C.4 Type Description examples
  1. A 128-bit signed integer.

<opc:OpaqueType Name="Int128" LengthInBits="128" ByteOrderSignificant="true">

<opc:Documentation>A 128-bit signed integer.</opc:Documentation>

</opc:OpaqueType>

  1. A 16-bit value divided into several fields.

<opc:StructuredType Name="Quality">

<opc:Documentation>An OPC COM-DA quality value.</opc:Documentation>

<opc:Field Name="LimitBits" TypeName="opc:Bit" Length="2" />

<opc:Field Name="QualityBits" TypeName="opc:Bit" Length="6"/>

<opc:Field Name="VendorBits" TypeName="opc:Byte" />

</opc:StructuredType>

When using bit fields, the least significant bits within a byte shall appear first.

  1. A structured type with optional fields.

<opc:StructuredType Name="MyStructureValue">

<opc:Documentation>A value with an associated timestamp, and quality.</opc:Documentation>

<opc:Field Name="ValueSpecified" TypeName="Bit" />

<opc:Field Name="StatusCodeSpecified" TypeName="Bit" />

<opc:Field Name="TimestampSpecified" TypeName="Bit" />

<opc:Field Name="Reserved1" TypeName="Bit" Length="29" />

<opc:Field Name="Value" TypeName="Variant" SwitchField="ValueSpecified" />

<opc:Field Name="Quality" TypeName="Quality" SwitchField="StatusCodeSpecified" />

<opc:Field Name="Timestamp"

TypeName="opc:DateTime" SwitchField="SourceTimestampSpecified" />

</opc:StructuredType>

It is necessary to explicitly specify any padding bits required to ensure subsequent fields line up on byte boundaries.

  1. An array of integers.

<opc:StructuredType Name="IntegerArray">

<opc:Documentation>An array of integers prefixed by its length.</opc:Documentation>

<opc:Field Name="Size" TypeName="opc:Int32" />

<opc:Field Name="Array" TypeName="opc:Int32" LengthField="Size" />

</opc:StructuredType>

Nothing is encoded for the Array field if the Size field has a value ≤ 0.

  1. An array of integers with a terminator instead of a length prefix.

<opc:StructuredType Name="IntegerArray" DefaultByteOrder="LittleEndian">

<opc:Documentation>An array of integers terminated with a known value.</opc:Documentation>

<opc:Field Name="Value" TypeName="opc:Int16" Terminator="FF7F" />

</opc:StructuredType>

The terminator is 32,767 converted to hexadecimal with LittleEndian byte order.

  1. A simple union.

<opc:StructuredType Name="Variant">

<opc:Documentation>A union of several types.</opc:Documentation>

<opc:Field Name="ArrayLengthSpecified" TypeName="opc:Bit" Length="1"/>

<opc:Field Name="VariantType" TypeName="opc:Bit" Length="7" />

<opc:Field Name="ArrayLength" TypeName="opc:Int32"

SwitchField="ArrayLengthSpecified" />

<opc:Field Name="Int32" TypeName="opc:Int32" LengthField="ArrayLength"

SwitchField="VariantType" SwitchValue="1" />

<opc:Field Name="String" TypeName="opc:String" LengthField="ArrayLength"

SwitchField="VariantType" SwitchValue="2" />

<opc:Field Name="DateTime" TypeName="opc:DateTime" LengthField="ArrayLength"

SwitchField="VariantType" SwitchValue="3" />

</opc:StructuredType>

The ArrayLength field is optional. If it is not present in an encoded value, then the length of all fields with LengthField set to "ArrayLength" have a length of 1.

It is valid for the VariantType field to have a value that has no matching field defined. This simply means all optional fields are not present in the encoded value.

  1. An enumerated type.

<opc:EnumeratedType Name="TrafficLight" LengthInBits="32">

<opc:Documentation>The possible colours for a traffic signal.</opc:Documentation>

<opc:EnumeratedValue Name="Red" Value="4">

<opc:Documentation>Red says stop immediately.</opc:Documentation>

</opc:EnumeratedValue>

<opc:EnumeratedValue Name="Yellow" Value="3">

<opc:Documentation>Yellow says prepare to stop.</opc:Documentation>

</opc:EnumeratedValue>

<opc:EnumeratedValue Name="Green" Value="2">

<opc:Documentation>Green says you may proceed.</opc:Documentation>

</opc:EnumeratedValue>

</opc:EnumeratedType>

The documentation element is used to provide human readable description of the type and values.

  1. A nillable array.

<opc:StructuredTypen Name="NillableArray">

<opc:Documentation>An array where a length of -1 means null.</opc:Documentation>

<opc:Field Name="Length" TypeName="opc:Int32" />

<opc:Field

Name="Int32"

TypeName="opc:Int32"

LengthField="Length"

SwitchField="Length"

SwitchValue="0"

SwitchOperand="GreaterThanOrEqual" />

</opc:StructuredType>

If the length of the array is −1 then the array does not appear in the stream.