Analog signals exhibit amplitude that varies along a continuum between minimum and maximum values. Digital signals, of course, exist at separate predetermined levels or increments. The most common implementation is binary, used to represent Boolean algebra and truth-table entries. The unit of information is either high or low with nothing between.
Theoretically any pair of voltage levels, one of which not necessarily need be zero, may be chosen to represent logic low and logic high. By convention transistor-transistor logic (TTL) power supplies output about 5 V. The maximum permissible is 5.25 V. If levels rise above that, semiconductors may be damaged. If there is +2 to +5 V on a NOR gate input terminal, the device will respond to logic low inputs by outputting logic high because it is an inverter. A logic-high output ranges from 2.4 to +5 V. A logic-low output is a voltage level between 0 and 0.4 V.
When the object of interest belongs to the CMOS family a supply voltage from +3 to +15 V will be tolerated. With a supply voltage between 3 and 10 V, the logic-high level will be 0.7 times whatever the supply voltage may be. The logic-low voltage will be 0.3 times the supply voltage. In both instances there will be a 0.5-V tolerance.
With a supply voltage between 10 and 18 V, the logic-low and logic-high levels are the same fraction of the supply voltage. The tolerance rises to 1.0 V.
For both TTL and CMOS logic families, either a logic probe, a DVOM or an oscilloscope may be used for troubleshooting purposes. On a typical logic probe there is a slide switch for choosing between TTL and CMOS logic, and the switch must be moved to the correct position before taking a reading.
In the foregoing, note that there are bad or uncertain regions in TTL and CMOS outputs. In such instances, circuit behavior is unpredictable. But don’t be too quick to blame the semiconductor, because the defect may well be in the outside circuitry.
In addition to go/no-go voltage checks made with a logic probe or DVOM, temperature measurements are useful in ascertaining the operating status of an IC. You can feel with your finger the insulated package, first taking voltage measurements on the circuit board to see if there is hazardous electrical energy that has inadvertently entered the picture. If the device feels unusually hot or not warm at all, you’ve got trouble. Again, the defect may be either internal or in the outside circuitry.
The tasks of digital troubleshooting, debugging and building of prototypes require a way of detecting the presence of output pulses and ascertaining whether they are logic high or logic low. The tools of choice are oscilloscope, multimeter and logic probe. In this type of work, each has advantages. All three are viable. For now we’ll consider the logic probe.
Qualitative analysis, difficult debugging tasks and advanced lab work all require a mixed signal or mixed domain oscilloscope because it can find digital anomalies and correlate them to analog performance, for example, in the power supply.
The multimeter can detect the presence or absence of logic high and low signals. Probing, however, is sometimes difficult in close quarters. In exacting situations it may be difficult to interpret the readout while holding both probes. For quickly checking multiple digital outputs, the logic probe is accurate and easy to use. With a fine tip, the convenient pencil body facilitates fast go, no-go measurements, and for this reason the logic probe is routinely used.
Setup is simple. There are variations in different models, but typically red and black leads with alligator clips connect to on-board dc power supply terminals, red to positive and black to negative. A second slide switch is marked MEM and PULSE. MEM (memory) is similar to the HOLD function of a full-featured multimeter or clamp-on ammeter. Begin tests in the PULSE mode. Touch the probe to possible outputs, either IC pins or circuit nodes. You can move around quickly and get an idea of what is going on.
Memory mode is useful when pulses are too quick or infrequent to see. Touch the probe to the point to be tested and move the slide switch to MEM. The pulse LED lights at the first new transition. It remains lighted until it is reset by moving the slide switch to PULSE.
Most logic probes have no battery. LEDs and internal circuitry receive their power from the circuit under test. Separate LEDs, red for high and green for low, indicate the logic status, and this readout is supplemented by a two-tone audio beeper.
It is worthwhile to look at the specifications of a typical logic probe:
Working voltage: 4-18 Vdc
Frequency response: 20 MHz
TTL logic high: >2.3 ± 0.2 Vdc
TTL logic low: CMOS logic high: 70% VCC ± 10%
CMOS logic low: 30% VCC ± 10%
Minimum detectable pulse width: 25 nsec
Input impedance: 1 MΩ
Input overload protection: ± ac or dc V, 25 nsec
Supply voltage protection: ± 20 Vdc
Pulse indicator flash time: 500 msec
Operating temperature 32 – 122°F
Notice the high input impedance. The logic probe will have minimal effect on the circuit under test.
However, the capacitive loading specification is not shown. It becomes important at higher frequencies for a digital signal that is changing. For example, if the logic probe Input impedance is 20 pF, the capacitive reactance loading of the logic probe attached to the circuit is:
· At 1 MHz 7,958 Ω
· At 10 MHz 796 Ω
· At 100 MHz 79.6 Ω
There are, however, situations where a logic probe is ineffective. An example is when a circuit is removed (electrically) from other sections. In this instance there may be no input signal. If there are an odd number of NOT gates, there may be a logic-high state at the output, but this piece of information is not really definitive. What is required is a means for injecting one or more pulses at the right time and place so the circuit can be monitored and information acquired. That’s where a logic pulser comes in.
It resembles the logic probe in that it has a body or housing to which a slender conductive point attaches. And like the logic probe, the logic pulser also has red and black leads with alligator clips that are intended to be connected to the positive and negative dc power supply rails.
A typical logic pulser has, in its internal circuitry, an output transistor that is protected by a 1-kΩ resistor that limits the current in the probe and in the device under test. Accordingly, the pulser can touch any pin of an IC without fear of damaging the pulser or semiconductor.
If the equipment being tested has known good output devices, they will show the results of the injected pulses. In the absence of such indicators, a logic probe is needed. By moving one or both of these diagnostic tools from point to point, a large amount of information can be quickly accumulated, preparing the way for more definitive oscilloscope tests.
Christopher Smith says
Hi. I’m new to logic probes. I purchased a logic probe a while back but never got round to trying it out till now. The probe I have came with a separate red probe with a alligator clip on the end, the probe has three silver what looks like tiny pillars as its a probe and pulser, I’m not sure what or how to use the separate red probe. I’m having problems finding this out on the Internet and was wondering if you could help. Thank you