Abstract:
First and second devices may simultaneously communicate bidirectionally with each other using only a single pair of LVDS signal paths. Each device includes an input circuit and a differential output driver connected to the single pair of LVDS signal paths. An input to the input circuit is also connected to the input of the driver. The input circuit may also receive an offset voltage. In response to its inputs, the input circuit in each device can use comparators, gates and a multiplexer to determine the logic state being transmitted over the pair of LVDS signal paths from the other device. This advantageously reduces the number of required interconnects between the first and second devices by one half.

Description:
[0001]     This application claims priority from Provisional Application No. 60/733,571, filed Nov. 4, 2005. 
     
    
     FIELD OF THE DISCLOSURE  
       [0002]     This disclosure relates in general to circuit input and output differential signaling interfaces and in particular to interfaces based on Low Voltage Differential Signaling (LVDS).  
       BACKGROUND  
       [0003]     Today LVDS signaling is being used in a myriad of circuit communication applications, including: (1) system to system communication via cable connections, (2) board to board communication via backplane connections, and (3) IC to IC communication via board or other substrate level connections. The present disclosure anticipates LVDS signaling applications to be extended, beyond these known applications, to include signaling applications between circuits (vender and custom circuits (cores)) embedded within ICs as well. The benefits of LVDS signaling over other (single ended) signaling schemes include; (1) improved signaling noise immunity, (2) lower signaling power consumption, and (3) higher signaling speeds. The drawback of LVDS signaling is that it doubles the number of connection required between a sending circuit and a receiving circuit. The present disclosure, in at least one aspect, eliminates this connection doubling drawback. Conventional LVDS signal communication occurs between an LVDS driver and an LVDS receiver over a pair of signal paths. The pair of signal paths may support bidirectional communication between two driver and receiver pairs, but not simultaneously.  
       BRIEF SUMMARY  
       [0004]     The present disclosure discloses a design of LVDS drivers and receivers such that a pair of LVDS drivers and receivers can simultaneously communicate over a single pair of signal path leads.  
         [0005]     A first device comprises a signal source  1 , a signal destination  2 , a LVDS driver, an input circuit, termination resistor, and a resistor in series with each of the differential signal paths. The input circuit receives inputs from the LVDS signal path and from an output from source  1 . The input circuit provides input to destination  2 .  
         [0006]     A second device  502  comprises a signal source  2 , a signal destination  1 , an LVDS driver, an input circuit, a termination resistor, and a resistor in series with each of the differential signal paths. The input circuit receives inputs from the LVDS signal path and from an output from source  2 . The input circuit provides input to destination  1 .  
         [0007]     A first example input circuit comprises an inverter with its input coupled to the output from the source circuit, a differential receiver with its non-inverting and inverting inputs coupled to the differential signal path, a window comparator with A and B inputs coupled to the differential signal path, and a multiplexer with a first input coupled to the output of the inverter, a second input coupled to the output of the differential receiver, a control input coupled to the output of the window comparator, and an output coupled to the input of the destination circuit.  
         [0008]     A second example input circuit comprises an inverter with its input coupled to the output from the source circuit, a window comparator with A and B inputs the differential signal path, and a multiplexer. The multiplexer has a first input coupled to a fixed logic high, a second input coupled to a fixed logic low, a third input coupled to the output of the inverter, a first control input coupled to an output C of the window comparator, a second control input coupled to an output D of the window comparator, and an output coupled to the input to destination circuit.  
         [0009]     One example circuit that could be used as window comparator comprises a first comparator with its non-inverting input coupled to the input A, its inverting input coupled to the input B, and its output coupled to the output C. A second comparator has its non-inverting input coupled to the input B, its inverting input coupled to the input A, and its output coupled to output D.  
