Appendix A - Local/Global OPC Options

The OPC XML-DA specification defines various options, which can be global and/or local and are associated with one or more OPC operations. A detailed description of these options can be found in [OPCXMLDA]. However, the specification is somehow complicated and some of these options can not be easily found. Moreover these options will be represented by specific Python data types.

In order to ease the client/server development with PyOPC, table 2 and 3 outline what options are used by which functions. The letters ``G'' and ``L'' indicate if the option is global or local for an OPC operation.

Table 2: OPC Options and Operations

Table 3: OPC Options and Operations

Moreover an alphabetical reference of all available OPC options is given in this appendix, which outlines the functionality of an OPC option and moreover denotes, by which Python data type it is represented in the PyOPC framework.

BrowseFilter

(string) Limits the returned elements during a browse operation. Allowed values are all, branch, item.

ClientItemHandle

(string): An identifier of the item in a request message. The ClientItemHandle is returned along with the requested item.

ClientRequestHandle

(string): An identifier of the client request.

ContinuationPoint

(string): A browse option for specifying secondary browse requests.

DataBufferOverflow

(bool): Indicates if some item changes were lost during subsequent subscription polls

Deadband

(float): The percentage of an item value change which has to be exceeded so that the item ``has changed'', meaning that the item will be sent during the next subscription poll.

DiagnosticInfo

(string): Additional server specific information in case of an error.

Description

(string): A verbose description of an item property

ElementNameFilter

(string): Limits the returned elements during a browse operation

EnableBuffering

(bool): Denotes if changed values should be buffered in case of a subscription

ErrorText

(string): Verbose error description in case of an item error

HasChildren

(bool): Indicates if a browse element has child elements

HoldTime

(datetime): Time to wait until the subscription poll response is sent

InvalidServerSubHandles

(list of strings): A list of invalid ServerSubHandles

IsItem

(bool): Indicates if a browse element is an item

ItemName

(string): The ItemName is part of the namespace of an OPC item. Together with the ItemPath, it forms a unique identification of an item.

ItemPath

(string): The ItemPath is part of the namespace of an OPC item. Together with the ItemName, it forms a unique identification of an item.

LimitField

(string): Transports the limit status of an OPC item and may be one of the following string: none, low, high, constant.

LocaleID

(string): Requested locale for the return message.

MaxAge

(long): Denotes how old the item value may be in milliseconds. MaxAge can be utilized for item caching in the server.

MaxElementsReturned

(long): Maximum amount of returned elements during a browse request.

MoreElements

(bool): Indicates that there are more elements than the returned ones in a browse response message

Name

(string/QName): This option is used as an identifier for a browse element (string type) or a property (QName type).

ProductVersion

(string): A version string of the OPC server

PropertyNames

(list of QNames): A list of item properties that should be returned.

QualityField

(string): The quality of an OPC item value. [OPCXMLDA] specifies a predefined list of allowed qualities, such as ``good'', ``uncertain'' or ``bad''.

RcvTime

(datetime): The time the server received the request.

ReplyTime

(datetime): The time the server returned the response.

RequestDeadline

(datetime): The time until the server response has to be issued.

RequestedSamplingRate

(long): The time in milliseconds in which the server checks for value changes in case of a subscription.

ReqType

(QName): With this options, the client may specify the data type of an OPC item value. The available data types are listed in [OPCXMLDA].

ResultID

(QName): In case of an item error, this option specifies the error type.

ReturnAllItems

(bool): Indicates if the server should return only the changed or all OPC items during a subscription poll

ReturnAllProperties

(bool): Indicates if all item properties should be returned

ReturnDiagnosticInfo

(bool): Return additional server specific information in case of an error

ReturnErrorText

(bool): If True, the OPC server returns a verbose error description in case of an item error.

ReturnItemName

(bool): Indicates whether the ItemName is returned by the server

ReturnItemPath

(bool): Indicates whether the ItemPath is returned by the server

ReturnItemTime

(bool): Indicates whether the Timestamp of an OPC item is returned by the server

ReturnPropertyValues

(bool): Indicates whether all item property values should be included in a response message

ReturnValuesOnReply

(bool): Specifies if the item values should be included in the response message

RevisedLocaleID

(string): In case the requested locale is not implemented by the server, it is revised. The revised locale is sent back to the client by the RevisedLocaleID option.

RevisedSamplingRate

(long): If the OPC server does not the requested subscription sampling rate, it returns a revised rate in this option.

ServerState

(string): The current status of the OPC server, the values may be one of the following: running, failed, noConfig, suspended, test, commFault

ServerSubHandle

(string): An identifier of an OPC subscription

ServerSubHandles

(list of strings): A list of ServerSubHandles.

StartTime

(datetime): The time the OPC server was started

StatusInfo

(string): Provides additional server information.

SubscriptionPingRate

(long): Maximum time in milliseconds between SubscriptionPolledRequest operations. If the ping rate is exceeded, the subscription will be canceled.

SupportedLocaleIDs

(string): String that contains all supported locales by the server

SupportedInterfaceVersions

(string): Supported versions of the OPC XML-DA standard, currently only ``XML_DA_Version_1_0'' is allowed

Timestamp

(datetime): The time when the OPC item value was sampled

Value

(anyType): The value of an OPC item or OPC property

ValueTypeQualifier

(QName): In case the value is date/time based, it will identifiy the exact XML-Schema data type.

VendorField

(long): A numeric value that matches the OPC Vendor Bit Field

VendorFilter

(string): Limits the returned elements during a browse operation

VendorInfo

(string): Vendor specific server information

WaitTime

(long): Time in milliseconds which the server should wait during a pending subscription poll for item value changes