Abstract:
A system and method to detect ground faults in a multi-processor communication system that includes a communications link that extends between the processors. The processors isolate different sections of the link. Measurements made with different isolated sections establish the location of the fault. The processors can also make measurements of the link to establish the magnitude of a shunt impedance on the link. Where an abnormal value of shunt impedance is detected, the respective devices can activate or deactivate solid state isolation circuitry to disconnect a section of the link exhibiting the abnormal level of shunt impedance.

Description:
FIELD OF THE INVENTION 
     The invention pertains to diagnostic devices and methods for use with communication links. More particularly, the invention pertains to such devices and methods which can be used to determine the presence or absence of shunt impedances as well as the presence or absence of ground faults on communication links. 
     BACKGROUND OF THE INVENTION 
     Multiprocessor communication systems which provide bidirectional communication capabilities for each of the processors using a bidirectional communication link are known. Such systems are often associated with alarm or monitoring functions. 
     One known form of such a system incorporates a common control unit which is connected to a two-wire electrical cable which extends throughout a region to be monitored. A plurality of detectors can be coupled across the wires of the link at locations corresponding to portions of the region to be monitored. Additionally, output devices can also be coupled across the wires of the link and can be used to provide various output functions such as energizing or de-energizing solenoids or energizing or de-energizing audible or visual annunciators. 
     It has also been recognized that such communication links at times exhibit deteriorating performance due to shunt impedances or ground faults. While there have been approaches developed in the past which attempt to establish the location of one or more such conditions on Class A loops, it would be desirable to ascertain the existence of shunt impedances across the wires of the link or the location of ground faults in other link configurations. Preferably, it would be desirable to provide such functionality without having to substantially increase the base cost of such systems for a given number of detectors. Additionally, it would be preferred if the desired functionality can be provided without substantially increasing system complexity. 
     SUMMARY OF THE INVENTION 
     A multi-processor communication system, in one aspect, includes a common communication link which could be implemented as a two-wire cable. Coupled to the link are the members of a plurality of electrical devices some of which might be substantially identical. 
     Typical electrical devices would include ambient condition detectors such as smoke detectors, position detectors, motion detectors, gas detectors, humidity detectors, and the like. Other types of electrical devices could be used for energizing or de-energizing solenoids so as to implement output functions which could include locking or unlocking, opening or closing doors or windows, energizing or enabling audible or visible output devices of various types to convey messages, warnings or the like. In another aspect, the detectors can be clustered on one communication link, and some of the output devices, if desired, clustered on another communications link. 
     The electrical devices can each include a local control circuit, which could be implemented as a programmed processor, and associated instructions. In one aspect, the local control circuit is coupled to a solid state or mechanical switch which can be used to open circuit the communication link in the vicinity of the electrical unit. A common control unit, if present, can in turn include similar solid state or mechanical switches which under the control of electrical circuitry therein can isolate the communication link from that control unit. 
     In order to measure the shunt resistance of the communication line, the devices contain drive circuitry to drive a current across the two wires of the communication line. The current is driven in a manner that the devices connected across the wires will not interfere with the measurement of the shunt resistance across the two wires. 
     The devices measure the voltage drop produced across the wires by the current and determine if the line shunt resistance is greater than a predetermined value. If the shunt resistance is lower than the predetermined value, a trouble indication can be generated. 
     In a preferred aspect, the current is driven in a polarity opposite to the polarity present when delivering power to the devices. In this manner, each device appears to be an open circuit or at least a very large impedance when the polarity is reversed. The voltage developed across the line will then be determined by the line shunt resistance. 
     In another aspect, the current can be driven with the same polarity as the polarity present when delivering power to the devices. However, in this case, it would be driven at very low value such that the voltage developed across the wires of the communication link is less than the voltage value required to deliver power to the devices. Since the devices do not have sufficient voltage to draw power from the line, the voltage on the line will be determined by the line shunt resistance. 
     In a preferred apparatus, each device also contains an isolation circuit such that it can open circuit the link. The isolation circuit can be implemented using solid state semiconductor switches. 
     Since each device open circuits the link, each device only checks the portion of the link between itself and an adjacent device. If a shunt resistance across the two wires of the communication link is less than a predetermined value, a trouble indicator can be generated and the device address indicates the location of the shunt resistance. A voltage drive could be substituted for a current drive since these parameters are related. 
     When a low shunt resistance has been indicated, the device can be commanded to open circuit the link to isolate the section of the communication link that has a low shunt resistance. This will prevent it from adversely affecting the communication process. 
     In many systems, the communication link is not connected to the earth ground or external shielding around the link. A resistive leakage path, or ground fault, can occur from either or both wires of the communication link to earth ground. This may potentially cause communication problems. It would be desirable to be able to identify that this leakage resistance to earth ground, or ground fault, exists prior to encountering communications problems. 
     A preferred method includes checking to determine if a link exhibits a ground fault. In one embodiment, the communication link includes two conductors. 
     One conductor is connected to all devices and cannot be open circuited by the devices. Each device has a capability to open circuit the second conductor. 
     To test the link, the devices open circuit the second conductor at spaced apart locations on the link. When a respective device is between the ground fault and a predetermined unit, the ground fault will disappear relative to the predetermined unit. 
     When the ground fault is between the unit and the device, the ground fault will be detected by the unit. By determining which device is closest to the ground fault and still between the ground fault and the unit, the location of the ground fault can be determined. 
     The predetermined unit can correspond to another device. Alternately, if the system includes a common control unit, coupled to the link, to implement various system-wide communication functions, the predetermined unit can correspond to the common control unit. 
     A predetermined searching pattern can be used to determine the location of the ground fault. A binary search pattern will find the ground fault with the least number of steps. 
     One method of locating a ground fault to one of the conductors of a communication link includes selecting an initial location on the link. Where the initial location corresponds to an end of the link, another displaced location on the link is selected. The link is open circuited at the another location. 
     Each of the segments can then be tested by use of voltage-type test signals to determine which segment includes the fault. Relative of this segment which indicates or includes a fault, another displaced location is selected and the line is open circuited. Each of the shortened line segments is then tested to determine which segment includes the fault. As noted above, a binary search pattern can be used and will determine the location of the fault in a minimal number of steps. 
     In yet another aspect, the isolation switches of each of the electrical units can be opened. An attempt can then be made to cause a test current to flow between an earth ground and the common conductor of the communication line which is not open circuited. The presence of this test current can be detected using a ground fault detect circuit coupled between the earth ground and the continuous conductor of the communications link. The current can be applied by either the electrical units, acting in parallel, or from another source such as the ground fault detect circuit or a supply elsewhere in a common control unit coupled to ends of the communication link. 
     In either instance, the presence of a ground fault between the open circuited conductor of the link and the earth ground can be detected by one or more adjacent electrical devices. Non-adjacent electrical devices are isolated from the ground fault due to the open circuited isolation switches of the adjacent electrical devices. Hence, the location of the ground fault can be undetermined, between two adjacent electrical devices. In that instance, the ground fault can be isolated from the communication link by maintaining the isolation switches of the two adjacent electrical devices in their respective open circuit states and short-circuiting the isolation switches of the remaining electrical units thereby enabling normal communication to re-initiated. 
     Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a communication system in accordance with the present invention; 
     FIG. 2 is a block diagram of a portion of the communication link of FIG. 1 illustrating additional details of electrical units coupled thereto and circuitry for locating the presence of a shunt resistance; 
     FIG. 3 is a block diagram of a portion of an alternate form of the system of FIG. 1 configured so as to locate a detected ground fault; 
     FIG. 4 is a block diagram of a portion of another form of the system of FIG. 1 configured to carry out an alternate form of ground fault locating; and 
     FIG. 5 is a block diagram of yet another form of the system of FIG. 1 which can be used to locate a ground fault and 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While this invention is susceptible of embodiment in many different forms, there are shown in the drawing and will be described herein in detail specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated. 
     FIG. 1 illustrates a multi-processor communications system  10  which can be used for monitoring a plurality of conditions in one or more regions to be supervised. As illustrated, the system  10  includes a common control unit  12  which could be implemented as one or more interconnected programmed processors and associated, prestored instructions. 
     The unit  12  includes an interface for coupling, for example, to a communications medium  14 , illustrated in FIG. 1 for exemplary purposes only as an optical or electrical cable. 
     Coupled to medium  14  is a plurality of ambient condition detectors  18  and a plurality of control or function units  20 . It will be understood that the arrangement of the members of the pluralities  18  and  20  relative to the medium  14  is not a limitation of the present invention. The members of the plurality  18  can include intrusion sensors,position sensors, gas sensors, fire sensors such as smoke sensors, thermal sensors or the like, all without limitation. The members of the plurality  20  can include solenoid actuated control or function implementing units, display devices, printers or the like. 
     Also coupled to the control unit  12  via a medium  24 , illustrated for example as a pair of electrical cables, is a plurality  26  of output devices. These could include audible or visible output devices without limitation, speech output devices and the like. The devices  26  are intended to broadcast a message, which might indicate alarm condition, in one or more predetermined regions. 
     It will be understood that the purpose for which the system  10  is being used is not a limitation of the present invention. The invention can be incorporated into local area networks which, for example, only transfer information between various devices coupled thereto. The invention can also be incorporated into peer-to-peer type communications systems which do not incorporate an element that corresponds to the common control unit  12  without departing from the spirit and scope of the present invention. 
     FIG. 2 illustrates a system and method for locating a shunt resistance. Three devices  18   a, b, c  and the control unit  12  are illustrated. The following comments also apply to members of the plurality  20 ,  26 . 
     Each device, such as  18   a,  includes an isolator switch SWa. Each of the devices  18   a, b . . . n,    20   a, b . . . n,    20   a . . .    20   m  includes a control element which can be implemented, at least in part as a programmed processor P- 18   a ,- 18   b  . . .  18   n  and P- 20   a , - 20   b  . . . - 20   m  and associated instructions. Alternately, the members of the pluralities  18 ,  20 ,  26  can be implemented using various forms of hardwired logic. 
     The processors each include output control ports or drivers A 1 , A 2  and analog input port A 3 . The analog receiver A 3  includes an A/D converter. Alternately, the A/D converter can be a separate circuit external to the respective processor. The converter is used to detect a threshold value. The drivers (A 1 , A 2 ) could be a single drive pin from the respective processor or the output of an external drive circuit. 
     Each of the devices  18   a  . . .  20   m  includes a local power supply such as INTa, INTh . . . . Each local supply can receive electrical energy from link  14 . Alternately, each such supply can receive energy from another source such as a battery. 
     During the shunt test, all switches SW 12 - a,  SW 12 - b,  SWa . . . SWn are opened. The conductor  14   a  is thus open circuited at each device, including at the control unit  12 . 
     Next, the drive circuits (A 1 , A 2 ) are turned ON so that a current (I 1 , I 2 , I 3 ) flows to the conductor  14   b.  Current I 1  has no return path to the internal ground of device  18   a.  Hence, the conductor  14   b  is driven to a high positive voltage relative to that device&#39;s internal ground. This voltage is measured at input port A 3 . 
     If there is no shunt, all devices will produce a high positive voltage relative to their internal grounds. This indicates that there is no shunt resistance present across the link  14 . 
     On the other hand, if there is a shunt resistance at L to L′ between device  18   b  and device  18   c,  then there will be a return path for the I 2  and I 3  currents to return to their respective internal grounds through Q 2  and Q 1  respectively of these devices. The amplitude of voltage developed across the link  14  (across the shunt resistance) is then a function of the internal resistance R in the current path in each device. [V(common)−V(+)]=[VINTi−V(D 1 )−V(D 3 )−V(Q 1 )]*(shunt resistance)/(R/ 2 +shunt resistance). 
     The value of R is divided in two because device  18   b  is in parallel with device  18   c  so that R values are effectively in parallel. If the voltage developed across the link  14  is lower than a predetermined value, then the devices will recognize that a shunt resistance is present with a value that is lower than a predetermined value. A trouble indication can be generated. The devices may then leave their respective isolator switch, SWa, SWb . . . , open to prevent the shunt impedance from affecting communications on the two conductors  14   a, b.    
     All devices that do not detect a low shunt resistance, will close or short circuit their respective isolator switch. This will restore the connection of the conductor  14   a  to the other devices. Likewise, the control unit  12  will close SW 12 - a , - b  and restore power delivery and communication to the link  14 . Only switches SWb, SWc will stay open. 
     In many cases, a second pair of wires is connected from the last device, such as  20   m,  back to the control unit  12  so that power and communications can then reach device  18   c.  All devices will then receive power to continue normal operation. However, the shunt resistance from L to L′ is isolated from the communication line. 
     FIG. 3 illustrates a system  10 - 1  for detecting the location of a ground fault. Common components of system  10 - 1  have been assigned the same identification numerals as in FIG.  2 . In FIG. 3, processors P- 18   a′  . . . P- 18   n′  could be implemented using processors P- 18   a  . . . P- 18   n  as previously discussed relative to FIG. 2 but with expanded or additional instructions to carry out a ground fault test function. 
     In the system  10 - 1  control unit  12 ′ includes control switches SW 12 ′- a  and SW 12 ′- b.  These switches, could be implemented as field-effect transistors, bipolar transistors or any other type of controllable solid state or mechanical switch. The same comments apply to switches SWa, SWb, SWc . . . . 
