Abstract:
The present invention relates to an apparatus and a method for detecting an open circuit fault condition in a differential signal, and generating a fault detection signal. An open circuit fault condition is detected by employing weak current sources to pull the differential signal paths outside the valid ac common-mode range and toward the supply rails. If both signal paths can be pulled within a predetermined proximity to the supply rails by their respective weak current sources, an open condition fault is defined to exist and a fault detection signal is generated. The fault detection signal can be used by another device to report the fault condition.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and a method for detecting an open circuit fault condition in a common-mode differential signal. A fault detection signal is generated for use by another device to report the fault condition. 
     BACKGROUND OF THE INVENTION 
     Differential signaling has been in existence for many years. For example, teletypes were some of the first equipment to use differential signaling to communicate. One common differential signaling technique utilizes a current loop to send and receive information between a receiver and transmitter. 
     According to this differential signaling technique, information is represented by current sent in one direction around the loop, or the other direction around the loop. A pulse of current in the loop in one direction may correspond to a logic value of “1”, while a pulse in the opposite direction may correspond to a logic value of “0.” 
     Current loops have several advantages over other signaling techniques. For example, data sent utilizing a current loop can travel further than data sent through a common RS-232 interfaces. Current loop differential signaling techniques also provide protection against electrical interference. Additionally, current loop differential signaling techniques can reliably make connections when other communication techniques cannot. 
     One type of a current loop differential signaling technique is Low Voltage Differential Signaling (LVDS). LVDS is a differential signaling technique commonly used in data transmission systems. LVDS uses relatively low supply voltages; V DD  is generally in the range of 2.5 volts and V SS  is generally zero. The valid common-mode range for a LVDS receiver is generally between V SS +50 mV and V DD −50 mV. A low voltage differential signal produced by a line driver typically has peak-to-peak amplitudes in the range from 250 mV to 450 mV. The low voltage swing minimizes power dissipation, while maintaining high transmission speeds. Typical transmission speeds are over 100 Mbps (Mega-bits per second). 
     SUMMARY OF THE INVENTION 
     The present invention is directed to detecting an open circuit fault condition in a differential signal. More specifically, the present invention is directed to providing an apparatus and a method for detecting when a differential signal is floating outside the valid common-mode range, and generating an open circuit fault detection signal. An open circuit fault condition prevents a valid differential signal from being obtained. 
     Briefly stated, the invention detects the existence of an open circuit fault condition by employing weak current sources to pull the two portions of the differential signal in opposite directions. The invention is also buffered from the differential signal source. Buffered signals are produced in response to the pulled differential signal. Portions of the buffered signal are compared to reference signals. Open circuit fault condition signals are produced when the amplitude difference between portions of the buffered signal and the reference signals meet predetermined criteria. An open circuit fault detection signal is produced from a comparison of open circuit fault condition signals. The open circuit fault detection signal is available to other devices to communicate the existence of an open circuit. 
     According to another example of the invention, the fault detection occurs with loading, altering, and disturbing the differential signal source. 
     According to yet another example of the invention, the reference signals are a predetermined amplitude toward the DC common-mode voltage of the differential signal from the local power supply rails. 
     According to a further example of the invention, the presence of an open circuit fault condition is signaled by a high control signal. 
