Abstract:
A physical layer device is adapted to communicate with a cable medium and communicates with an indicator. The physical layer device includes a first input/output terminal, a first transceiver that communicates with the first input/output terminal that is adapted to be connected to a cable medium, and a cable tester that tests the cable medium and determines a cable status using a pretest module. The pretest module senses activity on the cable medium and enables testing if activity is not detected for a first period, and a test module that is enabled by said pretest module, transmits a test pilse on the cable medium, measures a reflection amplitude and calculates a cable length. The cable tester determines the cable status based on the measured amplitude and the calculated cable length, and the indicator indicates at least one of cable testing status during the test and the cable status after the trst. The indicator also inducates at least one of link presence, link absence, link acitivty, link duplex and link speed of the first input/output terminal during normal operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/331,221, filed Dec. 30, 2002, which is a continuation-in-part of U.S. patent application Ser. No. 10/165,467 filed Jun. 7, 2002, now U.S. Pat. No. 6,825,672. The disclosures of the above applications are incorporated herein by reference in their entirety. 
    
    
     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 communicates with different pins at each end. For example, a first pair communicates with 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  communicates with pin  2  at the other end. A line on pin  2  communicates with 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     ⁢          1     T   L              =     20   ⁢     LOG   10     ⁢              Z   L     +     Z   O           Z   L     -     Z   O                      
 
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.
 
     Consumers can now purchase lower cost switches, routers, network devices and network appliances that include physical layer devices with ports that are connected to cable. When connecting these network devices to cable, the same types of cabling problems that are described above may occur. In these lower cost applications, the consumer typically does not have a cable tester or want to purchase one. Therefore, it is difficult to identify and diagnose cable problems without simply swapping the questionable cable with a purportedly operating cable. If the purportedly operating cable does not actually work, the consumer may incorrectly conclude that the network device is not operating and/or experience further downtime until the cable problem is identified. 
     SUMMARY OF THE INVENTION 
     A physical layer device according to the present invention is adapted to communicate with a cable medium and includes at least one indicator that identifies at least one of link presence, link absence, link activity, link duplex and link speed of the first input/output terminal during normal operation. A first tranceiver communicates with a first input/output terminal and the cable medium and includes a cable tester that tests the cable medium and determines a cable status. The cable tester also uses the indicator to indicate at least one of cable testing status during the test and the cable status after the test. 
     In other features, the cable tester includes a pretest module that senses activity on the cable medium and enables testing if activity is not detected for a first period. A test module is enabled by the pretest module, transmits a test pulse on the cable medium, measures a reflection amplitude and calculates a cable length. The cable tester determines the status based on the measured amplitude and the calculated cable length. 
     The pretest module 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 cable tester determines the cable status using the lookup table, the reflection amplitude and the cable length. The cable status includes good, open and short cable statuses. 
     In still other features, the sets of reflection amplitudes define a plurality of windows including a first window that is 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 cable length when a test cable type is an open circuit. The second threshold is based on a second set of reflection amplitudes that are measured as a function of cable length when the test cable type is terminated using a first impedance having a first impedance value. 
     In still other features, a second window is defined by third and fourth thresholds. The third threshold is based on a third set of reflection amplitudes that are measured as a function of cable length when the test cable type is a short circuit. The fourth threshold is based on a fourth set of reflection amplitudes that are measured as a function of cable length when the test cable type is terminated using a second impedance having a second impedance value. A third window is defined between the second and fourth thresholds. 
     In yet other features, the cable medium is declared an open circuit when the reflection amplitude is in the first window for the cable length. The cable medium is declared a short circuit when the reflection amplitude is in the second window for the cable length. The cable medium is declared normal when the reflection amplitude is in the third window for the cable length. 
     In yet other features, an analog to digital converter (ADC) measures the reflection amplitude. The test module measures offset at the ADC, subtracts the offset from the reflection amplitude, and zeroes the reflection amplitude below a floor. The floor has a first value during a first period after the test pulse and a second value during a second period after the first period. 
     In still other features, the physical layer device includes a plurality of input/output terminals and the cable tester includes a test actuator that triggers the test during operation of the physical layer device for unlinked input/output terminals but not unlinked input/output terminals. The cable tester enters a test fail status when the cable medium is active for a period greater than a test fail period. The cable tester is integrated with the tranceiver in a single integrated circuit. The indicator includes at least one of an audio indicator and a visual indicator. The indicator includes at least one of a speaker, a light emitting diode (LED), and an incandescent light. 
     In still other features, the physical layer device includes a detector that detects a power over Ethernet (POE) device and a power supply. A switching device selectively provides power from the power supply over the cable medium when the detector detects the POE device. The cable tester delays testing of the cable medium when the detector detects the POE device until the switching device provides the power. 
     In still other features, the physical layer device implement one of a switch, a router, a computer, a laptop, a smart videocassette recorder, an IP telephone, a fax machine, a modem, a television, a stereo, and a hand-held device. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of a cable tester according to the prior art; 
         FIG. 2  is a functional block diagram of a cable tester according to the present invention; 
         FIG. 3  is a state diagram of a pretest state machine; 
         FIG. 4  is a state diagram of a first test state machine for a cable tester for a media that transmits and receives on the same wire; 
         FIG. 5  is a state diagram of a second test state machine for a cable tester for a media that does not transmit and receive on the same wire; 
         FIG. 6  is a waveform diagram illustrating a time-based receiver floor; 
         FIG. 7  is an exemplary cable reflection amplitude vs. cable length relationship for a first type of cable; 
         FIG. 8  is a functional block diagram of an exemplary network device that includes one or more physical layer devices and that includes a hardware or software based cable testing switch for initiating cable testing; 
         FIG. 9  is a flowchart illustrating steps for performing a cable test for the exemplary network device in  FIG. 8 ; 
         FIG. 10A  is a functional block diagram of an exemplary power over Ethernet (POE) device; 
         FIG. 10B  is a flowchart illustrating steps for performing a cable test for the exemplary network device in  FIG. 8  when POE devices are possibly connected at remote cable ends; 
         FIG. 11  is a functional block diagram of an exemplary network device that includes one or more physical layer devices and that initiates cable testing at power on; 
         FIG. 12  is a flowchart illustrating steps for performing a cable test for the exemplary network device in  FIG. 11 ; 
         FIG. 13  is a flowchart illustrating steps for performing a cable test for the exemplary network device in  FIG. 11  when POE devices are possibly connected at remote cable ends; 
         FIGS. 14A–14E  illustrate exemplary LEDs during testing cable testing; 
         FIG. 15  illustrates the exemplary LEDs showing the results of cable testing; and 
         FIG. 16  illustrates exemplary LEDs of a network device that includes more than one LED per port. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify the same elements. 
     Referring now to  FIG. 2 , a cable tester  20  according to the present invention is shown. The cable tester  20  is capable of testing 10/100BaseT cable, 1000BaseT cable, and/or other cable media. For example, 10/100BaseT includes two pairs of twisted pair wires and 1000BaseT cable includes four pairs of twisted pair wires. A transmitter  28  and a receiver  30  are coupled to the I/O interface  26 . A test module  32  includes state machines for testing a media  34  such as cable. The test module  32  can be implemented in combinatorial logic, using discrete circuits, and/or using a processor and memory that executes testing software. 
     The test module  32  includes a pretest state machine or module  50 . The test module  32  also includes a first test state machine or module  52  and/or a second test state machine  54 . One or more lookup tables  56  containing cable empirical data are also provided as will be described below. The cable tester  20  may also include a display  58  for presenting fault status, cable length and/or reflection amplitude data. A cancellation circuit  59  cancels the test pulse when testing on media that transmits and receives on the same wire such as 1000BaseT. The cancellation circuit  59  is not used when testing media that transmits and receives on different wires such as in 10/100BaseT. The cancellation circuit  59  can be a hybrid circuit. 
     Referring now to  FIG. 3 , the pretest state machine  50  is illustrated in further detail. On reset, the pretest state machine  50  moves to a wait enable state  100 . Pair is set equal to zero and testover is set equal to one. When a test enabled signal is received, the pretest state machine  50  transitions to a wait powerdown state  102 . A powerdown timer is incremented and test_over is set equal to zero. The powerdown timer should have a period that is sufficient to bring a link down. When the powerdown timer exceeds a first period P 1 , the pretest state machine  50  transitions to a first timer start state  104 . 
     A first timer is set equal to zero and a blind timer is incremented. The blind timer waits for a blind timer period to allow a sufficient amount of time for transitions between pairs. Typically several clock cycles are sufficient. When wire_activity is high, the pretest state machine  50  transitions to a signal find state  106  and resets a second timer. Wire_activity is present when a signal on the wire is above a predetermined threshold. 
     When wire_activity is low in the signal find state  106 , the pretest state machine  50  transitions back to the signal find state  106  and resets the second timer. If the second timer is greater than a second period P 2 , the pretest state machine  50  transitions to a test state  110 . Tdrwrstart is set equal to one. If a test pass signal is received, the pretest state machine  50  transitions to a test over state  114 . Pair is incremented, tdrwrstart is set equal to zero, and the register is recorded. 
     If pair is less than 4 for 1000BaseT operation or 2 for 10/100BaseT operation, the pretest state machine  50  transitions from the test over state  114  to the first timer start state  104 . If pair is equal to 4 for 1000BaseT operation or 2 for 10/100BaseT operation, the pretest state machine  50  transitions from the test over state  114  to the wait enable state  100 . 
     In the first timer start state  104 , the pretest state machine  50  transitions to the test state  110  if the first timer is greater than a third period P 3 . In the signal find state  106 , the pretest state machine  50  transitions to the test over state  114  if the first timer is greater than the third period P 3 . 
     In a preferred embodiment, the first period P 1  is preferably 1.5 s, the second period P 2  is equal to 5 ms, and the third period is equal to 125 ms. Skilled artisans will appreciate that the first, second and third periods P 1 , P 2  and P 3 , respectively, may be varied. The P 3  is preferably selected based on a worst case spacing of link pulses. P 2  is preferably selected to allow testing between fast link pulses (FLP). FLP bursts have a length of 2 ms and a spacing of 16 ms. By setting P 2 =5 ms, the delay is a total of 7 ms, which is approximately half way between FLPs. P 1  may be longer than 1.5 seconds if required to bring the link down. 
     Referring now to  FIG. 4 , the first test state machine  52  for media that transmits and receives on the same wire is shown. The cancellation circuit  59  cancels the transmit test pulse. On reset, the first test state machine  52  transitions to a wait start state  150 . Peak is set equal to zero and cutoff is set equal to peak/2. When tdrwr_start_r rising edge is received from the pretest state machine  50 , the first test state machine  52  transitions to a detect offset state  154 . tdr_sel_pulse is set equal to 1 to generate a pulse and start a timer. The pulse is preferably a 128 ns pulse having a 2V amplitude. 
     After an offset is subtracted from tdr_in, the first test state machine  52  transitions to a detect peak state  158 . Peak stores the current value of tdr_in. If tdr_in is less than or equal to peak/2, the first test state machine  52  transitions to a detect cutoff state  162  where distance is set equal to a counter. If tdr_in is greater than peak, the first test state machine  52  transitions to state  158  and peak is replaced by a new tdr_in. If a timer is greater than a fifth period P 5 , the first test state machine  52  transitions to a test over state  166  where peak/distance is calculated, tdr_pass is set equal to 1, and tdr_sel_pulse is set equal to 0. 
     While in the detect cutoff state  162 , the first test state machine  52  transitions to the detect peak state  158  if tdr_in&gt;peak. While in the detect peak state  158 , the first state machine  52  transitions to the test over state  166  if the timer is greater than the fifth period P 5 . In a preferred embodiment, P 5  is equal to 5 μs. 
     Referring now to  FIG. 5 , the second test state machine  54  is shown in further detail. On reset, the second test state machine  54  transitions to a wait start state  200 . Peak is set equal to zero, cutoff is set equal to peak/2, and distance is set equal to 0. When tdrwr_start_r rising edge is received from the pretest state machine  50 , the second test state machine  54  transitions to a detect offset state  204  where tdr_in=filtered magnitude and tdr_sel_pulse is set equal to 1. The second test state machine  54  transitions to a first detect peak state  208  where peak1 is set equal to max of tdr_in. 
     If tdr_in is less than peak1/2, the second test state machine  54  transitions to a second detect peak state  212  and sets peak2 equal to maximum of tdr_in. If tdr_in is less than peak2/2, the second test state machine  54  transitions to a detect cutoff state  216 . Distance is set equal to a counter. If a fourth timer is greater than a fourth period P 4 , the second test state machine  54  transitions to a test over state  220 . Peak/distance is calculated, tdr_pass is set equal to 1, and tdr_sel_pulse is set equal to 0. 
     In the detect cutoff state  216 , if tdr_in is greater than peak2, the second test state machine  54  transitions to the second peak detect state  212 . In the second detect peak state  212 , if the fourth timer is greater than P 4 , peak2 is equal to 0 and peak1 is greater than a threshold, the second test state machine  54  transitions to a second test state  224 . In the second test state  224 , tdr_sel_half_pulse is set equal to 1 to send a half pulse. The second test state machine  54  transitions from the second test state  224  to the test over state  220 . 
     In the first detect peak state  208 , if the fourth timer is greater than P 4 , the second test state machine  54  transitions to the test over state  220 . In the second detect peak state  212 , if the fourth timer is greater than P 4 , peak2=0, and peak1 is less than or equal to a second threshold, the second test state machine  54  transitions to the test over state  220 . 
     The link is brought down and the pretest state machine  50  waits until the line is quiet. For each pair, the cable tester  20  generates a TDR pulse and measures the reflection. In 10/100BaseT media, after the test is enabled, the pretest state machine  50  waits until the line is quiet. A pulse is generated and the reflection is measured. The status receiver and transmitter pairs are determined sequentially. For the first pair, the receiver is preferably in MDIX mode and the transmitter is preferably in MDI mode. For the second pair, the receiver is preferably in MDI mode and transmitter is preferably in MDIX mode. 
     The pretest state machine  50  ensures that the line is quiet before the pulse is transmitted. After the test is enabled, the pretest state machine  50  waits P 1  (such as 1.5 seconds or longer) to make sure that the link is brought down. The pretest state machine  50  determines whether there is activity on a first pair (MDI+/−[0] for 1000BaseT network devices and RX for 10/100BaseT products). 
     In a preferred embodiment, activity is found when activity minus systemic offset such as a noise floor that is calculated in states  154  and  204  is greater than a predetermined threshold. If there is no activity for P 2  (such as 125 ms), the pretest state machine  50  proceeds to the test state and sends a pulse on the selected pair. If there is activity on the pair and the line is quiet for 5 ms afterwards, the pretest state machine proceeds to the test state. The test fail state is reached and a test failure declared if the line has not been quiet for more than 5 ms during a 125 ms period. If a test failure is declared on the first pair or the TDR test is completed for the pair, the same procedure is conducted on MDI+/−[1], MDI+/−[2], MDI+/−[3] sequentially for 1000BaseT devices and the TX pair for 10/100BaseT devices. 
     In 1000BaseT devices, the original 128 ns test pulse is cancelled by the cancellation circuit  59 . The pulse received at the ADC output is the reflection. The test pulse preferably has 2V swing. Before testing, the offset on the line is measured and is subtracted from the received ADC value. 
     Referring now to  FIG. 6 , the cancellation circuit  59 , which can be an analog hybrid circuit, does not perfectly cancel the test pulse. To prevent false reflection identification, a 250 mv floor within 32 clock cycles (125 Mhz clock) and a 62.5 mv floor after 32 clock cycles are used to allow a residual of cancellation of the test pulse and noise to be filtered. The peak value on the line is detected for 5 μs. The amplitude of reflection is the maximum magnitude that is detected. The amplitude is adjusted according to the sign of the reflection. The distance to the reflection is located at 50% of the peak. 
     The cable status is determined by comparing the amplitude and the calculated cable length to the lookup table  56  for the type of cable being tested. The measured reflection amplitude falls into a window. There are two adjustable thresholds for open circuit and short circuit cable. The open threshold is preferably based on experimental data, which can be produced by refection amplitudes for CAT3 and CAT5 cable that is terminated with a first impedance value such as 333 Ohms. 
     The default short circuit threshold is based on experimental data of refection amplitudes for CAT3 and CAT5 cable that is terminated with a second impedance value such as a 33 Ohms. As can be appreciated, the lookup table  56  may contain data for other cable types. Other impedance values may be used to generate the thresholds. 
     If measured amplitude falls between open and short circuit thresholds, the cable status is declared normal. If the amplitude is above the open threshold, the cable status is declared an open circuit. If the amplitude is below a short circuit threshold, the cable status is declared a short circuit. The cable status, reflection amplitude and cable distance are stored and/or displayed. 
     In the second test state machine, the original test pulse is not cancelled. Both the original pulse and the reflection are monitored. When an open circuit is located near the cable tester, the two pulses may be overlapping, which may cause saturation in the ADC. The test state machine preferably sends out a 128 ns pulse that has a 1V swing. The offset on the line is measured and subtracted from the received ADC value. A 250 mv floor is used within 32 clock cycles (125 Mhz clock) and a 62.5 mv floor is used after 32 clock cycles so that the residual of cancellation and noise can be filtered. Signals below the floor are considered to be 0. The peak value on the line is detected for 5 μs. As can be appreciated, the test pulse can have longer or shorter durations and amplitudes. 
     The first peak that is observed should be the test pulse. The amplitude of reflection is the maximum magnitude detected after the test pulse is detected. The distance of reflection is at 50% cutoff of the peak. If another pulse is not detected after the test pulse and the magnitude of the test pulse (when a counter reached a predetermined period) is greater than a preset threshold, the cable tester decides whether there is an open cable that is located relatively close or a perfectly terminated cable by sending a second test pulse that has one-half of the magnitude of the first test pulse. 
     If the maximum magnitude on the line is greater than ¾ of the original pulse, there is an open circuit that is located relatively close. Otherwise, if the first peak is detected after a predetermined number of clock cycles, the cable tester  20  declares an open circuit. If the first peak is within after the predetermined number of clock cycles, the cable tester  20  declares a perfectly terminated cable. In one exemplary embodiment, the predetermined number of clock cycles is 33. 
     The cable status is determined by comparing the amplitude and distance of reflection to the lookup table  56  based on the type of cable being tested. There are two adjustable thresholds for open and short circuit cable. The default open threshold is from the experimental data of refection amplitudes for CAT3 and CAT5 cable terminated with a first impedance value such as 333 Ohms. The default short circuit threshold is from the experimental data of refection amplitude of CAT3 and CAT5 cable that is terminated with a second impedance value such as 33 Ohms. Other impedance values may be employed for generating thresholds. 
     If the measured amplitude falls between open and short circuit thresholds, the cable status is declared normal. If the amplitude is above the open circuit threshold, the cable status is declared an open circuit. If the amplitude is below a short circuit threshold, the cable status is declared a short circuit. The cable status, reflection amplitude and cable length are stored and/or displayed. 
     Referring now to  FIG. 8 , the cable tester can be implemented in an exemplary network device  300  that includes a physical layer device  308  and a cable tester or cable test module (CTM)  312 , as described above. The network device  300  can be a switch  304  that includes an n port physical layer device  308  and a cable test module (CTM)  312 . While the switch  304  is shown, any other network device  300  that contains a physical layer device, a port and the CTM can be used. For example, the network device  300  may be a network appliance, a computer, a switch, a router, a fax machine, a telephone, a laptop, etc. 
     Cables  314 - 1 ,  314 - 2 , . . . , and  314 - n  can be connected to the switch  304  using connectors  318 - 1 ,  318 - 2 , . . . , and  318 - n , such as RJ-45 connectors or any other suitable connector type. The switch  304  can be connected to other network devices such as, but not limited to, computers, laptops, printers, fax machines, telephones and any other network device or network appliance. 
     In the embodiment shown in  FIG. 8 , the network device  300  includes a software or hardware based switch  324  that is used to trigger the cable test during operation. The network device  300  also includes one or more light emitting diodes (LEDs)  326 - 1 ,  326 - 2 , . . . , and  326 - n . If a single LED per port is used, the LEDs  326  are fully burdened during normal use. For example, the LEDs  326  are used to display the presence or absence of a link, link speed, link activity and other information during normal (non-cable-testing) use. While LEDs are shown, any other audio and/or visual indicator can be used. For example, audible tones from a speaker or other audio device can be used to indicate cable status. If the network device includes illuminated switches, the illumination of the switches can be flashed, brightened, dimmed or otherwise used to indicate cable status. Still other indicators include incandescent lights. 
     Referring now to  FIG. 9 , steps for operating the network device  300  are shown generally at  330 . Control begins with step  332 . In step  334 , control determines whether the test switch  324  has been pushed. If the test switch has not been pushed, control loops back to step  334 . Otherwise, control continues with step  336  where control sets the port equal to 1. 
     Control determines whether the link associated with a current port is up in step  338 . If not, control performs the cable test on the designated port in step  340 . Control continues from step  340  or step  338  (if true) with step  342  where control determines whether all ports have been tested. If not, control continues with step  344 , increments the port, and continues with step  338 . If all ports are tested as determined in step  342 , control displays the results for the tested port(s) in step  346  using the LEDs and control ends in step  348 . If the network device  300  has only one port, steps  336 ,  342  and  344  can be skipped. As can be appreciated by skilled artisans, the cable test can be executed sequentially for each port as set forth above or simultaneously for all ports. 
     Referring now to  FIGS. 10A and 10B , additional steps are performed when the network device may be connected to power over Ethernet (POE) devices or data terminal equipment (DTE), which will be collectively referred to herein as POEs. Examples of POEs include computers (notebooks, servers and laptops), equipment such as smart videocassette recorders, IP telephones, fax machines, modems, televisions, stereos, hand-held devices, or any other network device requiring power to be supplied over the cable. These devices typically include a filter or other circuit that is connected across center taps of transformers at the POE end of the cable. If not accommodated by the cable test module, the filters or other circuits that are used by the POEs may cause the cable test to generate inaccurate results. 
     Referring now to  FIG. 10A , an exemplary network device  350  provides cable power to an exemplary cable-powered POE  351 . The network device  350  includes a controller  352  that communicates with a signal generator  353 , a detector  354  and a selector switch  355 . The signal generator  353  communicates with a transmitter  356  having an output that communicates with a secondary of a transformer  357 . The detector  354  communicates with a receiver  359  having an input that communicates with a secondary of a transformer  360 . The selector switch  355  selectively connects center taps of primaries of the transformers  357  and  360  to a power source  361 . 
     Pair A of a cable  362  communicates with a primary of a transformer  363 . A secondary of the transformer  363  communicates with a selector switch  364 , which selects either a receiver  365  or a filter  366 . Pair B of the cable  362  communicates with a primary of a transformer  367 . A secondary of the transformer  367  communicates with the selector switch  364 , which selects either the transmitter  368  or the filter  366 . 
     A load  371  and a controller  372  are connected across center taps of the primaries of the transformers  363  and  367 . The load  371  includes, for example, the load of the receiver  365 , the transmitter  368  and other circuits in the cable-powered POE device  351 . The controller  372  controls the position of the selector switch  364 . In a de-energized state or when power is not supplied over data the cable  362 , the selector switch  364  connects the secondaries of the transformer  363  and  367  to the filter  366 . Typically the filter  366  is a low-pass filter. 
     The controller  372  detects when the network device  350  supplies power to the cable  362 . Since the load  371  is in parallel with the controller  372 , power is also supplied to the load  371  at the same time as power is supplied to the controller  372 . When power is supplied to the controller  372 , the selector  364  is controlled to connect the secondary of the transformer  363  to the receiver  365  and the secondary of transformer  367  to the transmitter  368 . At substantially the same time, power is supplied to the receiver  365 , the transmitter  368  and the other circuits of cable-powered POE device  351 . At this point, the cable-powered POE device  351  can begin autonegotiating with the network device  350 . 
     The cutoff frequency of the low-pass filter  366  filters out fast link pulses (FLPs). Without the filter  366 , when the POE  351  communicates with a non-POE enabled network device, the FLPs generated by the non-POE network device could be sent back to the non-POE network device. The non-POE network device may receive the FLPs that it sent and attempt to establish a link with itself or cause other problems. The filter  366  will also adversely impact the cable test. Thus, the network device  350  transmits test signals having pulse widths greater than FLPs, which will pass through the low-pass filter  352 . Once the selector switch closes, the network device  350  performs cable testing. 
