Understand the System
September 1971 Popular Electronics

September 1971 Popular Electronics Table of Contents Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

The extreme level of complexity and consolidation of circuit functions in today's functional integrated circuit (IC) blocks makes it so that people with almost no instruction or experience in circuit and system design can assemble and make work some pretty impressive creations. The days of vacuum tubes and early discrete semiconductors required a designer to know how to properly bias and interface various sections of circuits and systems. Nowadays, with the ready availability of impedance-matched amplifiers, filters, mixers, couplers, detectors, and other pre-packaged components, even RF and microwave frequency systems are within the reach of relative amateurs (including but not necessarily amateur radio operators). Likewise, people interested in digital and microprocessor circuits and systems have access to many pre-packaged sensors, stepper motors, displays, programmable logic blocks, imaging, and processor boards.

JK-RS Flip-Flop, Voltage Regulator

It's Not Necessary to Know All the Circuits so Long As You Understand the System

By Walter H. Buchsbaum

What makes one man "better" at electronics than another? Is it education? Apparently not, since we all know some guy just out of high school who can figure out what's wrong with a complex circuit long before his neighbor who has his BS in EE. Is it work experience? No, again, because there's always the kid fresh from school who can diagnose a fault in a piece of electronic gear while the "old timer" is still studying the schematic. Is it just plain "brightness?" The latter is often used to explain why some people seem to be so much faster in figuring out how things work. But what is brightness? What seems to enable one man to "think faster" than another?

Without going into philosophy or psychology, let's consider the words "concept or systems understanding." This is not a difficult subject because the bright people really use it all the time - often without even knowing it. Systems, or concept, understanding permits one man to take a fast look at a faulty piece of gear and come up with a solution while someone else has to use test instruments and work all day to arrive at the same conclusion. What bugs most of us is that we often feel we know more about basic electronics than the other guy, yet he amazes us with his insight into the problem.

What Is the Function?

Take a look at the schematic in Fig. 1. Without reading the caption, what is the circuit and how does it work? If that stumps you, try the simpler circuit in Fig. 2. If you have. difficulty analyzing these circuits, don't feel too bad. They happen to be integrated circuits, and while you were wasting your time trying to figure them out, all the information you really needed was "function" (what does it do?), "input and output" (what are the signals like?) and "specifications" (how is it tested?). This information is readily available from the manufacturer's data sheets. You don't have to figure out a thing; it was all done for you long ago by the IC designers.

The entire circuit shown in Fig. 1 can be represented by a "black box" such as that shown in Fig. 3; and this gives you all the information you need to know about that complicated circuit. The information in Fig. 3 says: When certain voltages (0 or 1) are present on the J and K inputs and a toggle pulse is applied, the flip-flop may or may not change states depending on what the J-K or R-S voltages are with respect to each other. Isn't that a lot simpler than trying to figure out what every transistor in Fig. 1 is doing at each instant? Consider the time saved.

Now take another look at Fig. 2 and its black box equivalent in Fig. 4. The information in Fig. 4 says: If the input voltage level is correct, the output voltage must be as specified or the IC is defective (discounting the discrete components). Of course, in a system with several black boxes, some other factors enter the picture, but the basic idea of the black box equivalent remains valid.

The two examples above show that you do not have to understand the detailed operation of complex circuits to figure out what is wrong when a problem occurs. All you need to understand is the concept - what role each black box plays in the overall scheme. This is what we call concept understanding. It is also called systems science or systems engineering. Whatever it is called, there is much dispute - even among teachers - as to whether it is a science, a field of engineering, or a mixture of math, logic, art, and magic.

"Christmas-Tree" Approach

One type of concept understanding was exemplified by the famous "Christmas Tree" approach. At the top of the tree was a statement of the problem. The tree's branches then consisted of procedural statements such as "if this happens, proceed to that" and "if this does not happen, proceed there." In this way, signal flow was followed.

Since we do not have a Christmas tree diagram for every electronic device ever built, however, we must construct one in our mind each time we tackle a system - regardless of its complexity. This is the aim of concept understanding. Once the basic flow has been established mentally, a deviation from that flow (which we call improper operation) can be traced to its origin. Obviously, this means that we must have a good knowledge of how the system is supposed to operate.

The idea of systems understanding is used in this magazine in nearly all of the articles involving digital integrated circuits. Logic diagrams are used to show the operation of the system rather than the extreme complexity of every detail within the IC's.