6 _ _ _ _ _ _ _ _ _ _ _ S~~TECH
Microprocessor Troubleshooting
(Part 2)
Part 1 of this 3-part series covered "Logic Probes" and "Logic
Probe Applications."
SIGNAL INJECTION
Static measurements of a digital circuit with the logic probe
alone may not be enough to isolate a circuit fault. An open or
short may still result in a correct static condition. However,
the circuit will malfunction under dynamic conditions. A more
complete check can be made by applying a test signal to a
circuit and then observing the output with a logic probe.
Special signal injection equipment is necessary to meet the
needs of the low input impedance TTL logic circuits so
commonly used today. To see why this is necessary, let's
briefly review the operational characteristics of the output
circuitry of a TTL gate.
Figure 6 shows a typical output portion of a TTL logic gate
circuit. The totem pole output stage contributes to the fast
switching time of a TTL circuit but, at the same time, causes
some problems when injecting test signals. The transistors
conduct alternately, depending upon whether the circuit is in
the 1 or the zero state. As shown in Figure 7 A, when the
output stage is in the 1 state, 01 conducts and serves as the
source to all the gates connected to the output. When the
output stage is in the zero state, as shown in Figure 78, 02
conducts and serves as a current sink to all gates connected
to that output. Due to the fact that either 01 or 02 is
conducting heavily and both are directly connected to the
output, the signal lines in the TTL output circuit are of very low
impedance. This means that both the input and output
circuits of a TTL gate are of low impedance. Therefore,
conventional signal generators cannot properly drive the TTL
circuits, and other equipment had to be developed.
One of the devices develped for logic circuit signal injection is
the Hewlett-Packard 546 Logic Pulser. This is a digital pulse
generator contained in a probe of the same general appear-
ance of the logic probe. However, instead of indicator lamps,
the logic pulser has a pulse button. This pulse button is a
switch that is used to provide different types of output pulses
from the logic pulser. A variety of output signal configurations
is provided by tapping the pulse button a specified number of
times. Figure 8 lists the capabilities of the 546 Logic Pulser.
For example, if the pulse button on the logic pulser is tapped
once, a single output pulse is obtained. Holding the pulse
button down produces a continuous 100-Hertz pulse stream.
If the button is tapped once and then held, the logic pulser
develops a 100-pulse burst. Tapping the button twice and
then holding it causes a 10-Hertz pulse stream to be
produced. Tapping the button three times and then holding it
produces 10-pulse bursts. Tapping the button four times and
then holding itwill produce a 1-Hertzcontinuousstream. With
the variety of signals available, the integrated circuits in a
logic system can be stimulated and their outputs observed
under pulse conditions with a logic tracer.
GATE
OUTPUT
FIGURE 6
•o•
SOURCE
SINK
A
B
FIGURE 7
STATE