/A double 5-19 5 Working with OPC Data Conversion of Values Written to an OPC Server When you write values to the OPC server using the write or writeasync function, you can provide any MATLAB data for the write operation. When you write data to an OPC server, the following data conversions take place: 1 OPC Toolbox software converts the value into the equivalent COM Variant according to Table 5-1. If any disallowed data type is encountered (for example, if you attempt to write a MATLAB structure), an error will be generated. 2 The COM Variant is sent to the OPC server. 3 The OPC server will attempt to convert the COM Variant to the server item's canonical data type, using COM Variant conversion rules. If the conversion fails, the server will return an error. Conversion of Values Read from an OPC Server When an OPC server returns values for a server item to MATLAB, the OPC server will first convert the value to the COM Variant equivalent of the data type specified by the daitem object's DataType property. If the conversion fails, an error message is returned with the value. When OPC Toolbox software receives the value, the COM Variant is converted to the equivalent MATLAB data type according to Table 5-1. Handling Arrays for Item Values The OPC Specification supports arrays of values being written to a server item, and read from a server item. However, a specific server item may not accept an array of values. The behavior of the server in that case is server-dependent. For example, one server may use only the first value of the array. Another server may return an error when attempting to write an array of values to a server item that only supports a scalar value. OPC Toolbox software is not able to determine if a server item accepts only scalar values. For all of the data types listed in Table 5-1 that can be converted between MATLAB and a COM Variant, scalar and array data are permitted by the toolbox. However, the OPC Specification supports only one-dimensional arrays of data. Higher dimension MATLAB arrays are flattened into a one-dimensional vector when writing data to the OPC server. 5-20 6 Using Events and Callbacks You can enhance the power and flexibility of your OPC application by using event callbacks. An event is a specific occurrence that can happen while an OPC Data Access client object (opcda client object) is connected to an OPC server. The toolbox defines a set of events that include starting, stopping, or acquiring records during a logging task, as well as events for asynchronous reads and writes, data changes, and server shutdown notification. When a particular event occurs, the toolbox can execute a function that you specify. Certain events can result in one or more callbacks. [. . . ] "Low EU" Present only for `analog' data. Represents the lowest value likely to be obtained in normal operation. Also used by servers that support non-zero DeadbandPercent values for a group. "High Instrument Range" Represents the highest value that can be returned by the instrument. [. . . ]