Figure 4-6.-Maintenance dependency charts: (A) Single event line showing dependency marker

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

following sections:

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

triangle

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

4-6