Testability and Diagnosability Terminology
Group – A collection of functions or failure modes for which
diagnostics can detect a fault and can isolate the fault to that collection,
yet cannot further isolate the fault to any subset of the collection.
Self-Test –Self-test to that degree of fault detection and
isolation which can be achieved entirely under computer control, without human
Test – That performance assessment, fault detection, diagnosis,
isolation, and prognosis which is performed with a minimum of reliance on human
Failure – A failure whose severity is slight enough to be
outweighed by the advantages to be gained by normal use of the system.
– A system or component whose inputs, outputs, and general function are known
but whose contents or implementation are unknown or irrelevant. (See also white
box, grey box, and glass box).
Scan – A technique of testing dense electronics circuits
created by the Joint Test Action Group (JTAG), whose name is often used
synonymously with Boundary Scan, that creates "virtual test-points" on a board
that does not have test point access physically built in. Boundary Scan was
adopted as standard IEEE 1149.1.
Self-Test (BIST) – Tiny tester models that allow an integrated
circuit to test itself.
Test (BIT) – An internal automatic or
semiautomatic feature in a system or subsystem designed to detect, diagnose or
isolate system failures by interrogating a system or monitoring system
Test Equipment (BITE) – Any device which is part of an equipment
or system and is used for the express purpose of testing the equipment or
system. (MIL-STD-1388-1). Hardware which is identifiable as performing the
built-In test function; a subset of BIT (MIL-STD-2165).
– An event or mechanism that can cause two or more failures or failure
indications to occur simultaneously.
Isolation – Also called Single Fault Isolation. A method of
fault isolation that proceeds under the assumption that all "failed" tests in a
diagnostic session are due to a single malfunction or fault. Although
frequently used as the basis for diagnostic analyses (such as testability)
during the design phase, c ommon cause isolation cannot consistency produce
accurate results when multiple components are simultaneously malfunctioning
during diagnosis. The degree to which diagnostic accuracy suffers depends on
the likelihood of multiple failures (taking into consideration the possibility
of dependent failures and the length of the interval between diagnostic
sessions) and the extent to which single failures can produce the same test
signature as multiple failures.(Compare with Multiple Failure Isolation).
Failure – A failure of apparently independent components or
communication links due to an initiating event that affects them all.
Test (DFT) – The practice of adding hardware hooks to
integrated circuits in order to facilitate effective, inexpensive testing.
Design for Testability – The aspects of the product design process
whose goal is to ensure that the testability of the end product is competently
and sufficiently developed.
Mechanism – The means of methods by which a failure can be
discovered by an operator under normal system operation or can be discovered by
the maintenance crew by some diagnostic action.
The process of identifying the nature or cause of a phenomenon.
Accuracy – A measure of the ability of a diagnostic
strategy or procedure to detect and isolate failures correctly across all of
the potential fault combinations for a given system, device or process.
Diagnostic Accuracy—which takes into consideration the possibility of
simultaneously malfunctioning components—is expressed as the
probability-weighted ratio of fault combinations for which all faults can
be detected and isolated to fault groups that contain them, to the total number
of possible fault combinations. (Compare with Isolation Accuracy)
Ambiguity – The situation that arises when diagnostics are
able to detect a fault, but the smallest fault group to which that fault can be
isolated contains more than one repair item.
Coverage – The ratio of failures detected (by a given test
program, test procedure, or set of tests) to the entire
theoretically-detectable failure population, expressed as a
probability-weighted percentage. Also called Fault Coverage or Fault Detection
Diagnostic Engineering – The engineering discipline through which the
diagnostic capability of a system or device is developed, assessed and
Health Monitoring – A method of performing diagnostics in which
status data (sensor readings, test results, etc.) gathered from a health
management system (e.g., IVHM) is used as a batch to "seed" a dynamic
diagnostic session, to automatically respond to tests in a static diagnostic
procedure, or to develop a customized diagnostic procedure to be applied to the
given fault combination.
Inference Engine – An inference engine that draws diagnostic
Procedure – A structured combination of tasks, tests,
observations, and other information used to localize a fault or faults.
Reasoner – A reasoning system used to perform diagnostics.
Session – The single application of a diagnostic procedure or
diagnostic reasoner to diagnose a fault. (See also iterative diagnostics).
Strategy – The computational logic associated with making
automated diagnostic assessments from a set of observations or data.
– The process of correlating the results of multiple tests to determine
overall system status and generating hypotheses for maintenance
Diagnostics– Diagnostics in which the order of testing is
determined during run time based on the current mission objectives,
component MTTFs, and available resources (test equipment, personnel). Fault
groups are also developed dynamically, with the diagnostics interpreting each
test as it is performed. Although dynamic diagnostics offer great flexibility
and efficiency, they can only be verified inductively. (Compare with Static
Diagnostics and Semi-Dynamic Diagnostics).
Diagnostics – Any portion of the system's
diagnostic capability that is an integral part of the prime system or support
– Any portion of the entire system's diagnostic capability
that is not embedded.
– The loss of ability of a system, device or process to perform a required
function. The manifestation of a fault. (See also Hardware Failure and Software
Latency – The elapsed time between
fault occurrence and failure indication. (MIL-STD-2165)
– The characteristic manner in which a failure occurs. Within a failure mode
diagnostic model, failure modes represent specific ways in which a system,
device or process can fail. (Also see Object Failure Mode).
– A warning reported by a diagnostic or health management system indicating the
existence of an operational fault when that fault does not exist. False alarms
can be reduced or sometimes completely eliminated by a full and accurate
diagnostic analysis and validated run-time diagnostics / health management
Removal – The removal of a good part suspected as being defective
due to inconclusive diagnostics (e.g., diagnostic ambiguity), inaccurate
diagnostic information, inefficient IETM information, or inadequate maintainer
training. False removals contribute to high spares consumption, high
turn-around times, low operational availability, and high RTOK rates.
– The set of faults that exist at the time of a diagnostic session.
The ratio of failures detected (by a given test program, test procedure,
or set of tests) to the entire theoretically-detectable failure population,
expressed as a probability-weighted percentage. Also called Diagnostic Coverage
or Fault Detection Coverage.
Detection – The process of identifying and reporting the presence
of one or more faults within a system, device or process.
Coverage – The ratio of failures detected (by a given test
program, test procedure, or set of tests) to the entire
theoretically-detectable failure population, expressed as a
probability-weighted percentage. Also called Diagnostic Coverage or Fault
Detection (FD) Metrics – Metrics that quantify
the likelihood of a fault being detected, including Fault Coverage.
Detection, Isolation and Remediation (FDIR) Metrics – Metrics that
quantify the likelihood that a fault will be detected, isolated and remediated.
– A set of suspected faults (in terms of functions or failure modes) at
some stage of a diagnostic session. (See also Isolated Fault Group)
Injection – A technique whereby the
effectiveness of tests and diagnostics can be assessed by creating a simulated
fault in a design and seeing the effect of that simulated fault on circuit
outputs, test results, and diagnostic conclusions.
Isolation – The process of determining the location of a fault to
the extent necessary to effect repair. (MIL-STD-721C)
Isolation (FI) Metrics – Metrics that quantify the
likelihood of a fault being isolated to a single component.
Localization – The process of determining
the approximate location of a fault.(MIL-STD-721C)
Resolution – The process of resolving a fault, taking into account
both the diagnostics (testing) and maintenance philosophy (replacement).
Resolution (FR) Metrics – Metrics that quantify the
likelihood of a fault being resolved with a single replacement.
Testing – A testing methodology that focuses on the
expected functionality of the item being tested (as opposed to focusing on
failures reported by BIT). Also known as behavioral testing.
– A system or component whose purpose or function is fully disclosed, yet for
which implementation details are not available. Offers the diagnostic
visibility of a white box, with the design protection of a black box. (See also
grey box). In some arenas, the term glass box is used synonymously with white
– A system or component for which some, but not all, knowledge of how it
behaves (beyond the interface) has been disclosed. A grey box stands in
the middle ground between black boxes and white boxes. (See also glass
Failure – The inability of the equipment to perform its expected
functions due to a condition caused by operational, maintenance, physical or
other environments. (Compare with Software Failure).