    
    
     A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detail description of presently a preferred embodiment of the invention, and to the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a LVDS driver and receiver; 
     FIG. 2 is a graph illustrating LVDS signal swing and reference signals; 
     FIG. 3 is a block diagram illustrating an exemplary operating environment; 
     FIG. 4 is a block diagram illustrating the LVDS open circuit fault condition detector processing the high and low differential signals; 
     FIG. 5 is a block diagram illustrating the LVDS open circuit fault condition detector processing the high side of a differential signal; 
     FIG. 6 is a block diagram illustrating the LVDS open circuit fault condition detector processing the low side of a differential signal; 
     FIG. 7 is a block diagram illustrating an exemplary LVDS open circuit fault condition detector apparatus overview; 
     FIG. 8 is a schematic diagram illustrating an exemplary S+ fault condition detector; 
     FIG. 9 is a schematic diagram illustrating an exemplary S− fault condition detector; 
     FIG. 10 is a schematic diagram illustrating an exemplary converter; 
     FIG. 11 is a schematic diagram illustrating an exemplary output stage; and 
     FIG. 12 is a schematic diagram illustrating an exemplary embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanied drawings, which form a part hereof, and which is shown by way of illustration, specific exemplary embodiments of which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     Definitions 
     Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The term “connected” means a direct electrical connection between the things that are connected, without any intermediary devices. The term “coupled” means either a direct electrical connection between the things that are connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means one or more passive and/or active components that are arranged to cooperate with one another to provide a desired function. The term “signal” means at least one current signal, voltage signal or data signal. The meaning of “a”, “an,” and “the” include plural references. The meaning of “in” includes “in” and “on.” 
     Unless otherwise indicated, the type of transistors is generally not designated in the drawings, specifications, and claims herein. For the purposes of this invention, p-type and/or n-type transistors may be used unless expressly indicated otherwise. The transistors may be bipolar devices, MOS devices, GaAsFET devices, JFET devices, as well as one or more components that are arranged to provide the function of transistors. 
     Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or is inconsistent with the disclosure herein. 
     Overview 
     Under certain conditions, a LVDS device may experience a fault condition where its signal path is open, shorted, or terminated by abnormal means. A fault condition is produced when the differential signal is floating, or when the signal&#39;s peak-to-peak signal swing is very low or near zero (i.e., a short circuit AC condition.). When the signal path is open, the received signal may migrate to the supply voltages, or remain within the valid common-mode range. Any fault condition prevents a valid signal from being received by the receiver. As a result, the output signal of the device is unknown and undeterminable. This situation is not desirable. 
     Briefly described, the present invention detects an open circuit condition fault where the signal is outside the valid common-mode range. The fault is detected without loading or shifting a valid LVDS differential signal. A fault detection signal is provided for use by other circuits to report an open circuit condition. This invention does not address detecting other forms of differential signaling fault conditions. 
     As used herein, the terms “fault” and “fault condition” include the situation when the path in a differential signaling device is open and the differential signal is outside the valid common-mode range. The term “normal condition” means those situations not included within “fault condition.” A “normal condition” is where the signal remains within the valid common-mode range, such as regular operating conditions when the differential signal is valid, and other conditions when the signal remains inside the common-mode range but is otherwise invalid. 
     The operating environment for the open circuit fault detector will be described below with reference to FIGS. 1 and 3. 
     Illustrative Environment 
     FIG. 1 is an exemplary schematic diagram illustrating a LVDS driver and receiver system being monitored by a LVDS open circuit fault condition detector. The system includes driver  101 , receiver  130 , differential communication line pair  120 , load R 135 , and open circuit fault condition detector  400 . 
     Driver  101  includes a current source and two pairs of transistors, M 103b  and M 105a  driving current in one direction, and M 103a  and M 105b  driving current in the opposite direction. A typical current produced by driver  101  could be in the range of 4.0 mA. Differential line pair  120  couples driver  101  to receiver  130 . Receiver  130  has high input impedance and can be any device configured to accept a LVDS input. Load R 135  is a termination load that is connected across the LVDS inputs of receiver  130 , and can be in the range of 100 ohms. Receiver  130  detects a voltage signal that is driven across load R 135 . When driver  101  switches directions, the current flow changes direction across R 135 , and the signal across R 135  changes polarity. Receiver  130  detects the change in polarity as a “high” or “low” logic state (i.e., logic “1” or logic “0.” 
     In operation, open circuit fault condition detector  400  monitors the differential signal (S id ) input to receiver  130  without significantly loading it. 
