Rockwell Automation AADvance Controller Troubleshooting and Maintenance Manual - Manual (Page 92)

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8-8 Document: 553634
(ICSTT-RM406F_EN_P) Issue: 089:
Troubleshooting and Maintenance Manual (AADvance
Controller)
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 an analogue 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.
SIL
Safety Integrity Level. One of four possible
discrete levels, defined in IEC 61508 and IEC
61511, for specifying the safety integrity
requirements of the safety functions to be
allocated to a safety-related system. SIL4 has
the highest level of safety integrity; SIL1 has
the lowest.
The whole of an installation (of which the
AADvance system forms a part) must meet
these requirements in order to achieve an
overall SIL rating.