UA Part 15: Safety - 7 Safety communication layer services and management

7 Safety communication layer services and management

Figure 10 gives an overview of the safety communication layer and its interfaces. It thereby also shows the scope of the specification of OPC UA Safety in part 15. The main function of the OPC UA Safety layer services is the state machine which handles the protocol. The state machines interact with the following interfaces:

- The Safety Application Program Interface (SAPI) is accessed by the safety application for exchanging safety data during runtime.

- The Safety Parameter Interface (SPI) is accessed during commissioning for setting safety parameters such as IDs or the timeout value in the SafetyConsumer.

- The non-safety related Diagnostics Interface (DI) can be accessed at runtime for troubleshooting the safety communication.

- the OPC UA platform interface (OPC UA PI) connects the SCL to the non-safe OPC UA stack and is used during runtime.

The interfaces (SAPI, SPI, DI and OPC UA PI) described in this clause are abstract and informative. They represent logical data inputs and outputs to this layer that are necessary for the proper operation of the state machine. No normative, concrete mappings are specified. The concrete implementations are vendor-specific and may not exactly match the abstract interfaces described.

Figure 10 – Safety communication layer overview

7.2 OPC UA Platform interface (OPC UA PI)

The state machines of OPC UA Safety are independent from the actual OPC UA services used for data transmission. This is accomplished by introducing a so-called OPC UA Mapper, serving as an interface between the safety communication layer and the OPC UA stack.

The mapper can either make use of OPC UA Client/Server and remote method invocation or the publishing of and subscribing to remote variables as defined in OPC 10000-14. The requirements on the implementation of the mapper are implicitly given in Clause 6 (OPC UA Information Model).

7.3 SafetyProvider interfaces

Figure 11 shows an overview of the SafetyProvider interfaces. The SAPI is specified in Clause 7.3.1, the SPI is specified in Clause 7.3.2.

Figure 11 – SafetyProvider interfaces

7.3.1 SAPI of SafetyProvider

[RQ7.1] The SAPI of the SafetyProvider represents the safety communication layer services of the SafetyProvider. Table 24 lists all inputs and outputs of the SAPI of the SafetyProvider. Each SafetyProvider shall implement the SAPI as shown in Table 24, however, the details are vendor-specific.

Table 24 – SAPI of the SafetyProvider

SAPI Term	Type	I/O	Definition
*SafetyData*	Structure	I	This input is used to accept the user data which is then transmitted as SafetyData in the SPDU. NOTE: Whenever a new MNR is received from a SafetyConsumer, the state machine of the SafetyProvider will read a new value of the SafetyData from its corresponding Safety Application and use it until the next MNR is received. NOTE: If no valid user data is available at the Safety Application, ActivateFSV is expected to be set to “1” by the Safety Application.
*NonSafetyData*	Structure	I	Used to consistently transmit non-safety data values (e.g. diagnostic information) together with safe data, see Clause 8.2.1.10
*EnableTestMode*	Boolean	I	By setting this input to “1” the remote SafetyConsumer is informed (by Bit 2 in ResponseSPDU.Flags, see Clause 6.2.2) that the SafetyData are test data, and are not to be used for safety-related decisions. NOTE: The OPC UA Safety stack is intended for implementation in safety devices exclusively, see Clause 4.2.
*OperatorAckProvider*	Boolean	I	This input is used to implement an operator acknowledgment on the SafetyProvider side. The value will be forwarded to the SafetyConsumer, where it can be used to trigger a return from fail-safe substitute values (FSV) to actual process values (PV), see Annex B.2.4.
*OperatorAckRequested*	Boolean	O	Indicates that an operator acknowledge is requested by the SafetyConsumer. This flag is received within the RequestSPDU.
*ActivateFSV*(Fail-safeSubstituteValues)	Boolean	I	By setting this input to “1” the SafetyConsumer is instructed (via Bit 1 in ResponseSPDU.Flags, see Clause 6.2.2) to deliver FSV instead of PV to the safety application program. NOTE: If the replacement of process values by FSV should be controllable in a more fine-grained way, this can be realized by using qualifiers within the SafetyData, see Clause 5.3.
*SafetyConsumerID*	UInt32	O	This output yields the ConsumerID used in the last access to this SafetyProvider by a SafetyConsumer (see Clause 6.1.3). NOTE: all safety-related checks are executed by OPC UA Safety. The safety application is not required to check this SafetyConsumerID.
*MonitoringNumber*	UInt32	O	This output yields the monitoring number (MNR). It is updated whenever a new request comes in from the SafetyConsumer. NOTE: all safety-related checks are executed by OPC UA Safety. The safety application is not required to check this Monitoring number.
*SafetyProviderID*	UInt32	I	For dynamic systems, this input can be set to a non-zero value. In this case, the SafetyProvider uses this value instead of the value from the SPI parameter SafetyProviderIDConfigured. If the value is changed to “0”, the value of parameter SafetyProviderIDConfigured from the SPI will be used (again).See Figure 11, Clause 3.2, and Clause 11.1.1. For static systems, this input is usually always kept at value “0”.
*SafetyBaseID*	GUID	I	For dynamic systems, this input can be set to a non-zero value. In this case, the SafetyProvider uses this value instead of the value of the SPI parameter SafetyBaseIDConfigured. If the value is changed to “0”, the value of parameter SafetyBaseIDConfigured from the SPI will be used (again).See Figure 11, Clause 3.2, and Clause 11.1.1. For static systems, this input is usually always kept at value “0”.

