Figure 4-6.-Maintenance dependency charts: (A) Single event line showing dependency marker; (B) Multiple event lines showing
parallel and unique functional Items.
definitions of its key terms and symbols are given in the
or cabinet locations. Each horizontal line results in an
action, such as a lamp lighting, a synchro rotating, a
meter indicating, or an indication of signal availability.
EVENTAn action or an availability of a signal at
These actions are called events. Each horizontal line
a point in a circuit. The event may be characterized as
(event line) is a representation of the circuit that
motor running, temperature normal, lamps lit, lamps
develops the event on its line. The MDC has the unique
out, instruments indicating, signal or voltage available
advantage of permitting the simple display of many
or not available, relay solenoids or thrusters energized,
events and their relationships in a limited space.
and so on.
The technique of isolating a fault is based upon a
FUNCTIONAL ENTITYA group of circuit
positive approach. It is an analysis of circuitry to verify
elements that together form a basic functional circuit,
whether the things that should have happened did
such as a filter network, a voltage divider network, an
happen. The event, if normal, is either readily
amplifier stage, an oscillator stage, and a flip-flop stage,
observable or its signal can be measured. If either the
CIRCUIT ELEMENTAn individual piece or part
action or signal is not present, components or circuits
for which no further breakdown can be made insofar as
upon which the event is dependent can be readily
fault isolation is concerned. Relay or switch contacts,
ascertained. With other methods of troubleshooting, it
relay coils, resistors, capacitors, motors, and fuses are
was impractical to present all the combinations that
examples; printed-circuit boards are not.
could cause a malfunction; for example, a trouble in 1
of 43 events resulting from parallel actions could
DEPENDENCY MARKERThe solid black
represent approximately one trillion possible
is used as a dependency marker. On an
symptoms. Accordingly, the negative approach to fault
event line, it shows that an action or availability of a
isolation or the so-called symptom-probable-
signal occurring on its line is dependent upon the
cause-remedy method is inadequate for the complexities
occurrence of an action or availability of a signal
that often occur in electronic circuits.
directly above its apex. The signal or action above the
dependency marker must be available and within
MDCs, often more detailed and involved than the
specification for the event on the line of the dependency
samples shown in figure 4-6, views A and B, are required
marker to result, if all the circuits and parts symbolically
to represent the complex circuitry of modern electronics
represented along the line are also performing properly.
systems. To help you understand an MDC, the