Workshop

Using a Multimeter
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Because it’s expensive to get someone in to pull a cable back into the wall, there may be times when
you’ll want to verify that a cable is in fact broken (that is, that the problem you’re experiencing is due to
the cable rather than stemming from some other source, such as EMI or a bad component). In such cases,
you can perform a continuity check (that is, you can check whether the current flows through the cable
from one end to the other) by using a multimeter, a device that measures electrical voltage, amperage,
and resistance. You can buy a multimeter for around $10 at Radio Shack or any other electronic supply
store.
A multimeter will also tell you whether a cable is open (broken) or closed (okay). After you turn the
multimeter’s continuity function on, you can see how it works by simply touching the test leads together.
You should hear a tone. (This is why testing a cable is sometimes referred to as toning it out.) The tone
indicates that a continuous electrical path exists between the leads.
If you have both ends of the cable in your hands, you can attach the leads to the same pin of the cable on
both sides. (Most Ethernet cables are straight through—that is, there’s a one-to-one relationship of pins;
the same is true of Token-Ring cables.) You should hear a tone when you do this. If you don’t, walk one
of the leads from pin to pin. If you still don’t hear a tone, the cable is broken.
If you hear a tone from more than one pin, this is also a bad thing. This indicates that somewhere along
the line, two wires have been shorted together.
Looping Back
If you don’t have both ends of the cable in your hands, what can you do? After all, you need to connect
both leads to the cable. The answer is to connect two pins together at one end of the cable to create one
long wire (see Figure 8.1). Then connect the leads to each end of the long wire (both ends of the long
wire will be on the same end of cable, as shown in the figure).
Figure 8.1 Continuity check on a cable in the wall.