7.3.2 SPI of SafetyProvider

[RQ7.2] Each SafetyProvider shall implement the parameters and constants [RQ7.3] as shown in Table 25. The parameters (R/W in column “Access”) can be set via the SPI, whereas the constants (R in column “Access”) are read-only. The mechanisms for setting the parameters are vendor-specific. The attempt of setting a parameter to a value outside its range, or of the setting of a read-only parameter, shall not become effective, and a diagnostic message should be shown when appropriate. The values of the constants depend on the way the SafetyProvider is implemented. They never change and are therefore not writable via any of the interfaces.

Table 25 – SPI of the SafetyProvider

Identifier	Type	Range	Initial Value (before configuration)	Access	Note
SafetyProviderIDConfigured	UInt32	0 - 0xFFFFFFFF	0x0	R/W	Provider-ID of the SafetyProvider that is normally used, see Clause 3.2 and Clause 11.1.1. For dynamic systems, the safety application program can overwrite this ID by providing a non-zero value at the input SafetyProviderID of the Safety Provider’s SAPI. This runtime value can be queried using the SafetyProviderIDActive parameter. See note on configured and active values at Table 13. Note: if both the values provided at the SPI and the SAPI are 0x0, this means that the SafetyProvider is not properly configured. SafetyConsumers will never try to communicate with SafetyProviders having a SafetyProviderID of 0x0, see Transitions T13/T27 in Table 30 and the macro <ParametersOK?> in Table 28.
SafetyBaseIDConfigured	GUID	any value which can be represented with sixteen bytes	all sixteenbytes are 0x00	R/W	Base-ID of the SafetyProvider that is normally used, see Clause 3.2. and Clause 11.1.1. For dynamic systems, the safety application program can overwrite this ID by providing a non-zero value at the input SafetyBaseID of the SafetyProvider’s SAPI. This runtime value can be queried using the SafetyBaseIDActive parameter. See note on configured and active values at Table 13. Note: if both the values provided at the SPI and the SAPI are 0x0, this means that the SafetyProvider is not properly configured. SafetyConsumers will never try to communicate with SafetyProviders having a SafetyBaseID of 0x0, see Transitions T13/T27 in Table 30 and the macro <ParametersOK?> in Table 28. See Clause 11.1.1 for more information on GUID.
SafetyProviderLevel	Byte	0x01 - 0x04	n.a.	R	The SIL the SafetyProvider implementation (hardware & software) is capable of, see Figure 12. NOTE: It is independent from the generation of the SafetyData at SAPI. NOTE: the SafetyProviderLevel is used to distinguish devices of a different SIL. As a result, SPDUs coming from a device with a low SIL will never be accepted when a SafetyConsumer is parameterized to implement a safety function with a high SIL.
SafetyStructureSignature	UInt32	0 – 0xFFFFFFFF	0x0	R/W	Signature of the SafetyData structure, for calculation see Clause 8.2.3.4Note: “0” would not be a valid signature and thus indicates a SafetyProvider which is not properly configured. SafetyConsumers will never try to communicate with SafetyProviders having a SafetyStructureSignature of 0x0, see Transitions T13/T27 in Table 30 and the macro <ParametersOK?> in Table 28.
SafetyStructureSignatureVersion	UInt16	0x1	0x1	R/W	Version used to calculate SafetyStructureSignature, see Clause 8.2.3.4
SafetyStructureIdentifier	String	all strings	“” (the empty string)	R/W	Identifier describing the data type of the safety data, see Clause 8.2.3.4.
SafetyProviderDelay	UInt32	0x0 – 0xFFFFFFFF	0x0	R/W	In microseconds (µs). It can be set during the engineering phase of the SafetyProvider or set during online configuration as well. SafetyProviderDelay is the maximum time at the SafetyProvider from receiving the RequestSPDU to start the transmission of ResponseSPDU, see Clause 10.1. The parameter SafetyProviderDelay has no influence on the functional behavior of the SafetyProvider. However, it will be provided in the OPC UA information model of a SafetyProvider to inform about its worst-case delay time. The value can be used during commissioning to check whether the timing behavior of the SafetyProvider is suitable to fulfill the watchdog delay of the corresponding SafetyConsumer. NOTE: This value does not need to be generated in a safety-related way.
SafetyServerImplemented	Boolean	0x0 / 0x1	n.a.	R	This read-only parameter indicates whether the SafetyProvider has implemented the server part of OPC UA Client/Server communication (see Clause 4.3): 1: Server for OPC UA Client/Server communication is implemented. 0: Server for OPC UA Client/Server communication is not implemented.
SafetyPubSubImplemented	Boolean	0x0 / 0x1	n.a.	R	This read-only parameter indicates whether the SafetyProvider has implemented the necessary publishers and subscribers for OPC UA PubSub communication (see Clause 4.3): 1: OPC UA PubSub communication is implemented. 0: OPC UA PubSub communication is not implemented.

