162 Rockwell Automation Publication ICSTT-RM447M-EN-P - July 2019
Glossary
S
safe state A condition which permitting the execution of a process demand. Usually
entered after the detection of a fault condition, ensuring that the effect of the
fault is to enable rather than disable a process demand.
safety accuracy The accuracy of an analogue signal in which the signal is guaranteed to have no
dangerous faults. If the signal drifts outside this range, it is declared faulty.
safety-critical state A faulted condition preventing the execution of a process demand.
sensor A device (or a combination of devices) that measures a process condition.
Examples are transmitters, transducers, process switches and position switches.
sequential function chart An IEC 61131 language dividing the process cycle into a number of specified
steps separated by transitions. See 'limited variability language'.
SFF Safe Failure Fraction. Given by (the total rate of safe failures plus the rate of
detected dangerous failures) divided by (the total rate of safe failures plus the
rate of detected and undetected dangerous failures).
SIF Safety Instrumented Function. A type of process control performing specified
functions to get (or maintain) a safe state of a process when unacceptable or
dangerous process conditions are detected.
SIL Safety Integrity Level. One of four possible levels, specified in IEC 61508 and
IEC 61511, for specifying the performance requirements of the safety
functions allocated to a safety-related system. SIL4 has the highest level of
safety integrity, SIL 1 has the lowest. For an installation to get a specified SIL
rating, all of the parts of the installation (together with the AADvance
controller) must fulfill these requirements.
SNCP Safety Network Control Protocol. A set of rules that allowing different
AADvance controllers in a system to share data. SNCP is a SIL 3 certified
protocol which provides a safety layer for an Ethernet network.
SNTP Simple Network Time Protocol. A set of rules for synchronizing the real-time
clocks of AADvance controllers and other devices over Ethernet networks.
structured text A high level IEC 61131-3 language with syntax equivalent to that of Pascal.
Useful for making complex procedures that cannot be expressed easily with
graphical languages.
synchronous A data communications term describing a serial transmission protocol. A pre-
arranged number of bits is expected to be sent across a line each second. To
synchronize the sending and receiving machines, a clocking signal is sent by the
transmitting computer. There are no start or stop bits.
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