         [0010]     The first comparator is designed such that the voltage on its non-inverting input must be greater than the voltage on its inverting input by an offset voltage (OSV) value (80 millivolts in this example) before the comparator output C will go high. The second comparator is designed such that the voltage on its non-inverting input must be greater than the voltage on its inverting input by an offset voltage (OSV) value (80 millivolts in this example) before the comparator D will go high. If the value difference on the A and B inputs is less than 80 millivolts, comparator outputs C and D go low. While 80 millivolts was used as an example OSV, any desired value of OSV may be used as well.  
         [0011]     Another circuit that could be used to realize the window comparator comprises a first comparator with its non-inverting input coupled to the A input and its inverting input coupled to a reference voltage (assumed to be 250 mv in this example), a second comparator with its non-inverting input coupled to the B input and its inverting input coupled to the reference voltage, an OR gate with a first input coupled to the output of the first comparator, a second input coupled to the output of the second comparator, and an output coupled to the C output.  
         [0012]     Another circuit that could be used to realize window comparator comprises a first comparator with its non-inverting input coupled to the A input, its inverting input coupled to a reference voltage (assumed to be 250 mv in this example), and an output coupled to the C output, a second comparator with its non-inverting input coupled to the B input, its inverting input coupled to the reference voltage, and an output coupled to the D output.  
         [0013]     Each device comprises a deserializer for receiving serial data from the input circuit, data receiving circuitry for inputting parallel data from the deserializer, a serializer for inputting serial data to the driver, and data providing circuitry for inputting parallel data to the serializer. The combination of the data receiving circuitry and deserializer represent one example design for a destination circuit or a source circuit.  
         [0014]     One of the devices also comprises clock output circuitry and an LVDS clock driver. The clock output circuitry provides a clock output to driver and outputs control (CTL) signals to operate the providing circuitry, serializer, deserializer, and receiving circuitry. The control (CTL) signals output to the serializer and deserializer from the clock output circuit will operate faster than the control signals to the receiving and providing circuits since they will be controlling the higher speed serial input and output operations occurring over the signal paths. The clock output circuit may employ use of clock and control signal modification circuits such as but not limited to; a phase lock loop, a phase shifter, a frequency divider, or a frequency multiplier. The clock driver is similar to the other drivers and drives LVDS clock outputs from the device on signal paths separate from the other signal paths. LVDS clocking is shown being used to provide high speed clock signals between the devices. If desired, single ended clocking could be used instead of the differential clocking shown, but the clocking frequency would be reduced between the devices. The device outputting the LVDS clock on the clock signal paths is assumed to be a master device.  
     
    
     BRIEF DESCRIPTION OF THE VIEWS OF THE DRAWINGS  
       [0015]      FIG. 1A  illustrates a prior art LVDS driver communicating a logic one to a prior art LVDS receiver.  
         [0016]      FIG. 1B  illustrates a prior art LVDS driver communicating a logic zero to a prior art LVDS receiver.  
         [0017]      FIG. 2B  illustrates a prior art example of a signal source circuit in one device communicating a logic 1 to a signal destination circuit in another device using an LVDS driver and receiver.  
         [0018]      FIG. 2B  illustrates a prior art example of a signal source circuit in one device communicating a logic 0 to a signal destination circuit in another device using an LVDS driver and receiver.  
         [0019]      FIG. 3  illustrates prior art example of signal source and destination circuits of one device communicating with signal source and destination circuits of another device using two pairs of LVDS signal paths.  
         [0020]      FIG. 4  illustrates prior art example of signal source and destination circuits of one device communicating with signal source and destination circuits of another device using a one pair of LVDS signal paths.  
         [0021]      FIG. 5  illustrates signal source and destination circuits of one device communicating with signal source and destination circuits of another device using one pair of LVDS signal paths according to the present disclosure.  
         [0022]      FIG. 6A  illustrates a first signal source communicating a logic 1 to a first signal destination simultaneous with a second signal source communicating a logic 1 to a second signal destination using one pair of LVDS signal paths according to the present disclosure.  
         [0023]      FIG. 6B  illustrates the electrical circuit model of the communication of  FIG. 6A .  
         [0024]      FIG. 7A  illustrates a first signal source communicating a logic 0 to a first signal destination simultaneous with a second signal source communicating a logic 0 to a second signal destination using one pair of LVDS signal paths according to the present disclosure.  
         [0025]      FIG. 7B  illustrates the electrical circuit model of the communication of  FIG. 7A .  
         [0026]      FIG. 8A  illustrates a first signal source communicating a logic 1 to a first signal destination simultaneous with a second signal source communicating a logic 0 to a second signal destination using one pair of LVDS signal paths according to the present disclosure.  
         [0027]      FIG. 8B  illustrates the electrical circuit model of the communication of  FIG. 8A .  
         [0028]      FIG. 9A  illustrates a first signal source communicating a logic 0 to a first signal destination simultaneous with a second signal source communicating with a logic 1 to a second signal destination using one pair of LVDS signal paths according to the present disclosure.  
         [0029]      FIG. 9B  illustrates the electrical circuit model of the communication of  FIG. 9A .  
         [0030]      FIG. 10  illustrates use of a single termination resistor in the LVDS signal paths of the present disclosure.  
         [0031]      FIG. 11A  illustrates a first example of how the input circuit of the present disclosure may be designed.  
         [0032]      FIG. 11B  illustrates one example of how the window comparator of the input circuit of  FIG. 11A  may be designed.  
         [0033]      FIG. 12A  illustrates a second example of how the input circuit of the present disclosure may be designed.  
         [0034]      FIG. 12B  illustrates one example of how the window comparator of the input circuit of  FIG. 12A  may be designed.  
         [0035]      FIGS. 13A-13D  illustrate a second example of how the window comparator of  FIG. 11A  may be designed.  
         [0036]      FIGS. 13E-13H  illustrate a second example of how the window comparator of  FIG. 12A  may be designed.  
         [0037]      FIG. 14  illustrates an example of two devices connected together via an LVDS data signal path and an LVDS clock signal path according to the present disclosure.  
         [0038]      FIG. 15  illustrates an example of two device connected together via plural LVDS data signal paths and an LVDS clock signal path according to the present disclosure.  
         [0039]      FIG. 16  illustrates a first example of a device connected to a debug, trace, or emulation controller via an LVDS data signal path and an LVDS clock signal path according to the present disclosure.  
         [0040]      FIG. 17  illustrates a second example of a device connected to a debug, trace, or emulation controller via an LVDS data signal path and an LVDS clock signal path according to the present disclosure.  
         [0041]      FIG. 18  illustrates an example of a device connected to an IC or die tester via an LVDS data signal path and an LVDS clock signal path according to the present disclosure.  
         [0042]      FIG. 19  illustrates an example of a device connected to an IC or die tester via a plurality of LVDS data signal paths and an LVDS clock signal path according to the present disclosure.  
         [0043]      FIG. 20  illustrates an example of a plurality of devices connected to an IC or die tester via LVDS data signal paths and LVDS clock signal paths according to the present disclosure.  
     
    
     DETAILED DESCRIPTION  
       [0044]      FIGS. 1A and 1B  illustrate the connection between a conventional LVDS driver  100  and receiver  102 . The driver has an input  114 , a non-inverting output  116  and an inverting output  118 . The driver is comprised of transistors, indicated as switches  104 - 110 , that are controlled by the input  114 . The receiver has a non-inverting input  120 , an inverting input  122 , and an output  124 . A first signal path connection  126  is formed between driver output  116  and receiver input  120 . A second signal path connection  128  is formed between driver output  118  and receiver input  122 . A termination resistor  112  is placed across the receiver inputs  120  and  122 .  
         [0045]     In  FIG. 1A , a logic high is input to the driver. In response, transistors  104  and  106  are turned on and transistors  108  and  110  are turned off. In this arrangement current flows from the driver current source through transistor  104 , termination resistor  112 , and transistor  106 . The direction of the current flow develops a voltage across termination resistor  112  such that the voltage at the receiver input  120  is more positive than the voltage at receiver input  122 . In response to this input voltage, the receiver outputs a logic high on output  124 .  
         [0046]     In  FIG. 1B , a logic low is input to the driver. In response, transistors  108  and  110  are turned on and transistors  104  and  106  are turned off. In this arrangement current flows from the driver current source through transistor  108 , termination resistor  112 , and transistor  110 . The direction of the current flow develops a voltage across termination resistor  112  such that the voltage at the receiver input  122  is more positive than the voltage at receiver input  120 . In response to this input voltage, the receiver outputs a logic low on output  124 .  
         [0047]      FIGS. 1A and 1B  are provided to illustrate the conventional operation of current mode LVDS driver and receiver. Voltage mode LVDS driver and receivers may also be used, but current mode LVDS drivers and receivers are the most common variety. The present disclosure may be used with either current or voltage mode LVDS drivers and receivers.  
         [0048]      FIGS. 2A and 2B  illustrate an LVDS connection formed between a first device  200  and a second device  204 . Devices  200  and  204  may be sub-systems in a system, boards in a backplane. ICs on a board or other substrate, or embedded core circuits in an IC. In  FIGS. 2A and 2B  and all following figures, the devices could represent; (1) a master device coupled to a slave device, (2) a master device coupled to another master device, or (3) a slave device coupled to another slave device.  
         [0049]     Example master devices include but are not limited too; a microprocessor, a digital signal processor, a Serdes serializer, a computer, a production or field tester, an emulation controller, and a trace/debug controller.  
         [0050]     Device  200  comprises a signal source  202  and an LVDS driver  100 . Device  204  comprises a signal destination  206  and an LVDS receiver  102 . The signal source  202  in device  200  may be any type of circuit that operates to output signals to the LVDS driver  100 . The signal destination  204  may be any type of circuit in device  204  that operates to input signals from LVDS receiver  102 . The source and destination circuits could be used to perform a myriad of operations including but not limited to; (1) a functional operation of the device, (2) a test operation of the device, (3) a debug operation of the device, (4) a trace operation of the device, and (5) an emulation operation of the device.  
         [0051]     One example signal source circuit could be a Serdes serializer that operates to input parallel data from another circuit within device  200  and to output that data serially to driver  100 . One example signal destination circuit could be a Serdes deserializer that operates to input serial data from receiver  102  and to output that data in parallel to another circuit within device  204 .  
         [0052]      FIG. 2A  illustrates a logic high being output from source  202  and received by destination  206 . As seen, the driver  100  outputs current from output terminal  116  to output terminal  118 , which develops a voltage across resistor  112  with the polarity being more positive on the receiver input  120  than receiver input  122 . The receiver  102  outputs a logic high to destination  206  in response to the polarity of the voltage across the resistor  112 .  
         [0053]      FIG. 2B  illustrates a logic low being output from source  202  and received by destination  206 . As seen, the driver  100  outputs current from output terminal  118  to output terminal  116 , which develops a voltage across resistor  112  with the polarity being more positive on the receiver input  122  than receiver input  120 . The receiver  102  outputs a logic low to destination  206  in response to the polarity of the voltage across the resistor  112 .  
         [0054]     In either  FIG. 2A  or  2 B the termination resistor  112  may exist within device  204  or it may exist external of device  204 . This is true for all following figures.  
         [0055]      FIG. 3  illustrates two devices  300  and  302  each having signal source  202  and destination  206  circuits and LVDS driver  100  and receiver  102  circuits. In this example, source  1   202  of device  300  can communicate with destination  1   206  of device  302  and source  2   202  of device  302  can communicate with destination  2   206  of device  300 . The communications can occur simultaneously since separate LVDS signal paths  304  and  306  exist between the devices. Having to use separate LVDS signal paths for simultaneous communication increases the interconnect between devices  300  and  302 .  