Figure 12 – Example combinations of SIL capabilities

The constant SafetyProviderLevel determines the value that is used for SafetyProviderLevel_ID when calculating the SPDU_ID, see Clause 8.2.3.3.

NOTE: SafetyProviderLevel is defined as the SIL the SafetyProvider implementation (hardware & software) is capable of. It should not be confused with the SIL-level of the implemented safety function. For instance, Figure 12 shows a safety function which is implemented using a SIL2-capable sensor, a SIL3-capable PLC, and a SIL1-capable actuator. The overall SIL of the safety function is considered to be SIL1. Nevertheless, the SafetyProvider implemented on the sensor will use the constant value “2” as SafetyProviderLevel, whereas the SafetyProvider implemented on the PLC will use the constant value “3” as SafetyProviderLevel.

The respective SafetyConsumers (on the PLC and the actuator) need to know the SafetyProviderLevel of their providers for being able to check the SPDU_ID (see Clause 8.2.3.2).

7.4 SafetyConsumer interfaces

Figure 13 shows an overview of the SafetyConsumer interfaces. The Safety Application Program Interface (SAPI) is specified in Clause 7.4.1, the Safety Parameter Interface (SPI) is specified in Clause 7.4.3.

Figure 13 – SafetyConsumer interfaces

7.4.1 SAPI of SafetyConsumer

The SAPI of the SafetyConsumer represents the safety communication layer services of the SafetyConsumer. Table 26 lists all inputs and outputs of the SAPI of the SafetyConsumer.

[RQ7.4] Each SafetyConsumer shall implement the SAPI as shown in Table 26, however, the details are vendor-specific.

