The following section details the generic MDISDiscreteInstrumentObjectType structure and defines the properties associated with it. Additional sections define a subtype MDISDiscreteOutObjectType that allows updates to the discrete value. This is in general a vendor and operator independent description, but all users of the MDISDiscreteInstrumentObjectType or MDISDiscreteOutObjectType can add vendor specific data. The vendor specific data should be defined as part of a subtype of the MDISDiscreteInstrumentObjectType or MDISDiscreteOutObjectType defined in this document. It is assumed that the subsea system is the Server and host of the instance of the MDISDiscreteInstrumentObjectType or MDISDiscreteOutObjectType. The DCS based system is the Client in the system. It is assumed that all interactions with the instance of the MDISDiscreteInstrumentObjectType are initiated by the Client and are directed to the Server.

The following section details the MDIS generic properties for the MDISDiscreteInstrumentObjectType. Implementation shall ensure adherence to Mandatory [M] aspects in order to comply with the MDIS interface standardisation. Optional [O] may or may not be implemented within a project. Figure 11 provides an overview of the MDISDiscreteInstrumentObjectType as defined by MDIS, including some nested types. This figure includes all items that are inherited from the MDISBaseObjectType.

Figure 11 - MDISDiscreteInstrumentObjectType & MDISDiscreteOutObjectType

Table 7 defines the structure of an MDISDiscreteInstrumentObjectType. Any vendor specified properties that have been implemented within a project should be documented within a similar format and supplied to the DCS vendor. The addition of vendor specific properties will result in a subtype of the MDISDiscreteInstrumentObjectType.

Table 7 - MDISDiscreteInstrumentObjectType

Attribute	Value
BrowseName	MDISDiscreteInstrumentObjectType
IsAbstract	False
References	Node Class	BrowseName	DataType	TypeDefinition	ModellingRule	RW
Subtype of the MDISBaseObjectType (see section 5.1.1)
HasComponent	Variable	State	UInt32	BaseDataVariableType	Mandatory	R
HasSubtype	MDISDiscreteOutObjectType

State – The state of the instance of MDISDiscreteInstrumentObjectType. This state is represented as a UInt32.

Table 8 defines the structure of an MDISDiscreteOutObjectType. Any vendor specified properties that have been implemented within a project should be documented within a similar format and supplied to the DCS vendor. The addition of vendor specific properties will result in a subtype of the MDISDiscreteOutObjectType.

Table 8 - MDISDiscreteOutObjectType

Attribute	Value
BrowseName	MDISDiscreteOutObjectType
IsAbstract	False
References	Node Class	BrowseName	DataType	TypeDefinition	ModellingRule	RW
Subtype of the MDISDiscreteInstrumentObjectType (see section 5.1.2)
HasComponent	Method	WriteValue	See 5.3.5		Mandatory

WriteValue – This Method allows a Client to change the value of State on an instance of MDISDiscreteOutObjectType. The Method will return any errors that occurred on setting the value. The Client shall verify that the State actually changed to confirm the update.

WriteValue Method (defined in Table 9) is used to change the value of the State Variable in an instance of the MDISDiscreteOutObjectType. The WriteValue operation applies to the object in the subsea system. Some systems will be able to report any errors immediately others will only be able to report that the operation was not refused. Clients are expected to monitor the State and ensure that the operation completed. If an error occurs after the Method has returned, a Fault flag shall be set and an appropriate FaultCode will be returned. The Fault (and FaultCode) will reset on the next successful WriteValue Method invocation.

Method Declaration

WriteValue (

[in] State UInt32

);

Table 9 - WriteValue Method parameters

Argument	Description
State	UInt32 value Variable, that indicates the target state of the Variable

Method result codes are defined as part of the Call Service (see OPC UA Services Part 4 – Services specification). They are described in Table 94 for ease of reference.

Comments

The WriteValue Method will change the value of the State Variable. The Method will report if any error occurs while writing the state of the Object. Table 10 specifies the AddressSpace representation for the WriteValue Method.

Table 10 - WriteValue Method AddressSpace Definition

Attribute	Value
BrowseName	State
References		Node Class	BrowseName	DataType	TypeDefinition	Modelling Rule
HasProperty		Variable	InputArguments	Argument[]	PropertyType	Mandatory