     FIG. 2 illustrates a typical LVDS signal swing that may appear across load R 135  at the input terminals of receiver  130  illustrated in FIG.  1 . For example, driving 4.0 mA through R 135  will produce a 400 mV input differential signal (S id ) across the input terminals of receiver  130  in FIG. 1. A typical LVDS system may have a common-mode voltage (V CM ) between +0.2V and +2.2V. As shown in FIG. 2, a typical LVDS signal may have a common-mode DC voltage (V CM ) centered at 1.5 V, a high output signal level (S h ) at 1.7V, and low signal output level (S l ) at 1.3V, yielding a differential signal (S d ) with a 400 mV peak-to-peak. 
     In the present invention, a fault condition is determined to occur when the input differential signal S d  (or S id  of FIG. 1) is outside the valid common-mode range. An embodiment of the present invention establishes detection ranges lying outside the valid common-mode signal range. Reference signals (Sref+, Sref−) define the detection ranges. 
     FIG. 3 is a block diagram illustrating an exemplary operating environment for a LVDS open circuit fault condition detector. A typical operating environment can include the LVDS receiver input buffer  330  (a component of LVDS receiver  130 , FIG.  1 ), open circuit fault condition detector  320 , output driver  340  (a component of LVDS receiver  130 ), and alternative output receivers including digital indicator  350 , CMOS signal driver  360 , and other devices  370 . 
     LVDS open circuit fault condition detector  320  and LVDS receiver input buffer  330  are coupled in parallel to the differential signal (IN). LVDS receiver input buffer outputs Intermediate signal. LVDS open circuit detector outputs signal OUT. Output driver  340  receives signal Intermediate and outputs signal Out. CMOS signal driver receives signal OUT and outputs signal CMOS Out. Digital Indicator receives signal CMOS Out and outputs a report. Other  370  is an alternative output circuit for receiving signal OUT and providing a signal to other circuits or devices. 
     The disclosures related to FIGS. 5-11 provide a more detailed description of the operation of the LVDS open circuit fault condition detector. 
     FIG. 4 is a block diagram illustrating an exemplary configuration of LVDS open circuit fault condition detector  320  processing the two sides of differential signal IN (S+, S−) (see S h  and S l  at FIG.  2 ). LVDS open circuit fault condition detector includes a S+ fault condition detector  440 , a S− fault condition detector  450 , and output stage  410 . 
     S+ fault condition detector  440  has an input coupled to differential signal S+ and an output coupled to output stage  410 . S− fault condition detector  450  has an input coupled to differential signal S− and an output coupled to output stage  410 . Output stage  410  receives inputs from S+ fault condition detector  440  and from S− fault condition detector  450 , and outputs fault condition control signal OUT (S CH , S CL ). 
     In operation, S+ fault condition detector  440  receives the high side of the differential signal (S+) and outputs comparator signal Sa. S− fault condition detector  440  receives the low side of the differential signal (S−) and outputs comparator signal Sb. Output stage  410  receives comparator signals Sa and Sb and outputs fault condition control signal OUT (S CH , S CL ). 
     FIGS. 5 and 6 are block diagrams illustrating exemplary configurations of S+ and S− fault condition detectors ( 440 ,  450 ) processing the two sides of the differential signal S d  (S+, S−) (see S h  and S l  at FIG.  2 ). In FIG. 5, the S+ fault condition detector  440  includes pull-up current source  520   a , S+ input buffer  530   a , level shifter (down)  510   a , and N-type comparator  540   a.    
     Pull-up current source  520   a  is coupled to signal S+. The input of S+ Input Buffer  530   a  is coupled to Signal S+ and it outputs signal S+(2). The input of level shifter (down)  510   a  is coupled to Signal Sref+, and it outputs signal Sref+(2). The inputs of N-type comparator  540   a  are coupled to signals Sref+(2) and S+(2), and it outputs signal Sa. 
     In operation, both pull-up current source  520   a  and S+ input buffer  530   a  are coupled to signal S+. Pull-up current source  520   a  is a current source having high impedance, and is arranged to “pull-up” signal S+ toward voltage supply V DD  when S+ is weak, as occurs in an open circuit fault. In a normal condition, pull-up current source  520   a  is not strong enough to load, alter, or disturb signal S+. S+ input buffer  540   a  has high input impedance and very small input capacitance to minimize disturbing S+. S+ input buffer  540   a  outputs signal S+(2). 