Table 26 – SAPI of the SafetyConsumer

SAPI Term	Type	I/O	Definition
*SafetyData*	Structure	O	This output either delivers the process values received from the SafetyProvider in the SPDU field SafetyData, or FSV.
*NonSafetyData*	Structure	O	This output delivers the non-safety process values (e.g. diagnostic information) which were sent together with safe data, see Clause 8.2.1.10
*Enable*	Boolean	I	By changing this input to “0” the SafetyConsumer will change each and every variable of the SafetyData to “0”1 and stop sending requests to the SafetyProvider. When changing Enable to “1” the SafetyConsumer will restart safe communication. The variable can be used to delay the start of the OPC UA Safety communication, after power on until “OPC UA connection ready” is set. The delay time is not monitored while enable is set to “0”.
*FSV_Activated*	Boolean	O	This output indicates via “1”, that on the output SafetyData FSV (all binary “0”) are provided. NOTE: If the ResponseSPDU is checked with error: ActivateFSV is set.
OperatorAckConsumer	Boolean	I	For motivation, see Clause 7.4.2. After an indication of OperatorAckRequested this input can be used to signal an operator acknowledgment. By changing this input from “0” to “1” (rising edge) the SafetyConsumer is instructed to switch SafetyData from FSV to PV. OperatorAckConsumer is processed only if this rising edge arrives after OperatorAckRequested was set to “1”, see Figure 20. If a rising edge of OperatorAckConsumer arrives before OperatorAckRequested becomes 1, this rising edge is ignored.
OperatorAckRequested	Boolean	O	This output indicates the request for operator acknowledgment. The bit is set to “1” by the SafetyConsumer, when three conditions are met: Too many communication errors were detected in the past, so the SafetyConsumer decided to switch to fail-safe substitute values. Currently, no communication errors occur, and hence operator acknowledgment is possible. Operator acknowledgment (rising edge at input OperatorAckConsumer) has not yet occurred. The bit is reset to “0” when a rising edge at OperatorAckConsumer is detected.
*OperatorAckProvider*	Boolean	O	This output indicates that an operator acknowledgment has taken place on the SafetyProvider. If operator acknowledgment at the SafetyProvider should be allowed, this output is connected to OperatorAckConsumer, see Annex B.2.4 and B.2.5. NOTE: If the ResponseSPDU is checked with error, this output remains at its last good value.
*TestModeActivated*	Boolean	O	The safety application program is expected to evaluate this output for determining whether the communication partner is in test mode or not. A value of “1” indicates that the communication partner (source of data) is in test mode, e.g., during commissioning. Data coming from a device in test mode may be used for testing but is not intended to be used to control safety-critical processes. A value of “0” represents the “normal” safety-related mode. Motivation: Test mode enables the programmer and commissioner to validate the safety application using test data.NOTE: If the ResponseSPDU check results in an error and the SafetyErrorIntervalTimer (see Clause 7.4.3) is also not expired, TestModeActivated is reset.
*SafetyProviderID*	UInt32	I	For dynamic systems, this input can be set to a non-zero value. In this case, the SafetyConsumer uses this variable instead of the SPI-Parameter SafetyProviderIDConfigured. This input is only read in the first cycle, or when a rising edge occurs at the input Enable. See also Table 27. If it is changed to “0”, the value of SPI parameter SafetyProviderIDConfigured will be used (again). For static systems, this input is usually always kept at value “0”.
*SafetyBaseID*	GUID	I	For dynamic systems, this input can be set to a non-zero value. In this case, the SafetyConsumer uses this variable instead of the SPI-Parameter SafetyBaseIDConfigured. This input is only read in the first cycle, or when a rising edge occurs at the input Enable. See also Table 27. If it is changed to “0”, the SPI-parameter SafetyBaseIDConfigured will become activated. For static systems, this input is usually always kept at value “0”.
*SafetyConsumerID*	UInt32	I	For dynamic systems, this input can be set to a non-zero value. In this case, the SafetyConsumer uses this variable instead of the SPI-Parameter SafetyConsumerID. This input is only read in the first cycle, or when a rising edge occurs at the input Enable. See also Table 27. If it is changed to “0”, the SPI-parameter SafetyConsumerID will become activated. For static systems, this input is usually always kept at value “0”.

7.4.2 Motivation for SAPI Operator Acknowledge (OperatorAckConsumer)

The safety argumentation assumes that random errors in the underlying OPC UA stack including its communication links are not too frequent, i.e., that its failure rate is lower than a given threshold, depending on the desired SIL (see Clause 11.3.1).

Whenever the SafetyConsumer detects a faulty message, it checks whether the assumption is still valid, and switches to fail-safe substitute values otherwise. Returning to process values then requires an operator acknowledgment.

Operator Acknowledge is expected to be initiated by a human operator who is responsible to check the installation, see Table 40, row “Operator Acknowledge”. For this reason, the parameter OperatorAckRequested is delivered by the SafetyConsumer to the safety application. See Clause B.2 for details on operator acknowledgment scenarios.

Timeout errors do only require an operator acknowledgment if operator acknowledgment is required by the safety function itself. In this case, SafetyOperatorAckNecessary is set to indicate that operator acknowledgments are required.

7.4.3 SPI of the SafetyConsumer

[RQ7.5] Each SafetyConsumer shall implement the parameters and constants [RQ7.5b] as shown in Table 27. The parameters (R/W in column “Access”) can be set via the SPI, whereas the constants (R in column “Access”) are read-only. The mechanisms for setting these parameters are vendor-specific. The attempt of setting a parameter to a value outside its range, or of the setting of a read-only parameter, shall not become effective, and a diagnostic message should be shown when appropriate. The SPI of the SafetyConsumer represents the parameters of the safety communication layer management of the SafetyConsumer. The values of the constants depend on the way the SafetyConsumer is implemented. They never change and are therefore not writable via any of the interfaces.

