Rockwell Automation Publication ICSTT-RM406J-EN-P - February 2021 91
PST Process Safety Time. The process safety time for the equipment under control
(denoted PSTEUC) is the period a dangerous condition can exist before a
hazardous event occurs without a safety system as a protection.
R
real A class of analogue variable stored in a floating, single-precision 32-bit format.
redundancy The use of two or more devices, each carrying out the same function, to
improve reliability or availability.
resolution The smallest interval measurable by an instrument; the level of detail which
may be represented. For example, 12 bits can distinguish between 4096 values.
RS-232-C, RS-422, RS-485 Standard interfaces introduced by the Electronic Industries Alliance covering
the electrical connection between data communication equipment. RS-232-C
is the most commonly used interface; RS-422 and RS-485 allow for higher
transmission rates over increased distances.
RTC Real-time clock.
RTU Remote terminal unit. The MODBUS protocol supported by the AADvance
controller for MODBUS communications over serial links, with the ability to
multi-drop to multiple slave devices.
S
safe state A state which enables the execution of a process demand. Usually entered after
the detection of a fault condition; it makes sure the effect of the fault is to
enable rather than disable a process demand.
safety accuracy The accuracy of a signal within which the signal is guaranteed to be free of
dangerous faults. If the signal drifts outside of this range, it is declared faulty.
safety-critical state A faulted state which prevents the execution of a process demand.
sensor A device or combination of devices that measure a process condition.
Examples are transmitters, transducers, process switches and position
switches.
sequential function chart An IEC 61131 language that divides the process cycle into a number of well-
defined steps separated by transitions. See 'limited variability language'.
SFF Safe Failure Fraction. Given by (the sum of the rate of safe failures plus the rate
of detected dangerous failures) divided by (the sum of the rate of safe failures
plus the rate of detected and undetected dangerous failures).
SIF Safety Instrumented Function. A form of process control that performs
specified functions to achieve or maintain a safe state of a process when
unacceptable or dangerous process conditions are detected.
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