Launching a New Type of Instrument
by Gerald Jacob, Senior Technical Editor
The simple hand-held, battery-powered logic probe has been a favorite diagnostic tool of logic circuit designers and troubleshooters for decades. When a conductor trace is contacted with the probe, an LED turns on and stays on to show the presence of a constant voltage level. When there is logic activity on the node, the LED flickers. Some logic probes also provide audible confirmation.
The basic logic probe primarily is used as an elementary "is this circuit alive?" or "which IC has toggling inputs but stuck outputs?" investigative tool. Although still useful for some applications, it is inadequate even for initially testing many of today’s high-density, complex, logic assemblies.
Millions of products, ranging from airplanes to children's toys, now contain tightly packaged digital circuits. This has made digital troubleshooting a large part of the workload for many engineers.
When first turning on or troubleshooting a digital circuit board, many questions present themselves: Is power applied? Is the clock present? Does the signal propagate through gates and flip-flops? Often, engineers must use a variety of diagnostic instruments—digital voltmeters, frequency counters, and timing analyzers—when perhaps all they want to do is to perform a quick initial status check.
According to Hewlett-Packard’s customer feedback, engineers often were frustrated by the amount of time required to hook up these instruments and the accompanying disruption to their design or troubleshooting thought processes. And these instruments often had probes that were too large for the increasingly miniaturized topologies of today's circuit boards.
Clearly, a new solution would be welcome: a hand-held instrument that provided the ease of use of the old logic probe, including a blinking LED, but that would make measurements and present a digital waveform display. Also, it would have small probes to access today’s fine-pitch devices.
Designing the Product
Responding to these customer requirements, HP set out to develop the LogicDart, a logic probe that represents the first in a new category of instruments. The instrument provides data on timing relationships, logic activity, supply voltages, and continuity. It combines many of the features of a digital voltmeter, a frequency counter, a logic probe, and a timing analyzer into one instrument and operates in several modes.
In the investigate mode, you can monitor logic activity while measuring voltage and frequency and make timing measurements at the same time. You do all this without changing probes or instrument setup.
In the analyze mode, as many as three channels of logic activity can be viewed simultaneously. You can trigger on edge, pattern, or edge/pattern combinations. With a sample rate of 100 MS/s, you have up to 10-ns resolution. Movable cursors and pan and zoom allow you to measure time between events. Levels are displayed as high, low, or tristate.
Logic activity is indicated by two LEDs, and an audible beeper checks for high, low, and toggling. You also can select several different logic families including TTL, CMOS, and ECL or set up your own thresholds for custom logic.
DC voltage and frequency are displayed simultaneously without switching modes. You can measure up to 35 V with 3½-digit resolution and frequencies up to 33 MHz.
In the compare mode, the differences between two waveforms can be viewed. For instance, you can compare a known-good waveform to a second waveform and store up to 10 waveform displays. An optional portable printer gives you hard copy of the waveforms you have monitored.
Since the LogicDart represents a new product class, HP recognized that some users may require special help. A five-minute tutorial presented on a card and a demo circuit were developed in addition to the conventional instruction manual and on-screen help.
Before embarking on full-scale production of LogicDart, HP wanted to make sure the instrument satisfied users’ expectations and requirements. To accomplish this, the company embarked on a field-trial program. A number of users would be asked to evaluate LogicDart in conjunction with their jobs. To obtain the maximum number of evaluators using a limited quantity of LogicDarts, the company devised an interesting approach.
LogicDarts were sent to approximately 100 potential users. The instruments were theirs to keep if they, as well as 10 colleagues, would evaluate the unit for one day each. This increased the potential number of evaluators to 1,100. In addition, LogicDarts were offered for free trials, yielding additional potential evaluators.
To gather the reactions of the evaluators, a questionnaire was developed by HP. The questionnaire asked the evaluators to rate how easy it was to learn to use the LogicDart and to rate its ergonomics. Detailed ratings were requested about the relative importance of the various functions. Other sections of the questionnaire dealt with the features that were most appreciated, least appreciated, and suggestions regarding additional capabilities.
As part of HP’s effort to get objective feedback about LogicDart, the company offered EE an opportunity to interview several of the evaluators. As a result of these interviews, EE discovered two classes of evaluators:
Those who had an immediate need for a compact logic signal analysis tool.
