Abstract:
A signal monitor device that detects a signal propagating on a signal line and that generates a timestamp when the signal is detected. The timestamp may be used in a variety of applications including measuring the propagation delays on signal lines and determining the timing in a system.

Description:
BACKGROUND  
       [0001]     Signal lines may be used to carry a wide variety of electrical signals in a wide variety of systems. Examples of signal lines include wires, coaxial cables, twisted pairs, optical fibers, etc. Signal lines may carry analog signals, digital signals, encoded signals, data packets, etc.  
         [0002]     It may be desirable in a variety of circumstances to determine a time when an electrical signal propagating on a signal line reaches a particular point along the signal line. For example, the times when an electrical signal reaches different points along a signal line may be used to determine a propagation delay of the signal line. In another example, the times when different electrical signals in a system reach different points along different signal lines in the system may be used to determine the overall timing of the system.  
         [0003]     Oscilloscopes, logic analyzers, and similar instruments may be used to detect the presence of an electrical signal at a particular point along a signal line. Unfortunately, oscilloscopes, logic analyzers, and similar instruments are relatively complex and expensive and may not be practical to use in some environments and physical locations.  
       SUMMARY OF THE INVENTION  
       [0004]     A signal monitor device is disclosed that detects a signal propagating on a signal line and that generates a timestamp when the signal is detected. The timestamp may be used in a variety of applications including measuring the propagation delays on signal lines and determining the timing in a system.  
         [0005]     Other features and advantages of the present invention will be apparent from the detailed description that follows.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention is described with respect to particular exemplary embodiments thereof and reference is accordingly made to the drawings in which:  
         [0007]      FIG. 1  shows one embodiment of a signal monitor device according to the present teachings;  
         [0008]      FIG. 2  shows an embodiment of a signal monitor device that provides active sensing of a signal on a signal line;  
         [0009]      FIG. 3  shows an embodiment of a signal monitor device that includes circuitry for inserting a timed signal onto a signal line;  
         [0010]      FIG. 4  shows a system that employs a set of signal monitor devices according to the present teachings.  
     
    
     DETAILED DESCRIPTION  
       [0011]      FIG. 1  shows one embodiment of a signal monitor device  10  according to the present teachings. The signal monitor device  10  includes a signal detector  40 , a timestamp latch  42 , a clock  44 , and a device controller  46 . In one embodiment, the clock  44  maintains a time-of-day. The device controller  46  includes circuitry that is adapted from communication via a network  100 . The network  100  may be a wire-based network or a wireless network. The network  100  may be a local area network, e.g. Ethernet.  
         [0012]     The signal monitor device  10  includes a pair of connectors  20  and  22  that enable the signal monitor device  10  to be inserted in the path of a signal line  30  in between portions  30   a  and  30   b  of the signal line  30 . The connectors  20  and  22  are adapted to the physical implementation of the signal line  30 . For example, if the signal line  30  is a coaxial cable then the connectors  20  and  22  are adapted for coaxial cables, e.g. BNC connections. In other embodiments, the connectors  20  and  22  may be adapted for connections to wires, twisted pairs, optical fibers, transmission lines of a local area network, e.g. Ethernet, to name a few examples.  
         [0013]     The signal monitor device  10  includes a signal line  32  that is a short length of the same signal medium of the signal line  30  so that a signal entering the connector  20  propagates along the signal line  32  and out the connector  22 . For example, if the signal line  30  is a coaxial cable then the signal line  32  is a short length of coaxial cable.  
         [0014]     The signal detector  40  senses the electrical states of the signal line  32  and detects a signal on the signal line  32 . In one embodiment, the signal detector  40  presents a relatively high impedance to the signal line  32 .  
         [0015]     In one embodiment, the signal detector  40  includes circuitry for detecting an edge, e.g. rising edge or falling edge, of a TTL signal on the signal line  32 . The signal detected by the signal detector  40  may be a predetermined pattern in a TTL signal, e.g. an encoded information signal. In another embodiment, the signal detector  40  includes circuitry for detecting an edge or a predetermined pattern of a low voltage differential signal (LDVS).  
         [0016]     In another embodiment, the signal detector  40  includes circuitry for detecting packets, e.g. Ethernet packets. The signal detector  40  may detect a packet header or other pattern on the signal line  32 . The signal detector  40  may include circuitry for decoding packets so that particular types of packets may be detected.  
         [0017]     When the signal detector  40  detects a signal on the signal line  32  it generates a signal  50  that causes the timestamp latch  42  to latch a time  52  from the clock  44 . The latched time in the timestamp latch  42  is provided to the device controller  46  as a timestamp  48 .  
         [0018]     The device controller  46  in some embodiments corrects the timestamp  48 . For example, the device controller  46  may correct the timestamp  48  by subtracting the propagation delay of a signal from the connector  20  to the signal detector  40  and the delay in latching the time  52  from the clock  44  in response to the signal  50 .  
         [0019]     The device controller  46  transfers the timestamp  48  via the network  100  in a message  70 . Other devices on the network  100  may receive the message  70  and use the timestamp  48  carried in the message  70  as appropriate. For example, a remote device on the network  100  may use the timestamp  48  carried in the message  70  in a determination of a propagation delay of the signal line  30  or in evaluating the timing of a system.  
         [0020]     For example, a controller  60  on the network  100  receives the message  70  via the network  100  and extracts the timestamp  48  carried in the message  70 . The controller  60  includes a clock  62  that provides a time-base for evaluating the timestamp  48 . For example, if the timestamp  48  is 10:01:22 AM and a local time in the clock  62  is 10:01:23 AM then it indicates that a signal was detected by the signal monitor  10  one second earlier. In many applications the resolutions of the clocks  44  and  62  are much higher than the one second resolution in this example.  
         [0021]     The signal monitor  10  and the controller  60  in one embodiment include mechanism for engaging in a time synchronization protocol to synchronize the times held in the clocks  44  and  62 . One example of a protocol for synchronizing the clocks  44  and  62  is the IEEE 1588 time synchronization protocol which includes the signal monitor  10  and the controller  60  exchanging timing messages via the network  100 .  
         [0022]     In an alternative to the connectors  20  and  22 , the signal monitor device  10  may include a sensor probe for sensing a signal on the signal line  30  without a physical connection to the signal line  30 . For example, a sensor probe may detect electromagnetic energy emanating from the signal line  30  or may detect stray light emitted from the signal line  30 .  
         [0023]      FIG. 2  shows an embodiment of the signal monitor device  10  that provides active sensing of a signal on the signal line  30 . The signal monitor device  10  in this embodiment includes an input circuit  80  and an output circuit  82  that buffer a signal received the signal line  30 . The input circuit  80  and the output circuit  82  also enable a determination of a direction of a signal on the signal line  30 .  
         [0024]     The input circuit  80  and the output circuit  82  are adapted to the physical implementation of the signal line  30 . In some embodiments, the input circuit  80  includes a physical interface circuit (PHY) for decoding a signal received via the signal line  30  and the output circuit  82  includes a PHY for encoding an output of the signal back onto the signal line  30 . For example, if the signal line  30  is an Ethernet line then the input circuit  80  and the output circuit  82  include circuitry for a media independent interface (MII). If the signal line  30  is LDVS then the input circuit  80  and the output circuit  82  include circuitry for isolating the signal detector  40  from LDVS. In other embodiments, the input circuit  80  and the output circuit  82  do not decode and encode but instead buffer a signal by matching the impedances of the signal line  30 .  
         [0025]      FIG. 3  shows an embodiment of the signal monitor device  10  that includes circuitry for inserting a timed signal onto the signal line  30 . The signal monitor device  10  in this embodiment includes a time signal generator  90  and a multiplexor (MUX)  92 .  
         [0026]     The device controller  46  programs a trigger time  94  into the time signal generator  90 . The trigger time  94  may be received in a message  72  via the network  100 , e.g. from the controller  60 . The time signal generator  90  generates a timed signal  98  when the local time  52  from the clock  44  matches the trigger time  94 . The MUX  92  provides the timed signal  98  to the output circuit  82  which applies it to the signal line  30 . The MUX is controlled by the device controller  46  via a signal  95  so that the device controller  46  may select either a signal received via the input circuit  80  or the timed signal  98 .  
         [0027]      FIG. 4  shows a system  200  that employs a set of signal monitor devices  210 - 213  according to the present teachings. The system  200  includes a set of devices  250 - 252  that are interconnected via a set of signal lines  220 - 224 .  
         [0028]     The signal monitor devices  210 - 213  enable a controller  240  to gather a view of system timing in the system  200 . For example, the signal monitor device  210  generates a timestamp Ta when the device  250  puts a signal onto the signal line  220 , the signal monitor device  211  generates a timestamp Tb when the signal generated by the device  250  reaches an input to the device  251 . The signal monitor device  212  generates a timestamp Tc when the device  251  puts a signal onto the signal line  222  and the signal monitor device  213  generates a timestamp Td when the device  252  puts a signal onto the signal line  224 .  
         [0029]     The controller  240  obtains the timestamps Ta-Td from the signal monitor devices  210 - 213  via a network  230 . The controller  240  includes a clock  260  that provides a time base for evaluating the timestamps Ta-Td. For example, the timestamps Ta and Tc and Td may correspond to events in the system  200 . The controller  240  may use the timestamps Ta-Td to adjust the timing in the system  200 , e.g. by reprogramming one or more the devices  250 - 252 , adjusting cable lengths, etc., to a achieve an overall system timing goal.  
         [0030]     In one embodiment, the clock  260  and the respective clocks in the signal monitor devices  210 - 213  synchronize their time-of-day using the IEEE 1588 time synchronization protocol by exchanging timing messages via the network  230 . Other time synchronization protocols may be used, e.g. NTP.  
         [0031]     The signal monitor devices  210  and  211  are also used to measure a propagation delay of the signal line  220 . For example, the signal monitor device  210  generates a first timestamp when the device  250  puts a signal on the signal line  220  and the signal monitor device  211  generates a second timestamp when that signal reaches the device  251 . The controller  240  obtains the first and second timestamps from the signal monitor devices  210  and  211  via the network  230  and determines the propagation delay of the signal line  220  in response to the first and second timestamps.  
         [0032]     The system  200  is provided as a simple example to illustrate a system that may benefit from the use of a signal monitor device according to the present teachings. A system that employs signal monitoring devices may be relatively complex and/or widely dispersed and may include locations or environments in which instruments such as oscilloscopes and logic analyzers are difficult if not impossible to use. One example of a system that may benefit from a signal monitoring device according to the present teachings is an automatic test equipment (ATE) system having a unit under test (UUT) and a set of instruments that are interconnected via signal lines, e.g. trigger lines. Signal monitoring devices according to the present teachings may be used to measure propagation delays on the trigger lines of an ATE system to determine overall timing of trigger signal distribution throughout an ATE system.  
         [0033]     The foregoing detailed description of the present invention is provided for the purposes of illustration and is not intended to be exhaustive or to limit the invention to the accurate embodiment disclosed. Accordingly, the scope of the present invention is defined by the appended claims.