General Provisions - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 17

universal computers, low operating speed, signifi-
cant labor expenditures in design and debugging of
application software, high probability of “hidden”
defects, programming errors and impossibility of
comprehensive testing of application software by
the developer did not allow using such computers
in NPP safety systems.
Significant extension of a set of software
performed functions for NPP units was provided,
first, by element base development—appearance
of available for use microprocessors, single-chip
microcomputer and microprocessor controllers,
including one or several microprocessors to-
gether with the relevant memory, for which IEC,
2008 suggested a more general title program-
mable electronic. (In this standard control, protec-
tion and monitoring system, based on one or
several programmable electronic devices, is called
a programmable electronic system).
It became possible to perform functional de-
composition- division of complex function into a
set of significantly simpler ones: the calculating
possibilities and storage capacity of appeared at
that time programmable electronic devices. For
examples, functions of input and input signals
transformation; comparison with set-points; func-
tions of logic processing and generation of control
commands; functions of continuous monitoring
of technical state, etc. could be assigned. Along
with functional decomposition, structural decom-
position, providing participation in performance
of one function of several sequentially or parallel
connected programmable electronic devices, was
used. For example, input and transformation func-
tions in case of large number of input signals can
be performed not by one, but by several simultane-
ously operating devices, among which all input
signals are distributed; diagnostics function of
technical state of system parts can be performed
under control of software, directly containing
in each of these parts; results of check can be
transferred for further processing, display and
archiving by specifically assigned programmable
electronic devices, etc.

Such an approach for building program-
mable electronic system, called “distributed” or
“decentralized” control, allowed refusing from
central computes and replacing them with a set
of programmable electronic devices, distributed
throughout the system. It is considered that such
a device individually has low complexity in the
sense of that all types of failures of each of its
components are clearly determined, and the de-
vice's behavior, in case of defects, is fully deter-
mined. The simplicity and relative independence
of performed functions cardinally simplified the
design of software for programmable electronic
devices of low complexity and allowed to refusing
from use of operation system, software interrupts,
drivers, etc. Important factors are lower probabil-
ity of errors during design and relatively simple
determination of defects in the process of software
testing (before supplying to the customer). So,
typical software features of modern I&C systems
are: orientation at many programmable electronic
devices, included in a system; storage of execut-
able programs directly in the memory of these
devices; elimination of the differences between
a system (operating) and application software;
combination of hardware and software design
for these devices, simultaneously performed by
one and the same organization; integration of
hardware and software directly on manufactur-
ing place and supply of programmable electronic
devices with “built-in” software. Distribution of
functions between hardware and software made
by developers is often not reflected in operating
documentation, i.e. is left unknown to the user.
This software can be changed only by replacing
the device, containing previous software, with a
new one with modified software (IEC, 2007,b).
As a result of the specified peculiarities,
it became possible to implement software for
implementation of main functions of I&C systems
important to NPP safety. In the second half of
the eighties programmable electronic systems,
developed by Electricite de France, performed
not only information and support functions,