Those who looked at the instrument as a general-purpose tool to perform many tasks that may or may not involve logic circuits.
The first class was motivated to quickly explore the functionality of the instrument. These evaluators reported that, with the help of the five-minute tutorial card, they learned how to use LogicDart in less than 10 minutes. They used it to solve problems it was designed to handle and were satisfied with its performance.
They also said that LogicDart possessed the features they needed. Prime examples of this class of evaluators were Nick Ditrolio of Drummond Scientific and Michael Branning of AVTEC.
Mr. Ditrolio bought the LogicDart to solve a particular troubleshooting problem on a portable communications system. "Previously, I was unable to properly trigger on start and stop bits. But when using the LogicDart, I had no difficulty triggering and solving my problem," he said.
"We bought it about the time we were designing a PCB using surface-mount devices," said Mr. Branning. "LogicDart had a good set of features in a very portable package, and the probes were designed for providing access to pretty small areas of the board. It had a good mix of functionalities."
But one instrument can never satisfy all. Some of these evaluators suggested that the maximum measurable frequency range should extend beyond the present 33 MHz.
Now for the other class of evaluators. Several of these engineers rated the LogicDart on the basis of how it performed tasks they currently were involved with—but for which the instrument was not designed. The additional features they wanted varied drastically, ranging from having an expanded voltage range far exceeding practical logic levels, to higher resolution, to the addition of a full-featured analog scope channel.
Some evaluators would have appreciated even a limited analog display capability. According to Glenn Kindred, an engineer at TRW: "LogicDart gives you the ability to move the threshold and show a waveform, so obviously it could take repetitive traces and build up a waveform picture. It wouldn’t have to be a high-speed scope, just something that could show you a little bit more of what the signal actually looks like."
The greatest difference of opinion focused on the probes. HP endeavored to develop probes that make reliable contact with fine-pitch device leads and traces—and they succeeded.
The probe accessories were praised by those who dealt with tightly packaged PCBs containing high-density, high-pin-out ICs, such as Chris Arnoult at Magnetic Data and AVTEC’s Mr. Branning. But others said they were not robust enough. Typically, these individuals did not use the LogicDart for dense logic-circuit tests. For instance, one engineer used the instrument to check out backplane wiring and another was troubleshooting large test fixtures. Since LogicDart was not targeted for such applications, these evaluations should not carry much weight in the final analysis of the instrument.
EE also contacted several of the early purchasers of LogicDarts and found a high level of satisfaction. The instruments were being used by development engineers to check out prototypes, test engineers to debug test equipment and production output, and software/hardware teams for initial investigation of timing issues.
They all liked the functionality of the instrument. Some users wanted a display expansion feature, a higher frequency range, or a low-resolution analog display capability. However, most felt that the instrument matched their requirements.
No one we talked to accessed all the features provided. For instance, most engineers said they rarely use the compare function. Others, such as TRW’s Mr. Kindred, use it very often. Similarly, while the majority of individuals we interviewed never used all three channels, Tom Ferreras of Lucent Technologies said this feature was invaluable.
"Using all three channels simplifies troubleshooting," explained Mr. Ferreras. "If a memory device failed and I needed to see where the failing address was, I could clip two leads on the select lines, use the other channel as a probe and go down the address line to see what bits were flying. That way, I could easily figure out at which address activity stops and determine which chip is bad."
The HP LogicDart is a compact and ergonomically designed instrument suited for first-level troubleshooting of logic circuits. It performs diverse measurements from a single probe tip and provides functions previously available only from a group of instruments.
Evaluators and users approved of the combination of functionality and compactness provided by LogicDart. One individual purchased a unit for home; another carries it back and forth between the office and home. At a company where 10 LogicDarts were purchased, the engineer we talked to said the users are satisfied with its functionality and performance.
Some caution is in order when drawing conclusions from these evaluations. The potential benefit of letting 11 individuals evaluate one unit may, on the surface, be substantial. However, the fact that their test and diagnostic requirements may not be congruent with the unit’s functionality can detract from the validity of their responses. Hewlett-Packard, (800) 452-4844.
Copyright 1998 Nelson Publishing Inc.