原创 The demise of the breakout box

One of the downsides of cables and connectors is that, by design, they typically conceal the copper wires they handle. While that's a good thing, it makes checking those wires for signal parameters—voltage, current, rise/fall, timing, and so on—difficult to impossible.

Enter the breakout box: a small, inexpensive, usually passive piece of test equipment. Breakout boxes bring those signals out to where they can be observed with probes, while also passing the signals through. I owned two of those, now lost somewhere. One was for the once-common 25-wire RS-232 connector (Figure 1), the second was for the four-wire telephone line using an RJ-11 connector (Figure 2).

Figure 1. The RS-232 breakout box saved many an engineer while troubleshooting the communications link.

Breakout boxes were fun to use. They made it fairly easy to observe a signal line with a voltmeter for basic checks, or an oscilloscope if needed. Some of the RS-232 boxes were more advanced, with LEDs on the signal lines to show their state and activity, and even small DIP switches so you could disable and then jumper key lines—all of which gave you the joyous feeling of being able to both observe and control.

Figure 2. Troubleshooting telecom lines is easy with an RJ-11 telco breakout adapter.

Now, though, it looks as if the simple breakout box has seen its best days. Today's high-speed, fast-slewing, small-swing signals—sensitive as they are to capacitance, load, and even temperature—can't tolerate the relatively heavy hand, figuratively speaking, of just any casual probing you might do with a breakout box. If you want to observe a signal going to or through a connector, you probably should have designed-in the necessary buffered test points in your prototype design and fabrication. Alternately, you'll likely need to get a relatively expensive test box that picks off the signals you want to observe, while not interfering with their paths and transitions in a way that the signals would notice.

There's not much we can do about this, as the reality of the physics of these signals is not something you can fool. It's the electronic T&M version of Heisenberg's uncertainty principle. We live in a world of fast-moving signals and their precision connectors, and they don't like to be touched. Even a benign scope probe or careless finger can upset the careful balance of inductance, capacitance, and other factors that the signal and connector were designed to play with nicely.

But I still think about those basic breakout boxes, and how much good they did in their days of glory. Certainly, there are some test tools that have been around "forever" and look as if they'll be staying around, whether in their original form (a basic, pencil-like logic probe, perhaps) or greatly improved implementation (DVMs, scopes). But there are some that are now relegated to the back of the closet, and that we probably won't be seeing or using much anymore.