     Switches  12 ′- a,    12 ′- b  open circuit respective conductors  14   a, b.  Unit  12 ′ also includes ground fault detect circuitry  12 ′- 1 . The structure and operation of circuitry  12 ′- 1 , which could be implemented in part with pre-programmed instructions used to control one or more processors in unit  12 ′, are discussed below. 
     One ground fault locating method involves starting at the unit  12 ′ and progressing out therefrom one device at a time. The steps include opening and closing the respective isolator switches SWa, SWb, SWC . . . and, therefore, open circuiting and short circuiting the conductors  14   a  at each device, until the ground fault no longer disappears during the time when the device open circuits the conductor  14   a.  This identifies the device closest to the ground fault which is still between the ground fault and the control unit  12 . If the ground fault is after the first device, then only two checks are needed. If the ground fault is after 500 devices, 501 checks will be needed. This method can be used with T-taps on the communication link. 
     In another method, the devices can be configured to drive a current into the earth ground. The location of the earth ground shunt resistance (ground fault) at L to L′ can be determined. 
     During this test SW 12 - a  is opened and SW 12   b  is closed in the control unit  12 . The isolation switch SWi in each device open circuits the conductor  14   a  at each device. Then, each device drives a current (I 1 , I 2 , I 3 ) into the conductor  14   b  which can then flow to the control unit(I) and out to the earth ground (I′). If there is no earth ground shunt resistance, then I′=0 and therefore I 1 =0, I 2 =0, and I 3 =0. 
     On the other hand, if there is an earth ground shunt resistance at L to L′, then current can flow through the earth ground shunt resistance to complete the current path back to devices adjacent to point L. In this case, I′=I 2 +I 3 , I 1 =0 because the isolator SWb in device  18   b′  prevents formation of a complete path back to device  18   a′.    
     Devices  18   b′  and  18   c′  both sense that a current is flowing internally and set an earth ground fault indicator or trouble signal. This trouble signal can then be transmitted to the control unit  12 ′ or some other device for indication and display. Since both device  18   b′  and device  18   c′  indicate a ground fault, then the earth ground shunt resistance L to L′ is determined to be between device  18   b′  and device  18   c′.    
     In another system  10 - 2  and method illustrated in FIG. 4, a ground fault circuit  12 ′- 2  is connected between the control unit  12 ′ and earth ground. The ground fault circuit  12 ′- 2  applies a voltage V′ to earth ground. This voltage (V′) may be applied continuously as a pulse or in some other form, including modulated signals. 
     If a ground fault is present, then, a current (I′) will flow into the earth ground connections. The ground fault location check is made when the isolator switches SWa, SWb . . . are opened. 
     When the isolator switches SWa, SWb . . . are open, current I′ will then flow through the shunt resistance, L to L′, and create a voltage at the input circuits B 2  of device  2  and B 1  of device  3 . The current will flow through the devices to the conductor  14   b  and return to the control unit  12 ′ and the ground fault circuit  12 ′- 1 . 
     Device  18   a  will not see any voltage at its input because the isolators prevent any currents from flowing to the conductor  14   a  coupled to device  18   a″.  Devices  18   b″  and  18   c″  then will report that they received ground fault signals. This will determine the location of the ground fault. 
     If there are isolators in the negative wire and not in the positive wire, then the same principles apply. The ground fault circuit can apply a voltage (V″) and a detection means in the devices senses a voltage developed by the devices adjacent to the shunt resistance. A negative V′ can be applied and the devices sense a voltage at the inputs B 1  or B 2 . 
     Two isolators can be used with each device if they have diodes across each of them so that current can flow in one direction as shown in FIG.  5 . The basic concept is that the isolators prevent other devices from detecting a ground fault signal. 
     Only the device(s) adjacent to the shunt resistance connection detect the ground fault signal. If there are isolators in the negative wire and not in the positive wire, then the same principles apply. The ground fault circuit can apply a voltage (V′) and a detection means in the devices senses a voltage developed by the devices adjacent to the shunt resistance. A negative V′ can be applied and the devices sense a voltage at the input B. 
     Other equivalent sensing means can be used such that only the devices adjacent to the shunt resistance detect the ground fault circuit signals when the isolators are open. Other than a pulse, modulated waveforms or other signals can be applied to the earth ground. Either a positive or negative voltage (V′) can be used, depending upon the type and polarity of the input circuit in the devices). 
     From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.