8 _ _ _ __ _ _ _ _ _ S~RJECH
digital ICs, it will be found that the most common failures are
generally of a catastrophic nature. Rarely do faults occur that
might cause a change in the propagation delay of a signal or
that otherwise affect the operation of the integrated circuit.
When preliminary investigation implies such faults may be
present, those cases can be handled with an oscilloscope
and conventional test equipment in the same manner as
when dealing with an analog circuit.
Troubleshooting a digital circuit involves the process of
determining the digital logic levels present at various points
in the circuit. Once a point is found where the logic levels are
not as they should be, the source of the trouble can be
determined by analyzing the input and output signals of the
integrated circuit being checked. Figure 3 shows a combina-
tion of two NANO gates which are used as part of a quad 2-
input NANO gate. Inputs A and B combine in ICA and the
output from the first gate (Point 0) combines with Input C in
IC 8 to form the resultant output at E. The associated truth
table shows the basic function of the individual NANO gates.
A logic high is indicated as a 1, and a logic low is indicated as a
0. In a NANO gate, the output is logic 0 only when both inputs
are logic 1. For all other input combinations, the output is
logic 1. As you become experienced in troubleshooting, you
will remember the various logic functions for individual gates.
When in doubt, consult the manufacturer's data.
To troubleshoot the circuit of Figure 3, start at the output
(Point E) and compare the logic levels present to those
specified by the manufacturer. In this example, logic levels
are shown in parentheses. You should first check with the
logic probe at test Point E, and if the signal obtained is a logic
1, the circuit is probably functioning normally. To be certain,
ground either Input A or Input B while observing Output E. If
Output E goes low, the circuit is functioning normally. A
further check, made at the inputs to IC 8 (test Points C and 0),
would show that with a zero logic level at test Point O and a 1
logic level at test Point C, the output should be logic 1. The
operation of ICA can be checked in a similar manner. Output O
from ICA should be logic zero when both Inputs A and Bare
logic 1.
For analog equipment, the proper method of looking for a loss
of output is to divide a piece of equipment into more than one
section. If a correct signal exists at one point, all circuitry that
feeds that point can be assumed to be functioning properly.
For digital circuits, you must exercise care in using this
technique, because some circuit faults could produce a
correct output at some points even when the input levels are
not correct. It is wise to make several checks at various input
and output points to determine if the circuit is functioning
properly or if there is a hidden fault. The logic probe, like any
other test inst rument, must be used wisely and the results
must be carefully analyzed in order to determine the nature of
a fault.
Open circuit conditions, either associated with an integrated
circuit itself or its printed circuit board, are common faults in
digital circuits. In Figure 4, the output lead from ICA is shown
to be open at the output connection. This has the effect of
floating the input to IC 8 and any other circuits connected to
(I) A
(II B
INPUTS
FIGURE 3
en....- - - - - '
FIGURE 4
OUTPUT
0
0
I
0
I
O
I
I
I
0