     Signals Sref+ and Sref− are predetermined to define the detection range with respect to the valid common-mode voltage and local supply values V DD  and V SS , respectively, as illustrated in FIG.  2 . Level shifter (down)  510   a  is configured to match the level shift of S+ input buffer  530   a , such that Sref+ is shifted down by the same magnitude as S+. Level shifter (down)  510   a  outputs signal Sref+(2). N-type comparator  540   a  receives input signals Sref+(2) and S+(2), and outputs comparator signal Sa. The disclosure related to FIG. 8 contains additional details concerning the configuration and operation of S+ fault condition detector  440 . 
     FIG. 6 is a block diagram illustrating an exemplary configuration of a S− fault condition detector  450  processing the S− side of differential signal S d  (S+, S−). The S− fault condition detector includes pull-down current source  520   b , S− input buffer  530   b , converter  660 , level shifter (up)  510   b , and P-type comparator  540   b.    
     Signal S− is coupled to pull-down current source  520   b  and S− input buffer  530   b . The output of S− input buffer  530   b  is coupled to an input of P-type comparator  540   b . The input of level shifter (up)  510   b  is coupled to Signal Sref−, and its output is coupled to an input of P-type comparator  540   b . The inputs of P-type comparator  540   b  are coupled to the outputs of level shifter (up)  510   b  and the output of S− input buffer  530   b , and it outputs signal S−(3). Converter  660  has an input coupled to P-type comparator  540   b  and outputs signal Sb. 
     In operation, the S− fault condition detector  450  of FIG. 6 is substantially similar to the S+ fault condition detector  440  of FIG.  5 . However, the two detectors use different type channel devices. In an embodiment, the S+ fault condition detector employs n-channel devices and the S− fault condition detector employs p-channel devices. S− fault condition detector  450  additionally includes converter  660  to invert the output of p-type comparator  540   b  so that comparator signals (Sa, Sb) can be used processed by the same common mode logic as more fully described in FIG.  11 . 
     In operation, both pull-down current source  520   b  and S− input buffer  530   b  are coupled to signal S−. Pull-down current source  520   b  is a current source having high impedance. It is arranged to “pull-down” signal S− toward voltage supply V SS  when S− is weak, as occurs in an open circuit fault. In a normal condition, pull-down current source  520   b  is not strong enough to load, alter, or disturb signal S−. S− input buffer  540   b  has high input impedance and very small input capacitance to minimize disturbing signal S−. S− input buffer  530   b  outputs signal S−(2). Level shifter (up)  510   b  is configured to match the level shift of S− input buffer  530   b , such that its output Sref−(2) is shifted up by the same magnitude as S−(2). 
     Level shifter (up)  510   b  outputs Sref−(2) maintaining the predetermined detection range with S−(2). P-type comparator  540   b  receives input signals Sref−(2) and S−(2), and outputs comparator control signal S−(3). The disclosure related to FIG. 9 contains additional detail concerning the configuration of P-type comparator  540   b  and output control signal S−(3). Converter  660  receives output control signal S−(3) and outputs comparator control signal Sb. The disclosure related to FIG. 10 contains additional detail concerning the configuration and operation of converter  660 . 
     FIG. 7 is a block diagram illustrating an exemplary configuration of an embodiment of the invention using components previously disclosed in FIGS. 4-6. The block titles, figure numbers, connections, and operation are the same as in FIGS. 4-6. Another embodiment can exchange n-channel devices for p-channel devices, and employ a converter ( 660 ) as is appropriate so that the inputs to output stage  660  are compatible with its logic circuitry. 
     FIG. 8 is a schematic diagram of an exemplary S+ fault condition detector  440  according to an embodiment of the invention. As shown, S+ fault condition detector  440  includes n-type transistors M 7 , M 8 , M 21 , and M 22 ; current sources I 6 , I 20 , I 29 , and I 50 : and loads R 802  and R 804 . 
