Logic probes: Troubleshooting to the component level [Jan 1988--Electronic Servicing & Technology]

By Conrad Persson [Persson is editor of ES&T.]

The logic probe can help a skillful technician troubleshoot a digital circuit down to the component level. The probe lets the technician know what kinds of signals are appearing at the pins of the ICs in the circuit. (Photo courtesy of Phillips ECG) A test unit such as this can apply test signals and interpret results by exercising a PC system through its I/O connections. It then outputs a message to guide the technician to the suspect pins of the suspect components. (Photo courtesy of Vu-Data Corp.)

Servicing personal computers has typically been a matter of locating the problem at the PC-board level and then replacing that board. Taken on a case by-case basis, this approach seems quick and eifficient. The inefficiency in this approach, which is not readily apparent, is the investment in keeping a stock of spare boards on hand. Another problem is that a board that appeared to be faulty turns out to be perfect.

The solution to this problem is to troubleshoot down to the component level and replace faulty components instead of board swapping. In some cases, this approach can require some sophisticated and expensive test equipment. In other cases, however, some very simple test equipment will do the job, or you might be able to find the faulty component without any test equipment at all (as mentioned in the Apple computer servicing article).

The logic probe, shown at left, indicates whether the logic state of the point being tested is at a logic high or low. A sophisticated probe will have indicators to show whether the test point is high or low or is exhibiting pulses.

Some will even have a memory or pulse stretcher that will show the presence of a 1-shot pulse that is of so short a duration that it is insufficient even to light the indicator, or it lights the indicator too briefly to be recorded by the human eye.

The indicators used to show the logic state of the pin being tested are usually LEDs. In some cases, a single LED is used to indicate any of the conditions (high, low or pulsing); in other cases, individual LEDs are used for each indication.

Let's say you're troubleshooting using a sophisticated logic probe that is capable of testing a number of different kinds of logic circuits: TTL, DTL, CMOS. You would connect the probe's power input to the power supply from which the circuitry being tested derives its power. Connecting the probe in this manner will indicate the approximate value of signal voltage that constitutes a logic low or high.

For example, the power-supply voltage for a CMOS logic circuit is 18V. A logic low in this circuit would be about 30% of that value or 5.4V. A logic high in this circuit would be about 70% of 18V or approximately 12.6V. If you suspect a specific IC, it would make sense to go directly to that IC once you have the logic probe connected into the circuit's power supply. Otherwise, use the time-tested method of starting approximately in the middle of the circuit, and let the results at the point guide you upstream or downstream a half circuit at a time.

Guided fault isolation

The logic probe all by itself is a valuable diagnostic tool. With a little help, it can be even more useful. One of the logic circuit testers that can help a technician use a logic probe to locate a faulty component is a guided fault isolation unit. This unit connects to the PC system board input/output connectors and, armed with the proper software, goes through a step-by-step test routine that exercises the computer circuits. When it encounters a condition that casts suspicion on that circuit segment, it outputs a message that tells the operator to check the condition of a specific pin of a specific IC. Your choice Just as with the servicing of television, audio and other consumer electronic equipment, servicing of PCs can be done on just about any level. You can swap boards (where the product is modular) or troubleshoot to the component level. If you choose to troubleshoot to the component level, you can use simple, inexpensive test equipment or more sophisticated, expensive test equipment.

Whichever you choose, the most important thing to bring to the test bench is good, solid information about the equipment you're servicing and sharply honed troubleshooting skills.

Also see: