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SCAN-LAN VI
8.2.4 L
ENGTH
The SCAN-LAN VI uses a method called Time Domain Reflectometry
(TDR) to measure cable length. With TDR, a signal is transmitted at one
end of a cable and, at the same time, a timer is started. The electrical
signal travels down the cable until it reaches a change in impedance (for
example, an open, a short, a fault in the cable, or a cable mismatch) and
part or all of the signal is reflected back. If the cable is properly terminated
(that is, the terminator is equal to the impedance of the cable), there will
be no reflection of the signal and you will not be able to determine the
length. Figure 8-10 illustrates the behavior of the electrical signal when it
reaches the end of the cable.
Figure 8-10. Using TDR to Locate Cable Faults.
When the reflected signal reaches the tester where the signal was
generated, the timer stops. By knowing the speed of the signal traveling
down the cable, you can determine the length of the cable.
Transmitted pulse
Reflected pulse
Cable
Transmitted pulse
Reflected pulse
open
short
Transmitted pulse
No reflected pulse
terminator
(matches cable
characteristic impedance)
Summary of Contents for SCAN-LAN VI
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