     Transistor M 7  has a base coupled to node N 854 , a drain coupled to node N 855 , and a source coupled to node N 860 . Transistor M 8  has a base coupled to node N 856 , a drain coupled to node N 858 , and a source coupled to N 860 . Transistor M 21  has a base coupled to signal Sref+, a drain coupled to V DD , and a source coupled to node N 854 . Transistor M 22  has a base coupled to signal S+, a drain coupled to V DD , and a source coupled to node N 856 . Loads R 802  and R 804  are coupled between V DD  and nodes N 855  and N 858 , respectively. Current sources I 6 , I 20 , and I 29  are coupled between nodes N 854 , N 860 , and N 856 , and V SS , respectively. Current source I 50  is coupled between V DD  and signal S+. 
     In operation, transistor M 22  functions as a source follower, acts as an input buffer isolating LVDS open circuit detector  320  from differential signal S+, and outputs single-sided signal S+(2). Transistor M 22  is a relatively small device with high impedance and very low capacitance input capacitance in the range of 45-60 fF. As a result, transistor M 22  does not does not add significant capacitance, or load, alter, or disturb signal S+. Current source I 50  is a weak, high impedance source that does not significantly load signal S+, and is arranged to “pull-up” signal S+ toward voltage supply V DD  when S+ is weak, as occurs in an open circuit fault. A pull-up can occur because a floating or open signal does not have any voltage holding it to any other value. In a normal condition, pull-up current source I 50  is not strong enough to disturb signal S+. Signal Sref+ is supplied by an external source, and its value is predetermined to provide a detection range between the high signal component (S h ) of the differential signal (S d ) and local power supply V DD  (See FIG.  2 ). For example, signal Sref+ could be set at 2.0 volts when V DD  is 2.5 volts and the high signal component (S h ) is 1.7 volts. This provides 300 mV of remaining headroom between a valid differential signal and signal Sref+ for detecting an open circuit fault. Transistors M 21  and M 22  are matched, and their current sources I 28  and I 29  are also matched. This equalizes the signal shift of S+ and Sref+ and maintains the predetermined detection range. Transistor M 22  outputs buffered signal S+(2) and transistor M 21  outputs signal Sref+(2). 
     Transistors M 7  and M 8  are a n-type differential pair, and act as a comparator in cooperation with loads R 802  and R 804 , and current source I 6 . Single-ended signal S+(2) is applied to the base of transistor M 8 . Reference signal Sref+(2) is applied to the base of transistor M 7 . Loads R 802  and R 804  are sized to allow approximately equal current flow through transistors M 7  and M 8  when signal Sref+(2) is substantially equal to signal S+(2). In an open fault condition, transistor M 8  has a larger base-to-source voltage than transistor M 7  because current source I 50  is able to pull up single-ended signal S+(2) into the detection range above signal Sref+(2). This causes more current to be steered through transistor M 8  and less current to be steered through transistor M 7 . Signal Sa at node N 858  is pulled down toward V SS  generating a low comparator signal Sa, representing a possible open circuit fault condition in signal S+. 
     In a normal condition, transistor M 8  has a smaller base-to-source voltage than transistor M 7  when single-ended signal S+(2) falls below signal Sref+(2). This causes less current to be steered through transistor M 8  and more current to be steered through transistor M 7 . Signal Sa at node N 858  is pulled up toward V DD  generating a high comparator signal Sa. Current source I 50  is not able to pull signal S+(2) up because S+ is tied to the differential signal voltage Sd. High comparator signal Sa represents a normal condition. 
     FIG. 9 is a schematic diagram of an exemplary S− fault condition detector  450  according to an embodiment of the invention. S− fault condition detector  450  employs p-type transistors and functions in substantially the same manner as S+ fault condition detector  440 , except that its output must be inverted to operate the current mode logic of output stage  410  (See FIGS. 10 and 11 for converter  660  and output stage  410 ). As shown, S− fault condition detector  450  includes p-type transistors M 10 , M 12 , M 13 , and M 14 ; current sources I 9 , I 10 , I 18 , and I 52 : and loads R 906  and R 908 . 
