6.3 Cable
The Ethernet Cable Test uses Time Domain Reflectometry (TDR) to validate and
detect open and short circuited CAT5/CAT5E cables. This test is convenient for
installation and troubleshooting of cables.
Time Domain Reflectometry Principle
The TDR method can be used for control of electrical cables. The TDR
transmits a short pulse which will be fully absorbed if the cable is perfectly
terminated. In case of open or short circuited cables, the pulse will be
reflected.
An open circuited cable will result in an increase of impedance.
A short circuited cable will result in a decrease of impedance.
These conditions will give a pulse reflection in an either positive or negative
mirrored pulse. If the time is measured and the speed of the signal in the
cable is known, a distance to the problem can be estimated.
The magnitude of the reflection (called the reflective coefficient P) can be
found from a formula involving the impedance to the load (Z
t
) and the
resistance in the media (Z
0
):
P = (Z
t
- Z
0
) over (Z
t
+ Z
0
).
P = 0 indicates no reflection. +1 indicates an open circuit, and -1 indicates a
short circuit.
Wiring of Ethernet Cables
A CAT5/CAT5E cable consists of 4 twisted pairs which typically are
terminated in a RJ45 connector.
CAT5
Defined in ANSI/TIA/EIA-568-A. The color code referring to this standard is
called T568A.
CAT5E
Defined in TIA/EIA-568-B. The color code referring to this standard is called
T568B.
6.3.1 Test Setup
When you go to the test setup of the Ethernet Cable Test, the following
screen is displayed.
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