Trouble at the Interface

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How the interface can cause problems.

Have you ever noticed when troubleshooting that many if not most problems occur where two different things interface with one another?  An interface is the place or boundary where two things meet or touch.  It is at this point of connection where problems naturally seem to occur.  Keep this in mind as it may be a way to identify poor design or failures in electronic circuits.  You may want to look at the interfaces first.

There are lots of examples in electronics, especially mechanical.  Relay contacts are a common instance.  Pitted, corroded, or burned contacts fail to connect.  Connectors and cables also come to mind.  Two conductors come together to make a connection.  A loose fit, broken or dirty contacts, or misalignment produce a high resistance or infinite resistance path.  An bad connector not only prevents a good contact but can also act as a  non-linear device and produce passive intermodulation (PIM) distortion that generates unwanted new signals into a device.

Another common problem is grounding where two different circuits with different grounds come together often producing an offset voltage or ground loop.  Such “galvanic” connections require transformer, optical or other isolation methods to fix this kind of problem.  You might even call the place where two conductors ALMOST come together an interface as capacitive or inductive coupling can cause a problem without physical contact.

Then there are the many different electrical interfaces that define the connections between circuits and equipment.  The interface could be RS-232, RS-485, USB, Ethernet, or fiber.  Besides the mechanical cable and connector issues, interfaces also produce electrical problems in timing, crosstalk, noise and interference.  Even a wireless link is an interface.

How about the interface between hardware and software?  This is always a big consideration in any design where an embedded controller is used, like in EVERY product.  Troubleshooting often leads to a question about whether a problem is in the hardware or software….or at their interface.

A really big interface problem these days is the man-machine connection.  How easy is a machine operated or repaired by man?  The man-computer or man-smartphone usage is clearly determined by a graphical user interface (GUI).  The GUI on an OS is really a big deal.  Touchscreens produce an unusually difficult interface problem.

But of course there are also lots of non-electronic examples of interface problems.  Any place where the water meets the land, an interfacing occurs that ultimately causes a problem.  Floods, erosion, and pollution are examples.  What about metal-glass attachments?  Or solder on a copper PCB pad?  Then there is adhesive that holds two things together. Thermal interfaces are another example.

Let’s not leave out people interfaces.  There is always the possibility of a clash of personalities, opinions, misunderstandings, philosophies and viewpoints when two humans try to communicate.  The same issues occur between different countries with their own unique cultures, objectives, and religions.  Think of interfaces between husband and wife, buyer and seller, company and employee, the individual and government.  Anyway, you get the idea.  And I am sure you have encountered some unusual interface issues in your own experience.

In thinking about a design or troubleshooting, first, always identify the interfaces and imagine how they can introduce a problem.  Then design to avoid or minimize the interface and figure out how to prevent or troubleshoot it in the future.

Just a thought for future reference.

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Blogs on topics such as wired and wireless networking.

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Lou Frenzel

Lou Frenzel is the Communications Technology Editor for Electronic Design Magazine where he writes articles, columns, blogs, technology reports, and online material on the wireless, communications...
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