     Transistor M 12  has a base coupled to node N 972 , a drain coupled to node N 976 , and a source coupled to node N 974 . Transistor M 13  has a base coupled to node N 980 , a drain coupled to node N 976 , and a source coupled to N 978 . Transistor M 10  has a base coupled to signal Sref−, a source coupled to V SS , and a drain coupled to node N 972 . Transistor M 14  has a base coupled to signal S−, a source coupled to V SS , and a drain coupled to node N 980 . Loads R 906  and R 908  are coupled between V SS  and nodes N 974  and N 978 , respectively. Current sources I 9 , I 11 , and I 18  are coupled between nodes N 972 , N 976 , and N 980 , and V DD , respectively. Current source I 52  is coupled between V SS  and signal S−. 
     In operation, transistor M 14  functions as a source follower, acts as an input buffer isolating LVDS open circuit detector  320  from differential signal S−, and outputs a differential signal S−(2). Transistor M 14  is a relatively small device with high impedance and very low capacitance input capacitance in the range of 45-60 fF. As a result, transistor M 14  does not does not add significant capacitance, or load, alter, or disturb signal S−. Current source I 52  is a weak, high impedance source arranged in approximately the same manner as current source I 50 . Current source I 52  does not significantly load signal S−, and is arranged to “pull-down” signal S− toward voltage supply V SS  when S− is weak, as occurs in an open circuit fault. A pull-down can occur because a floating or open signal does not have any voltage holding it to any other value. In a normal condition, pull-down current source I 52  is not strong enough to disturb signal S−. Signal Sref− is supplied externally, and like signal Sref+, its value is predetermined to provide adequate headroom between the low signal component (S l ) of the differential signal (S d ) and local power supply V SS  (See FIG.  2 ). For example, signal Sref− could be set at 0.5 volts when V SS  is 0.0 volts and the low signal component (S l ) is 1.3 volts. This provides 800 mV of headroom between the low signal component (S l ) of a valid differential signal and reference signal Sref− for detecting an open circuit fault. 
     Transistors M 10  and M 14  are matched, and their current sources I 11  and I 18  are also matched. This equalizes the signal shifts of S+ and Sref+ and maintains the predetermined detection range. Transistor M 22  outputs buffered signal S+2 and transistor M 21  outputs signal Sref+(2). 
     Transistors M 12  and M 13  are a p-type differential pair, and function as a comparator in cooperation with loads R 906  and R 908  and current source I 9 . Single-ended signal S−(2) is applied to the base of transistor M 13 . Reference signal Sref−(2) is applied to the base of transistor M 10 . Loads R 906  and R 908  are sized to allow approximately equal current flow through transistors M 12  and M 13  when signal Sref−(2) is substantially equal to signal S−(2). 
     In a fault condition, transistor M 13  has a lower base-to-source voltage than transistor M 12  because single-ended signal S−(2) has been pulled below signal Sref−(2) by current source I 52 . More current is steered through transistor M 13  and less current is steered through transistor M 12 . The plus side of signal S−(3) at node N 978  is pulled high toward V DD , and the minus side of signal S−(3) at node N 974  is pulled low toward V SS . The resulting high signal S−(3) represents a possible fault condition in signal S−detected by the invention. This signal must be inverted by converter  660 , as illustrated in FIG. 10, so that a compatible comparator signal Sb can be generated for use by the common-mode logic of output stage  410  as illustrated in FIG.  11 . 
     In a normal condition, transistor M 12  has a lower base-to-source voltage than transistor M 13  because current source I 52  is not able to pull-down signal S−(2). This causes more current to be steered through transistor M 12  and less current to be steered through transistor M 13 . As a result, the minus side of signal S−(3) at node N 974  is pulled high toward V DD , and the plus side of signal S−(3) at node N 978  is pulled low toward V SS . 
     FIG. 10 is a schematic diagram of an exemplary converter  660  according to an embodiment of the invention. Converter  660  includes n-type transistors M 15  and M 16 ; current source I 20 : and loads R 1010  and R 1012 . 
