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Glossary
Probability of a dangerous failure on
The average probability of a dangerous failure on demand.
demand (PFD)
Probability of dangerous failure per
The average frequency of a dangerous failure per hour.
hour (PFH)
recoverable fault A fault, which when properly handled by implementing the fault handling
mechanisms that are provided by the controller, does not force user logic
execution to be ended.
requested packet interval (RPI) How frequently the originating application requires the transmission of data
from the target application.
routine A set of logic instructions in one programming language, such as a ladder
diagram. Routines provide executable code for the project in a controller. Each
program has a main routine. You can also specify optional routines.
safe failure fraction (SFF) The sum of safe failures plus the sum of dangerous detected failures divided by
the sum of all failures.
safety Add-On Instruction An Add-On Instruction that can use safety application instructions. In
addition to the instruction signature used for high-integrity Add-On
Instructions, safety Add-On Instructions feature a SIL 2 or SIL 3 safety
instruction signature for use in safety-related functions.
safety application instructions Safety Instructions that provide safety-related functionality. They have been
certified to SIL 2 or SIL 3 for use in safety routines.
safety component Any object, task, program, routine, tag, or module that is marked as a safety-
related item.
safety input A combination of produced and consumed safety tags, mapped safety inputs,
and inputs from safety modules.
safety instruction signature The safety instruction signature is an ID number that identifies the execution
characteristics of the safety Add-On Instruction. The signature is used to verify
the integrity of the safety Add-On Instruction during downloads to the controller.
safety integrity level (SIL) A relative level of risk-reduction that is provided by a safety function, or to
specify a target level of risk reduction.
safety I/O Safety I/O has most of the attributes of standard I/O except it features
mechanisms that are certified to SIL 2 or SIL 3 for data integrity.
safety network number (SNN) Uniquely identifies a network across all networks in the safety system. You are
responsible for assigning a unique number for each safety network or safety
subnet within a system. The safety network number constitutes part of the
Unique Node Identifier (UNID).
safety partner The processor in a dual-processor controller that works with the primary
controller to perform safety-related functions in a SIL 3 system.
105
Rockwell Automation Publication 1756-RM012B-EN-P - April 2018
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