simulated operation of the software system (or of some significant portion of the system) to display significant aspects of the behaviour of the system, for instance applied to a requirements specification in an appropriate format or an appropriate high-level representation of the system design

NOTE Animation can give extra confidence that the system meets the real requirements because it improves human recognition of the specified behaviour.

element or a group of elements that independently perform(s) a function

EXAMPLE A two channel (or dual channel) configuration is one with two channels that independently perform the same function.

NOTE 1 The elements within a channel could include input/output modules, a logic system (see 3.4.5), sensors and final elements.

NOTE 2 The term can be used to describe a complete system, or a portion of a system (for example, sensors or final elements)

discipline of identifying the components of an evolving system for the purposes of controlling changes to those components and maintaining continuity and traceability throughout the lifecycle

failure whose probability cannot be expressed as the simple product of the unconditional probabilities of the individual events which caused it

NOTE Two events A and B are dependent, where P(z) is the probability of event z, only if:

P(A and B) > P(A) x P(B)

fractional decrease in the probability of dangerous hardware failure resulting from the operation of the automatic diagnostic tests

NOTE 1 The definition may also be represented in terms of the following equation, where DC is the diagnostic coverage, λDD is the probability of detected dangerous failures and λtotal is the probability of total dangerous failures:

NOTE 2 Diagnostic coverage may exist for the whole or parts of a safety-related system. For example diagnostic coverage may exist for sensors and/or logic system and/or final elements.

NOTE 3 The term safe diagnostic coverage, or diagnostic coverage including safe failures, is used to describe respectively the fractional decrease in the probability of safe hardware failure, or of both safe and dangerous hardware failures, resulting from the operation of the automatic diagnostic tests.

different means of performing a required function

EXAMPLE Diversity may be achieved by different physical methods or different design approaches.

Electrical/electronic/programmable electronic

based on electrical (E) and/or electronic (E) and/or programmable electronic (PE) technology

NOTE The term is intended to cover any and all devices or systems operating on electrical principles.

EXAMPLE Electrical/electronic/programmable electronic devices include:
— electro-mechanical devices (electrical);
— solid state non-programmable electronic devices (electronic);
— electronic devices based on computer technology (programmable electronic)

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

system which responds to input signals from the process and/or from an operator and generates output signals causing the EUC to operate in the desired manner

NOTE The EUC control system includes input devices and final elements