     Transistor M 15  has a base coupled to node N 978 , a drain coupled to node N 1092 , and a source coupled to node N 1090 . Transistor M 16  has a base coupled to node N 974 , a drain coupled to node N 1091 , and a source coupled to node N 1090 . Loads R 1010  and R 1012  are coupled between V DD  and nodes N 1091  and N 1092  respectively. Current source I 20  is coupled between node N 1090  and V SS . 
     In operation, converter  660  inverts Signal S−(3). The high and low sides of signal S−(3) are applied to the bases of differential pair transistors M 15  and M 16  respectively. Loads R 1010  and R 1012  are sized to allow approximately equal current flow through transistors M 15  and M 16  when the high and low sides of signal S−(3) are substantially equal. In the normal condition when the high side of signal S−(3) rises above the low side of signal S−(3), transistor M 15  has a larger base-to-source voltage than transistor M 16 , causing M 15  to drive more current than transistor M 16 . The greater current through M 15  and load R 1012  pulls signal Sb higher, and drives node N 1091  lower. In a fault condition when the low side of signal S−(3) rises above the high side of signal S−(3), transistor M 16  has a larger base-to-source voltage than transistor M 15 , causing M 16  to drive more current than transistor M 15 . The greater current through M 16  and load R 1010  pulls signal Sb lower, and drives node N 1091  higher. Converter  660  has inverted high signal S−(3) (fault condition) into a low comparator signal Sb. Likewise, a low signal S−(3) (normal condition) is inverted into a high signal Sb. 
     FIG. 11 illustrates a schematic diagram of an exemplary output stage  1100  according to an embodiment of the invention, and is an embodiment of output stage  410  as shown in FIG.  4 . Output stage  1100  includes transistors M 31 , M 32 , M 34 , and M 35 ; loads R 1120 , R 1122 , R 1124 , and R 1126 ; and current sources I 30  and I 32 . 
     Transistor M 31  has a base coupled to node N 1194 , a drain coupled to node N 1198 , and a source coupled to node N 1199 . Transistor M 32  has a base coupled to V DD , a drain coupled to node N 1195 , and a source coupled to node N 1193 . Transistor M 35  has a base coupled to node N 858 , a drain coupled to node N 1196 , and a source coupled to node N 1199 . Transistor M 34  has a base coupled to node N 992 , a drain coupled to node N 1196 , and a source coupled to node N 1199 . Current source I 30  is coupled between node N 1193  and local power supply V SS . Current source I 33  is coupled between node N 1199  and local power supply V SS . Loads R 1120 , R 1124  and R 1126  are coupled between local power supply V DD  and nodes N 1194 , N 1196 , and N 1198 , respectively. Load R 1122  is coupled between nodes N 1194  and  1195 . 
     In operation, output stage  1100  is similar to a wired current-mode logic (CML) NOR gate, or a three output comparator. Output stage  1100  includes a reference signal generator (Sref3). Signal Sref3 is provided by loads R 1120  and R 1122 , transistor M 32 , and current source I 30 . Transistor M 32  has a base biased at V DD , and functions as a voltage-controlled resistor. Transistor M 32  in conjunction with loads R 1120  and R 1122 , and current source I 30  forms a controlled voltage drop from local power supply V DD  to V SS , and outputs signal Sref3 at node N 1194 . The logic-type functionality resides in a differential pair formed by transistor M 31  and the parallel pair of transistors M 34  and M 35 . Loads R 1124  and R 1126  are approximately equal and function as pull-up devices for nodes N 1196  and N 1198 , respectively. Current source I 33  provides a constant current source at node N 1199 , which is coupled to the sources of transistors M 34 , M 35 , and M 31 . Signal Sref3 provides a uniform base-to-source bias voltage for transistor M 31 . The current through transistor M 31  establishes the level of signal S CL  at node N 1198 . Transistor M 31  is always enabled by Sref3 coupled to its base. 
