66..33 CableCable
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 PrincipleTime 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 = 00 indicates no reflection. +1+1 indicates an open circuit, and -1-1 indicates a
short circuit.
Wiring of Ethernet CablesWiring of Ethernet Cables
A CAT5/CAT5E cable consists of 4 twisted pairs which typically are terminated
in a RJ45 connector.
CAT5CAT5
Defined in
ANSI/TIA/EIA-568-A
. The color code referring to this standard is
called T568AT568A.
CAT5ECAT5E
Defined in
TIA/EIA-568-B
. The color code referring to this standard is called
T568BT568B.
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