CRC
<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, 3.10]
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 60050-192:2024, 192-03-02, modified – notes added]
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, new note to entry added]
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, 2123031, modified – insertion of "(usually octets)", deletion of notes and source]
PL
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, 3.1.35]
SCL
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
Note 1 to entry: The SCL provides several services, the most important ones being the SafetyProvider and the SafetyConsumer.
[SOURCE: IEC 61784-3:2021, 3.1.39 modified – “FAL” replaced by “OPC UA communication stack”, not to entry added]
worst case elapsed time following an actuation of a safety sensor connected to a fieldbus, until the corresponding safe state of its safety actuator(s) is achieved in the presence of errors or failures in the safety function
Note 1 to entry: This concept is introduced in IEC 617843:2021, 5.2.4 and is addressed by the functional safety communication profiles defined in the IEC 61784-3 series of documents.
[SOURCE: IEC 61784-3:2021, 3.1.44]
SIL
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 level1 has the lowest
Note 1 to entry: The target failure measures (see IEC 615084:2010, 3.5.17) for the four safety integrity levels are specified in Table 2 and Table 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 “SIL n 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 617843:2021, 5.3 and 5.4.
[SOURCE: IEC 61784-3:2021, 3.1.46]
SPDU
PDU transferred through the safety communication channel
Note 1 to entry: The SPDU may include more than one copy of the SafetyData 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, 3.1.47]