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MAINTENANCE
WJ-8718A/MFP
4.1.3.1.4
Logic Clip
The logic clip indicator displays the logic level present at all of the terminals of an
IC at the same time. The readout is 16 LEDs: the individual LED glows if the terminal is high,
and remains dark if the terminal is low. When used in combination with the logic pulser, the
simultaneous effect of input pulses on all of the terminals of the IC under test can be observed.
4.1.3.2
4.1.3.3
Symptoms of Integrated Circuit Failure
1.
Open Input - The output of a digital IC that has an opened input
(internal) will usually behave as if the input level is a static high.
The output of this IC will not respond to input pulses.
2.
Opened Output - The output level of a digital IC that has an opened
output (internal) will not respond to input pulses and will usually
assume a faulty level of approximately 1.5 volts.
This faulty level
will often have the same effect as a high level on the input of the
next IC in the circuit.
3.
Short Circuit between Input and Ground or Vcc -
If
the input is
shorted to ground, the device will react as though the input is low. If
the input is shorted to Vcc, the device will react as though the input
is high.
Remember that either of these conditions can affect the
output of the stage prior to this.
A digital voltmeter (DVM) or
oscilloscope can be used to distinguish between normal logic levels,
and ground or Vcc.
The logic pulser cannot override the Vcc or
ground potential.
4.
Short Circuit between Output and Ground or Vcc - As in the above
cases, the output will not respond to pulses at the input. A DVM can
be used to distinguish between normal logic levels, and ground or Vcc.
5.
Short Circuit between Two Inputs - This failure is not as easy to
detect. The device will behave normally unless one input tries to go
high while the other is low.
If
this occurs, the short circuit will hold
both inputs low.
6.
Internal Failure of Logic Circuits - The effect of this trouble is to
lock the output in either the high or low state. The output will not
respond to input pulses.
In-Circuit Tests
Digital ICs rarely have partial or intermittent failures; thus, failures can usually be
located by simple static tests. The circuit troubleshooting procedure will normally isolate a
failure down to the few ICs that could be involved in the circuit failure. Checking the various
nodes (branches common to one or more inputs and one or more outputs) is part of this
procedure. The following series of steps will serve to locate most faulty ICs.
4-10
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