Table 27 – SPI of the SafetyConsumer

Identifier	Type	Valid range	Initial Value (before configuration)	Access	Note
SafetyProviderIDConfigured	UInt32	0x0 - 0xFFFFFFFF	0x0	R/W	The SafetyProviderID of the SafetyProvider this SafetyConsumer normally connects to, see Figure 11 and Clause 3.2. For dynamic systems, the safety application program can overwrite this ID by providing a non-zero value at the input SafetyProviderID of the safety Consumer’s SAPI. This runtime value can be queried using the SafetyProviderIDActive parameter. See note on configured and active values at Table 14.
SafetyBaseIDConfigured	GUID	any value which can be represented with sixteen bytes.	All sixteen bytes are 0x00	R/W	The default SafetyBaseID of the SafetyProvider this SafetyConsumer uses to make a connection, see Clause 3.2. For dynamic systems, the safety application program can overwrite this ID by providing a non-zero value at the input SafetyBaseID of the SafetyConsumer’ s SAPI. This runtime value can be queried using the SafetyBaseIDActive parameter. See note on configured and active values at Table 14. See Clause 11.1.1 for more information on GUID.
SafetyConsumerIDConfigured	UInt32	0x0 - 0xFFFFFFFF	0x0	R/W	ID of the SafetyConsumer, see Clause 11.1.2. For dynamic systems, the safety application program can overwrite this ID by providing a non-zero value at the input SafetyConsumerID of the SafetyConsumer’s SAPI. This runtime value can be queried using the SafetyConsumerIDActive parameter. See note on configured and active values at Table 14.
SafetyProviderLevel	Byte	0x01 - 0x04	0x04	R/W	SafetyConsumer’s expectation on the SIL the SafetyProvider implementation (hardware & software) is capable of. See Clause 3.2, Clause 8.2.3.3, and Figure 12.
SafetyStructureSignature	UInt32	0x0 – 0xFFFFFFFF	0x0	R/W	Signature over the SafetyData structure, see Clause 8.2.3.4
SafetyStructureSignatureVersion	UInt16	0x1	0x1	R/W	Version used to calculate SafetyStructureSignature, see Clause 8.2.3.4 For the SafetyConsumer, this parameter is optional.
SafetyStructureIdentifier	String		“”	R/W	Identifier describing the data type of the safety data, see Clause 8.2.3.4. For the SafetyConsumer, this parameter is optional.
SafetyConsumerTimeOut	UInt32	0x0 – 0xFFFFFFFF	0x0	R/W	Watchdog-time in microseconds (µs). Whenever the SafetyConsumer sends a request to a SafetyProvider, its watchdog timer is set to this value. The expiration of this timer prior to receiving an error-free reply by the SafetyProvider indicates an unacceptable delay. See Clause 10.1
SafetyOperatorAckNecessary	Boolean	0x0 / 0x1	0x1	R/W	This parameter controls whether an operator acknowledgment (OA) is necessary in case of errors of type “unacceptable delay” or “loss”, or when the SafetyProvider has activated FSV (ActivateFSV). 1: FSV are provided at the output SafetyData of the SAPI until OA. 0: PV are provided at SafetyData of the SAPI as soon as the communication is free of errors. In case of ActivateFSV the values change from FSV to PV as soon as ActivateFSV returns to “0”. NOTE: This parameter does not have an influence on the behavior of the SafetyConsumer following the detection of other types of communication errors, such as data corruption or an error detected by the SPDU_ID. For these types of errors, OA is mandatory, see Clause 7.4.2.
SafetyErrorIntervalLimit	UInt16	6, 60, 600	600	R/W	Value in minutes. The parameter SafetyErrorIntervalLimit determines the minimal time interval between two consecutive communication errors so that they do not trigger a switch to FSV in the SafetyConsumer, see Clause 7.4.2. It affects the availability and the PFH/PFDavg of this OPC UA Safety communication link, see Clause 11.4.
SafetyClientImplemented	Boolean	0x0 / 0x1	n.a.	R	This read-only parameter indicates whether the SafetyConsumer has implemented the client part of OPC UA Client/Server communication (see Clause 4.3): 1: Client for OPC UA Client/Server communication is implemented. 0: Client for OPC UA Client/Server communication is not implemented.
SafetyPubSubImplemented	Boolean	0x0 / 0x1	n.a.	R	This read-only parameter indicates whether the SafetyConsumer has implemented the necessary publishers and subscribers for OPC UA PubSub communication (see Clause 4.3): 1: OPC UA PubSub communication is implemented. 0: OPC UA PubSub communication is not implemented.

7.4.4 Motivation for SPI SafetyOperatorAckNecessary

This parameter determines whether automatic restart (i.e., automatically switching back from fail-safe values to process values) is possible for the safety function or not. It is expected to be set to 1 for safety functions where automatic restart is not allowed and restart always requires human interaction.

If automatic restart of the safety function is safe, the parameter can be set to 0.