Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
2002-06-07
2004-11-30
Deb, Anjan (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S534000
Reexamination Certificate
active
06825672
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electronic diagnostic systems, and more particularly to testing equipment for cable used in a network.
BACKGROUND OF THE INVENTION
One goal of a network manager is to control total cost of ownership of the network. Cabling problems can cause a significant amount of network downtime and can require troubleshooting resources, which increase the total cost of ownership. Providing tools that help solve cabling problems more quickly will increase network uptime and reduce the total cost ownership.
Referring now to
FIG. 1
, conventional cable testers
10
are frequently used to isolate cabling problems. The cable testers
10
are coupled by a connector
12
(such as an RJ-45 or other connector) to a cable
14
. A connector
15
connects the cable to a load
16
. Conventional cable testers typically require the load
16
to be a remote node terminator or a loop back module. Conventional cable tests may generate inaccurate results when the cable is terminated by an active link partner that is generating link pulses during a test. The cable tester
10
performs cable analysis and is able to detect a short, an open, a crossed pair, or a reversed pair. The cable tester
10
can also determine a cable length to a short or open.
A short condition occurs when two or more lines are short-circuited together. An open condition occurs when there is a lack of continuity between ends at both ends of a cable. A crossed pair occurs when a pair is connected to different pins at each end. For example, a first pair is connected to pins
1
and
2
at one end and pins
3
and
6
at the other end. A reversed pair occurs when two ends in a pair are connected to opposite pins at each end of the cable. For example, a line on pin
1
is connected to pin
2
at the other end. A line on pin
2
is connected to pin
1
at the other end.
The cable tester
10
employs time domain reflection (TDR), which is based on transmission line theory, to troubleshoot cable faults. The cable tester
10
transmits a pulse
17
on the cable
14
and measures an elapsed time until a reflection
18
is received. Using the elapsed time and a cable propagation constant, a cable distance can be estimated and a fault can be identified. Two waves propagate through the cable
14
. A forward wave propagates from a transmitter in the cable tester
10
towards the load
16
or fault. A return wave propagates from the load
16
or fault to the cable tester
10
.
A perfectly terminated line has no attenuation and an impedance that is matched to a source impedance. The load is equal to the line impedance. The return wave is zero for a perfectly terminated line because the load receives all of the forward wave energy. For open circuits, the return wave has an amplitude that is approximately equal to the forward wave. For short circuits, the return wave has a negative amplitude is also approximately equal to the forward wave.
In transmission line theory, a reflection coefficient is defined as:
T
L
=
R_wave
F_wave
=
V
-
V
+
=
Z
L
-
Z
O
Z
L
+
Z
O
Where Z
L
is the load impedance and Z
o
is the cable impedance. The return loss in (dB) is defined as:
R
L
(
db
)
=
20
LOG
10
&LeftBracketingBar;
1
T
L
&RightBracketingBar;
=
20
LOG
10
&LeftBracketingBar;
Z
L
+
Z
O
Z
L
-
Z
O
&RightBracketingBar;
Return loss performance is determined by the transmitter return loss, the cable characteristic impedance and return loss, and the receiver return loss. IEEE section 802.3, which is hereby incorporated by reference, specifies receiver and transmitter minimum return loss for various frequencies. Additional factors that may affect the accuracy of the return loss measurement include connectors and patch panels. Cable impedance can also vary, for example CAT5 UTP cable impedance can vary ±15 Ohms.
SUMMARY OF THE INVENTION
A cable testing system and method according to the present invention tests cable and determines status. The test module includes a pretest state machine that senses activity on the cable and enables testing if activity is not detected for a first period. A test state machine is enabled by the pretest state machine, transmits a test pulse on the cable, measures a reflection amplitude and calculates a cable length. The test module determines the cable status based on the measured amplitude and the calculated cable length.
In other features, the pretest state machine enables testing if, during the first period, activity is detected and is subsequently not detected for a second period after the activity is detected. A lookup table includes a plurality of sets of reflection amplitudes as a function of cable length. The test module determines the cable status using the lookup table, the reflection amplitude and the cable length.
In yet other features, the sets of reflection amplitudes define a plurality of windows. Three windows are defined by first and second thresholds. The first threshold is based on a first set of reflection amplitudes that are measured as a function of length when the test cable type is terminated using a first impedance having a first impedance value. The second threshold is based on a second set of reflection amplitudes that are measured as a function of length when the test cable type is terminated using a second impedance having a second impedance value.
In still other features, the cable is declared an open circuit when the reflection amplitude is within the first window for the calculated cable length. The cable is declared a short circuit when the reflection amplitude is within the second window for the calculated cable length. The cable is declared normal when the reflection amplitude is within the third window for the calculated cable length.
In still other features, when testing cable that transmits and receives on different wires, the test module transmits the test pulse, measures offset, subtracts the offset from the reflection amplitude, and detects peaks. If a second peak is not detected after a first peak and the reflection amplitude of the first peak is greater than a first threshold, the test module transmits a second test pulse having a second amplitude that is less than a first amplitude of the first test pulse. If the reflection amplitude of a first peak after transmitting the second test pulse is greater than a second threshold, the test module declares a close open status. If the first peak is detected after a predetermined period after transmitting the second test pulse, the test module declares an open status. If the first peak is less than a predetermined threshold within the predetermined period after transmitting the second test pulse, the test module declares a perfectly terminated status.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
REFERENCES:
patent: 5461318 (1995-10-01), Borchert et al.
Intel, “LXT9784 Octal 10/100 Tranceiver Hardware Integrity Function Overview” Application Note, Jan. 2001.
Guo Yiqing
Lo William
Deb Anjan
Lair Donald M.
Marvell International Ltd.
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