<value> redundant data derived from, and stored or transmitted together with, a block of data in order to detect data corruption

<method> procedure used to calculate the redundant data

Note 1 to entry: Terms “CRC code” and “CRC signature”, and labels such as CRC1, CRC2, may also be used in this document to refer to the redundant data.

[SOURCE: IEC 61784-3:2021, 3.1]

discrepancy between a computed, observed or measured value or condition and the true, specified or theoretically correct value or condition

Note 1 to entry: Errors may be due to design mistakes within hardware/software and/or corrupted information due to electromagnetic interference and/or other effects.

Note 2 to entry: Errors do not necessarily result in a failure or a fault.

[SOURCE: IEC 615084:2010, 3.6.11]

termination of the ability of a functional unit to perform a required function or operation of a functional unit in any way other than as required

Note 1 to entry: Failure may be due to an error (for example, problem with hardware/software design or message disruption).

[SOURCE: IEC 615084:2010, 3.6.4, modified – notes and figures deleted]

abnormal condition that may cause a reduction in, or loss of, the capability of a functional unit to perform a required function

Note 1 to entry: IEV 1910501 defines “fault” as a state characterized by the inability to perform a required function, excluding the inability during preventive maintenance or other planned actions, or due to lack of external resources.

[SOURCE: IEC 615084:2010, 3.6.1, modified – figure reference deleted]

<information theory and communication theory> ordered sequence of characters (usually octets) intended to convey information

[SOURCE: ISO/IEC 2382:2015, 2123205, modified – insertion of "(usually octets)", deletion of notes and source]

discrete level used to specify the ability of safety-related parts of control systems to perform a safety function under foreseeable conditions

[SOURCE: ISO 138491:2023, 3.1.5]

probability of an error undetected by the SCL safety measures

[SOURCE: IEC 61784-3:2021 3.1]

statistical rate at which the SCL safety measures fail to detect errors

[SOURCE: IEC 61784-3:2021, 3.1]

communication layer above the OPC UA communication stack that includes all necessary additional measures to ensure safe transmission of data in accordance with the requirements of IEC 61508.

The SCL provides several services, the most important ones being the SafetyProvider and the SafetyConsumer.

[SOURCE: IEC 61784-3:2021, 3.1 modified]

worst-case elapsed time of a safety function, following an actuation of a safety sensor connected to a fieldbus, until the corresponding safe state of the safety function’s actuator(s) is achieved, in the presence of errors or failures.

Note 1 to entry: This concept is introduced in IEC 617843, 5.2.4 and is addressed by the functional safety communication profiles defined in that specification.

[SOURCE: IEC 61784-3:2021, 3.1 modified]

discrete level (one out of a possible four), corresponding to a range of safety integrity values, where safety integrity level 4 has the highest level of safety integrity and safety integrity level 1 has the lowest level of safety integrity

Note 1 to entry: The target failure measures (see IEC 615084:2010, 3.5.17) for the four safety integrity levels are specified in Tables 2 and 3 of IEC 615081:2010.

Note 2 to entry: Safety integrity levels are used for specifying the safety integrity requirements of the safety functions to be allocated to the E/E/PE safety-related systems.

Note 3 to entry:  A safety integrity level (SIL) is not a property of a system, subsystem, element or component. The correct interpretation of the phrase “SILn safety-related system” (where n is 1, 2, 3 or 4) is that the system is potentially capable of supporting safety functions with a safety integrity level up to n.

[SOURCE: IEC 615084:2010, 3.5.8]

measure to control possible communication errors that is designed and implemented in compliance with the requirements of IEC 61508

Note 1 to entry: In practice, several safety measures are combined to achieve the required safety integrity level.

Note 2 to entry: Communication errors and related safety measures are detailed in IEC 617843, 5.3 and 5.4.

[SOURCE: IEC 61784-3:2021, 3.1]

PDU transferred through the safety communication channel

Note 1 to entry: The SPDU may include more than one copy of the safety data using differing coding structures and hash functions together with explicit parts of additional protections such as a key, a sequence count, or a time stamp mechanism.

Note 2 to entry: Redundant SCLs may provide two different versions of the SPDU for insertion into separate fields of the OPC UA frame.

[SOURCE: IEC 61784-3:2021, 3.1]