     For additional details concerning these and other POE devices, see “Method and Apparatus for Detecting and Supplying Power by a First Network Device to a Second Network Device”, U.S. patent application Ser. No. 10/098,865, filed Mar. 15, 2002, and “System and Method for Detecting A Device Requiring Power”, WO 01/11861, filed Aug. 11, 2000, which are both incorporated by reference in their entirety. 
     Referring now to  FIG. 10B , steps for performing the cable test when the network device may be connected to POE devices are shown generally at  380 . Common steps from  FIG. 9  have been identified using the same reference number. If the link is not up in step  338 , control continues with step  382  where control determines whether the filter  366  is detected. Is false, control continues with step  340  as described above. If the filter  366  is detected, control powers up the POE device in step  384 . In step  386 , control determines whether the selector switch  355  is on. If not, control loops back to step  386 . Otherwise, control continues with step  340  as described above. 
     Referring now to  FIG. 11 , a network device  400  includes a physical layer device  408  and a cable tester  412 , as described above. For example, the network device  400  can be a switch  404  that includes an n port physical layer device  408  and a cable test module (CTM)  412 . However, any other network device that contains a physical layer device can be used. Cables  314 - 1 ,  314 - 2 , . . . , and  314 - n  can be connected to the switch  404  using connectors  318 - 1 ,  318 - 2 , . . . , and  318 - n , such as RJ-45 connectors or any other suitable connector type. The switch  404  can be connected to other network devices such as, but not limited to, computers, laptops, printers, fax machines, telephones and any other network device or POE. In the embodiment shown in  FIG. 11 , the network device  400  initiates the cable test when powered on by a power supply  416 . The cable test can be initiated manually and/or automatically on power up. The network device  400  also includes one or more LEDs  326 - 1 ,  326 - 2 , . . . , and  326 - n.    
     Referring now to  FIG. 12 , steps for operating the network device  400  are shown generally at  430 . Control begins with step  432 . In step  434 , control determines whether power is on. When power is on, control sets a port equal to 1 in step  436 . In step  438 , control performs the cable test as described above. In step  440 , control determines whether all of the ports have been tested. If not, control increments the port and returns to step  438 . If the network device has only one port, the steps  436 ,  440  and  442  may be skipped. Otherwise, control displays the results in step  444  and control ends in step  446 . As can be appreciated by skilled artisans, the cable test can be executed sequentially for each port as set forth above or simultaneously for all ports. 
     Referring now to  FIG. 13 , additional steps are performed when the network device may be connected to power over Ethernet (POE) devices as shown generally at  460 . Common steps from  FIG. 12  have been designated using the same reference number. In step  470 , control determines whether the filter  466  is detected. If false, control continues with step  438  as described above. If a filter is detected, control powers up the POE device in step  474 . In step  478 , control determines whether the switch is on. If not, control loops back to step  478 . Otherwise, control continues with step  438  as described above. 
     Referring now to  FIGS. 14A–14E , control successively tests each port. Each port may be associated with one or more LEDs. During normal operation, the LEDs are used to indicate the presence or absence of a link, link activity, link speed or any other information. These same LEDs are also used to indicate testing in progress and the results of the cable test. As can be appreciated, other than the addition of the cable test module, no other hardware needs to be added. 
     When testing, the CTM may optionally turn on, turn off, or blink one or more of the LEDs to designate that a cable test is occurring on the associated port. Each of the ports are tested one or more times sequentially, randomly or in any order. When the tests are complete, the network device indicates the results using the LEDs, for example as shown in  FIG. 15 . For example, turning on the LED associated with a port indicates that a good cable communicates with the port. Turning the LED off indicates an open circuit. Blinking the LED indicates a short. As can be appreciated, the on, off and blinking states or speed and LED color can be assigned in a different manner to cable states of good, open, and short. The LEDs can be monochrome or color. Color LEDs can be used to indicate additional information such as the relative location of the failure (such as near, intermediate, far or other distance ranges), the identification of the signal pair with the fault, whether the fault relates to impedance mismatch, and/or the magnitude of the measured impedance (such as low, medium, high, open). By using existing, fully burdened LEDs to indicate the results of the cable test, the present invention provides lower cost network devices with built-in cable testing. While only one LED per port is shown in  FIGS. 14 and 15 , the network device may also include additional LEDs that are associated with each port as shown in  FIG. 16 . 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.