Star Tech Journal

S*T J
MARCH 1981
7
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Data is often a stream of pulses, and it is important to determine if
the integrated circuits being tested are processing the pulse
streams in the correct fashion. To provide an analysis of the
dynamic operation of the circuits, much more sophisticated
equipment is necessary. A unit called a logic tracer is designed for
this purpose. The model 5004A Logic Tracer, manufactured by
Hewlett-Packard, has an indicator like that of a digital multimeter
with a four-character display, as shown in Figure 14.
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FIGURE/4
The logic tracer includes a sixteen-bit register whose action can be
started and stopped with pulses from the circuit being tested. The
resultant display of four alphanumeric characters represents a pulse
stream of sixteen bits from the circuit being checked. Each of the
displayed characters represents four of the binary bits of the
captured sixteen-bit pulse stream. To avoid an ambiguous display,
conventional hexadecimal characters are not used, but the
arrangement shown in Figure 15 is employed. A character such as
the letter "B" is not used because it would be difficult to determine
whether the display is indicating a B or an 8 when using a
segmented-bar type display.
The logic tracer is provided with probes that sample the system
clock pulses and start and stop pulses, and also with a data probe.
The start and the stop pulses derived from the system being tested
are chosen so that the unit can capture the required sixteen bits of
sequential data. Provisions are made to select either positive or
negative going clock, start, stop, and data pulses. As shown in
Figure 16, a particular sixteen-bit data stream will produce the
display shown. A different data stream will produce a completely
different display.
DATA
DISPLAY
0000
0001
0010
0 0 1 1
0100
0 101
0 1 1 0
0 1 1 1
1000
100 1
101 0
1 0 1 1
1 100
1 1 0 1
1 1 1 0
1 1 1 1
0
1
2
3
FIGURE 15
CLOCK
START
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STOP
DATA
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FIGURE 16
The application of such an instrument requires careful thought in
the original design of equipment that will be tested. The circuit
designer must provide convenient connecting points at various
stages in the circuitry to obtain stop pulses, start pulses, and data.
In addition, the display that will be obtained at each important
point must be identified. A diagnostic routine is also included so
that pre-determined fixed displays can be obtained at the specified
test points.
Figure 17 illustrates how the logic tracer displays are identified on
a circuit diagram. The manufacturer would indicate the test points
to which the stop, start, and clock probes would be connected.
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