Management (HM)– The capability of monitoring real-time sensors to
determine the health and performance of a system, subsystem, device or process.
It may or may not be hosted on the system being monitored.
Diagnostic Inference – An inference about a
function based on its relationship to functions at another indenture level. For
example, if a function of an assembly is determined to be fully operational,
then all child functions on components operating within that assembly
can be inferred to be fully operational. Conversely, a function on
an assembly can be inferred to be fully operational when all child
functions on components within that assembly have been determined to be
Diagnostic Inference – An inference about a function based on its
relationship to failure modes that affect that function. Likewise, an inference
about a failure mode based on its relationship to functions affected by that
failure mode. For example, if a function is determine to be fully operational,
then all failure modes that always or exclusively affect that function can be
inferred to be non-present. Conversely, when all failure modes that can
possibly affect a function have been determined to be non-present, then that
function can be inferred to be fully operational.
Engine – The component of a reasoning system that draws
conclusions based on available information and knowledge.
Diagnostics (ID) – A structure design and management process
to achieve the maximum effectiveness of a system's diagnostic capability by
considering and integrating all related pertinent diagnostic elements. The
process includes interfaces between design, engineering, testability,
reliability, maintainability, human engineering, and logistic support analysis.
Electronic Technical Manual (IETM)
– A tool providing information a technician needs to perform a
maintenance action, including (but not restricted to) test and repair
procedures, parts information and theory. The U.S. DoD has identified the
following six classes of IETM:
Health Management – A health management process that has been
integrated into the system, or device whose health is being monitored/managed.
Vehicle Health Management (IVHM) – Vehicle
health management. A term first used by NASA to indicate the integrated health
management system on a space vehicle.
Fault Group – A fault group that represents
the final results of a diagnostic session. In testability analysis, an isolated
fault group is referred to as an ambiguity group.
Isolation Accuracy –
A measure of the ability of a diagnostic strategy or procedure to isolate
detected failures correctly across all of the potential fault combinations
for a given system, device or process. Isolation Accuracy—which
takes into consideration the possibility of simultaneously malfunctioning
components—is expressed as the probability-weighted ratio of
fault combinations for which all detected faults can be detected and
isolated to fault groups that contain them, to the total number of possible
fault combinations containing at least one detected fault. (Compare with
Diagnosis – The process of diagnosing a system by iteratively
running a diagnostic engine and taking the appropriate corrective action for
the fault identified by each iteration. Iterative diagnostics do not attempt to
isolate all known failures with each iteration, but rather to isolate a single
failure to the extent necessary to effect repair.
Search – A method for prioritizing the replacement of
repair items in an isolated ambiguity group, based on isolated failure
probability. Testability analyses often include both fault isolation metrics
(based on testing only, without the use of lambda search) and fault resolution
metrics (which take into consideration both testing and serial replacement
prioritized using a technique like lambda search).
Diagnostics – Diagnostics performed using a Model-Based Reasoner.
Not to be confused with diagnostics based on a Diagnostic Dependency Model.
Reasoner (MBR) – An algorithm that creates diagnostic conclusions
by comparing measured values against propositions created from a model that is
dynamically adjusted to match changes to the system.
Isolation – A method of fault isolation that takes into
consideration the possibility of multiple, simultaneous malfunctions during
diagnostics. The likelihood of
multiple faults existing during diagnostics is influenced by
several criteria: the failure rates associated with individual faults, the
likelihood of dependent failures, and the length of the time interval between
diagnostic sessions. The diagnostic accuracy of procedures that incorporate
multiple-failure isolation is generally higher than those based on common cause
isolation. Although multiple-failure isolation can result in greater diagnostic
ambiguity than does common cause isolation, the ambiguity groups that result
from multiple-failure isolation often lend themselves to prioritized
replacement, thereby negating or reducing the effect of multiple-failure
isolation upon diagnostic performance.
Testing – The testing of an item with the item removed from its
normal operational environment. A testing approach that requires a system
to be taken out of service while tests are performed.
Testing – The testing of a system item using active processing to
detect faults while the system as a whole continues to provide its normal
or Predictive Diagnostics – The process of predicting the
occurrence of failures to a system, device or process based on predictable
time domain failures (such as mechanical wear). This is in contrast to non-time
domain, random failures that cannot be prognosed using known technology (such
as electronic failures).