     Comparator signals Sa and Sb are coupled to the bases of differential pair transistors M 35  and M 34  respectively. As illustrated in the preceding figures, at least one high comparator signal (Sa, Sb) represents a normal condition, and two low comparator signals (Sa, Sb) represent a fault condition. In a normal condition, at least one single-ended signal is not pulled into a detection range, resulting in at least one high comparator signal (Sa, Sb) being generated. In a fault condition, both single-ended signals are pulled into their respective detection ranges, and both comparator signals (Sa, Sb) are low. As a result, in normal LVDS condition, at least one comparator signal (Sa, Sb) is high at the bases of transistor M 34  or M 35 , respectively. This results in at least one of transistors M 34  or M 35  being on and steering current away from transistor M 31 . As a result, the current through load R 1126  decreases, and fault condition control signal S CL  is pulled high toward V DD . The current through load R 1124  increases, and fault condition control signal S CH  is pulled low toward V SS . Transistors M 34  and M 35  are sized so that the current steering results in S CH  being lower than S CL , indicating a normal LVDS condition. In a LVDS fault condition, both comparator signals (Sa, Sb) are low at the bases of transistors M 34  and M 35 . Both transistors M 34  and M 35  are turned off, current I 33  flows entirely through transistor M 31 . As a result, the current through load R 1198  increases, and fault condition control signal S CL  is pulled low toward V SS . The current through load R 1196  decreases, and fault condition control signal S CH  is pulled high toward V DD . Components M 34 , M 35 , M 31 , R 1124 , R 1126 , and I 33  are sized so that fault detection signal S CH  is greater than S CL  when Sa and Sb are low, constituting a fault detection signal. In an embodiment, the transistors are scaled with respect to each other such that transistors M 35  and M 31  are twice the capacity of M 34  (i.e., ((W/L) 35 =(W/L) 31 =2×(W/L) 34 ). 
     The components of output circuit  900  are arranged so that the difference between signal S CH  and signal S CL  is approximately 600 mV. In a normal condition (non-fault condition), output stage  1100  provides a fault condition control signal such that S CL  is greater than S CH  by approximately 600 mV. In a fault condition, LVDS fault condition detector  320  provides a fault condition control signal such that S CH  is greater than S CL  by approximately 600 mV. 
     The high fault condition control signal (S CH &gt;S CL ) constitutes a fault detection signal and is used by a reporting device to report an open circuit fault. In another embodiment, the fault detection signal is available for other devices such as a digital indicator to communicate the existence of an open circuit fault. See FIG. 3 for an illustration. 
     Embodiments of output stage  1100  can include many types of wired NOR gates, or three input comparators, to generate fault condition control signals S CH  and S CL . 
     FIG. 12 illustrates a schematic diagram of a LVDS open circuit fault condition detector  1100  according to an embodiment of the invention, and combines the embodiments illustrated in FIGS. 4-11 into one circuit. FIG. 11 also illustrates an embodiment of LVDS open circuit fault condition detector  320  illustrated in the block diagram of FIG.  3 . FIG. 11 also schematically illustrates an open circuit fault condition detector processing both the S ih  and S il  sides of a LVDS differential signal. Similar components are similarly labeled. 
     LVDS open circuit fault condition detector  1000  comprises S+ fault condition detector  440  illustrated in FIG. 8, S− fault condition detector  640  illustrated in FIG. 9, converter  660  illustrated in FIG. 10, and output stage  1100  illustrated in FIG.  11 . These components of LVDS open circuit fault condition detector are coupled at the nodes described in the figures relating to the particular components, and function as described in FIGS. 8-11. The part numbers are the same as in FIGS. 8-11 except the loads, where like numbers indicate approximately equal values. 
     Transistors pairs M 22  and M 29  are matched to transistors pairs M 21  and M 28  on the S+ side. Transistors pairs M 10  and M 11  are matched to transistors pairs M 14  and M 18  on the S− side. This matching balances the current flow and level shift occurring on each side of signal IN. 
     The operation of LVDS open circuit fault condition detector is described in conjunction with FIGS. 3-10. 
     The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.