OPC HDA Item Attributes

• Data Type — Specifies the data type for an item. See the definition of a particular Variant for valid values.

  Comparison of MATLAB and COM Variant Data Types

  MATLAB® Data Type	OPC Server Data Type (COM Variant Type)
  double	VT_R8
  single	VT_R4
  char	VT_BSTR
  logical	VT_BOOL
  uint8	VT_UI1
  uint16	VT_UI2
  uint32	VT_UI4
  uint64	VT_UI8
  int8	VT_I1
  int16	VT_I2
  int32	VT_I4
  int64	VT_I8
  cell	N/A
  struct	N/A
  object	N/A
  N/A	VT_DISPATCH
  N/A	VT_BYREF
  double	VT_EMPTY

• Description — Describes the item.

• Eng Units — Specifies the label to use in displays to define the units for the item (e.g., kg/sec).

• Stepped — Specifies whether data from the history repository should be displayed as interpolated (sloped lines between points) or stepped (vertically-connected horizontal lines between points) data. Value of 0 indicates interpolated.

• Archiving — Indicates whether historian is recording data for this item (0 means no).

• Derive Equation — Specifies the equation to be used by a derived item to calculate its value. This is free-form text.

• Node Name — Specifies the machine which is the source for the item. This is intended to be the broadest category for defining sources. For an OPC Data Access Server source, this is the node name or IP address of the server. For non-OPC sources, the meaning of this field is server-specific.

• Process Name — Specifies the process which is the source for the item. This is intended to the second-broadest category for defining sources. For an OPC DA server, this would be the registered server name. For non-OPC sources, the meaning of this field is server-specific.

• Source Name — Specifies the name of the item on the source. For an OPC DA server, this is the ItemID. For non-OPC sources, the meaning of this field is server-specific.

• Source Type — Specifies what sort of source produces the data for the item. For an OPC DA server, this would be "OPC". For non-OPC sources, the meaning of this field is server-specific.

• Normal Maximum — Specifies the upper limit for the normal value range for the item. It is used for trend display default scaling and exception deviation limit calculations.

• Normal Minimum — Specifies the lower limit for the normal value range for the item. It is used for trend display default scaling and exception deviation limit calculations.

• ItemID — Specifies the item ID.

• Max Time Interval — Specifies the maximum interval between data points in the history repository regardless of their value change. A new value shall be stored in history whenever the specified number of seconds have passed since the last value stored for the item.

• Min Time Interval — Specifies the minimum interval between data points in the history repository regardless of their value change. A new value shall not be stored in history unless the specified number of seconds have passed since the last value stored for the item.

• Exception Deviation — Specifies the minimum amount that the data for the item must change in order for the change to be reported to the history database.

• Exception Dev Type — Specifies whether the exception deviation is given as an absolute value, percent of span, or percent of value. The span is defined as High Entry Limit – Low Entry Limit.

• High Entry Limit — Specifies the highest valid value for the item. A value for the item that is above this limit cannot be entered into history. This is the top of the span.

• Low Entry Limit — Specifies the lowest valid value for the item. A value for the item that is below this limit cannot be entered into history. This is the zero for the span. What follows is a list describing the OPC specified attributes which may be supported by the server.