Star Tech Journal

S~R-TECH __ ______ _ __ 7
Figure 1 shows some typical TTL signal pulses. The level of
the pulses with respect to the high and low thresholds
determines the operation of the individual digital logic circuits.
A pulse level above the high threshold is in the high, or logic
one, state (positive logic), and a pulse level below the low
threshold is in the low, or logic zero, state (positive logic). A
pulse level in between the low and high threshold values
(between about .4 volts and 2.4 volts) is indeterminate. While
tracing signal pulses through a circuit, it is not important to
know the exact amplitude of a pulse, but whether it is either
above the high or below the low voltage level thresholds. If an
oscilloscope were used to check a pulse level, the user would
have to constantly measure signal amplitudes to determine if
either the high or low thresholds were met.
To simplify the measurement or determination of logic state
levels (both static and pulsed), a device referred to as a Logic
Probe is often employed. Figure 2 shows the basic appear-
ance of this instrument. Constructed in the shape of a probe,
the unit contains level sensing circuits, pulse sensing circuits,
and indicators. In the logic probe shown in Figure 2, three
separate LED indicators are included; one to represent the
high state (H), another the low state (L), and the third to
indicate the presence of pulses (P).
The use of the logic probe is quite simple. The unit is powered
from the circuit under test by connecting the power clips to
ground and a +s volt point. The probe tip is then connected to
the desired test point and the LEDs will indicate either the
presence of a high or low state or the presence of a pulse. The
probe generally includes a "pulse stretcher'' circuit that
allows observation of very narrow pulses. The probe includes
a memory circuit to capture a single narrow pulse and
indicate its presence by causing the P indicator to glow
continuously. Once a pulse has occurred, the pulse reset
switch is used to extinguish the P indicator and to reset the
pulse memory for another pulse.
If the signal level at the test point is not above the high level
threshold or below the low level threshold, but is in the
indeterminate state, both the high and low level indicators will
light simultaneously, indicating that condition. (In some
probes, the indeterminate state is indicated by neither LED
lighting. Refer to the manufacturer's operating instructions
for the particular type of probe you are using.) With the logic
probe, the signal level at any point in the circuit can be
determined without the need for making accurate measure-
ments and then checking if the measurements satisfy the
voltage threshold requirements. Most probes are designed
for use in TTL circuits with the standard TTL logic levels
shown in Figure 1. Some probes are available with an
adjustment feature that allows both the high and low level
thresholds to be varied so that a wide variety of nonstandard
logic systems can be analyzed.
Logical Probe Applications
As mentioned earlier, a logic probe provides a convenient
means of determining the signal conditions at various points
in a digital circuit. It automatically indicates whether the
signal is either logic high or logic low and it also indicates the
presence of a pulse. In analyzing the faults associated with
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