Health Management (PHM) – An approach to Health
Management that incorporates prognostic, as well as
Diagnostics – A type of diagnostic algorithm that generates
diagnostic conclusions rapidly enough to provide an immediate benefit to a
system during operation. Where algorithms become too complex for real-time
response, predictive diagnostics can be used supplementally.
System – A system that can combine elements of
information and knowledge to draw conclusions. (IEEE 1232-2002)
– The act of correcting an operational failure
through switching to a redundant system or a comparable function, or changing
the operational characteristics (such as a mission plan). Maintenance of the
failure functional item is deferred to a more suitable time (such as during
Diagnostics – A type of external diagnostics where the diagnostic
decision-making is done remotely to where the testing is performed.
– One or more parts considered as a single part for the purposes of
replacement and repair due.
Diagnostics – Diagnostics performed to diagnose faults in
a fielded system, device or process.
Diagnostics – Diagnostics in which
fault groups are developed dynamically (during run-time), yet which contains no
mechanism for dynamically selecting the order of testing. Although
semi-dynamic diagnostics may be based on a predetermined test order, the
maintainer may skip tests, postpone tests, or otherwise perform tests out of
order without impacting diagnostic accuracy. Although not as flexible as
dynamic diagnostics, semi-dynamic diagnostics are more adaptable to changing
maintenance environments than are statistic diagnostics. Furthermore, like
static diagnostics, the predetermined test order allows the diagnostics to
be relatively easily verified.
Fault Isolation – Also called Common Cause Isolation. A
method of fault isolation that proceeds under the assumption that all "failed"
tests in a diagnostic session are due to a single malfunction or fault.
Although frequently used as the basis for diagnostic analyses (such as
testability) during the design phase, single fault isolation cannot consistency
produce accurate results when multiple components are simultaneously
malfunctioning during diagnosis. The degree to which diagnostic accuracy
suffers depends on the likelihood of multiple failures (taking into
consideration the possibility of dependent failures and the length of the
interval between diagnostic sessions) and the extent to which single failures
can produce the same test signature as multiple failures. (Compare with
Diagnostics – Diagnostics which are based on a predetermined test
order and for which the diagnostic conclusions associated with each possible
test signature have been pre-computed. With static diagnostics, tests may not
be skipped, postponed, or otherwise performed out of order. The main benefit of
static diagnostics is that they are relatively easily verified. (Compare with
Dynamic Diagnostics and Semi-Dynamic Diagnostics).
Failure – The inability of the software to perform any further
functions as the result of a software fault. (Compare with Hardware Failure).
Fault – A bug or defect in the code that causes the software to
Health Management (SHM) – The capability of monitoring real-time
sensors to determine system health and performance.
Signature – The set of test results (Pass/Fail) that are
generated when diagnostics are performed for a given fault combination.
– A design characteristic which allows the status (operable, inoperable, or
degraded) of an item to be determined and the isolation of faults within the
item to be performed in a timely manner (MIL-STD-2165). A design characteristic
that allows its operational status to be determined and the isolation of faults
to be performed efficiently (IEEE Std 1522).
Analysis – The engineering practice associated with evaluating the
testability of a system, device or process.
Set (TPS) – The combination of test program, interface device,
test program instruction, and supplementary data required to initiate and
execute a given test of a UUT. (MIL-STD-2165)
Requirements Document (TRD) – An item specification that contains
the required performance characteristics of a UUT and specifies the test
conditions, values (and allowable tolerances) of the stimuli, and associated
responses needed to indicate a properly operating UUT. (MIL-STD-2165)
– The process of inferring behavioral properties of a product on the basis of
execution in a known environment with selected inputs and checking results.
Under Test (UUT) – An item for which
testability design and diagnostics must be developed.
Health Management (VHM) – An integrated health
management system on a vehicle that monitors/manages that vehicle's health.
Diagnostics – A type of remote diagnostics where interaction with
the diagnostic engine is achieved through a web interface.
White Box –
A system or component whose contents and implementation are fully disclosed.
(See also black box, grey box and glass box).