         [0056]      FIG. 4  illustrates two devices  400  and  402  each having signal source  202  and destination  206  circuits and LVDS driver  402  and receiver  102  circuits. LVDS drivers  402  are similar to LVDS drivers  100  with the exception that the LVDS drivers  402  have an enable input  404  and  406 . The enable input is used to enable or disable the output drive of driver  402 . If enable input  404  is set to enable driver  402  of device  400  and enable input  406  is set to disable driver  402  of device  402 , source  1   202  of device  400  can communicate with destination  1   206  of device  402 . Likewise, if enable input  404  is set to disable driver  402  of device  400  and enable input  406  is set to enable driver  402  of device  402 , source  2   202  of device  402  can communicate with destination  2   206  of device  400 . The communications cannot occur simultaneously but rather must occur at separate times since only one LVDS signal path  410  exists between the devices. Having to communicate at separate times decreases the communication bandwidth between the source and destination circuits of devices  400  and  402 .  
         [0057]     As seen in  FIG. 4 , the termination resistors  112  of each receiver  102  lie in parallel on the LVDS signal path  410 . This results in a parallel resistance termination (PRT)  412  on the signal path (indicated in dotted line). The value of PRT  412  is equal to the parallel resistance of resistors  112 . For example, if resistors  112  are 100 ohms, a typical value for LVDS termination resistors, PRT will be 50 ohms. Since current mode LVDS driver  402  output a constant current, a reduction in the signal path termination resistance (i.e. the 50 ohm PRT) will lower differential signaling voltages on the signal path  410  to the receivers  102 . Lowering differential signaling voltages can cause communication problems (i.e. lowers the differential noise margin) between an enabled driver and receiver in the LVDS signaling arrangement of  FIG. 4 . Therefore the differential signaling arrangement of  FIG. 4  should only be used in applications where noise is low and the signaling path  410  is short.  
         [0058]     The present disclosure provides a way to allow simultaneous source to destination communication between devices, like in  FIG. 3 , while requiring only a single LVDS signal path interconnect between devices, like in  FIG. 4 .  
         [0059]     The present disclosure provides a way to maintain appropriate LVDS signaling voltages (and noise margins) on an LVDS signal path where the termination resistance of the signal path is decreased due to the parallel arrangement of LVDS termination resistors, like in  FIG. 4 .  
         [0060]      FIG. 5  illustrates the LVDS signaling arrangement between devices  500  and  502  according to the present disclosure. Device  500  comprises a signal source  1   202 , a signal destination  2   206 , and LVDS driver  100 , an input circuit  504 , termination resistor  112 , and resistors  506  and  508 . The input circuit  504  receives inputs from LVDS signal path  514 , LVDS signal path  516 , and the output from source  1   202 . The input circuit  514  provides input to destination  2   206 . Resistor R 1   506  is placed in series between the driver output terminal  116  and signal path  514 . Resistor R 2   508  is placed in series between the driver output terminal  118  and signal path  516 .  
         [0061]     Device  502  comprises a signal source  2   202 , a signal destination  1   206 , and LVDS driver  100 , an input circuit  504 , termination resistor  112 , and resistors  510  and  512 . The input circuit  504  receives inputs from LVDS signal path  514 , LVDS signal path  516 , and the output from source  2   202 . The input circuit  514  provides input to destination  1   206 . Resistors R 3   510  is placed in series between the driver output terminal  116  and signal path  514 . Resistor R 4   512  is placed in series between the driver output terminal  118  and signal path  516 .  
         [0062]     If devices  500  and  502  are boards or other substrates is a system, resistors  506 - 512  could be discrete resistors placed, as shown, in series between the driver outputs  116  and  118  and board/substrate contacts connected to signal paths  514  and  516 .  
         [0063]     If devices  500  and  502  are ICs on a board or other substrate, resistors  506 - 512  could be poly or transistor channel resistances placed, as shown, in series between the driver outputs  116  and  118  and IC pads connected to signal paths  514  and  516 .  
         [0064]     If devices  500  and  502  are embedded core circuits in an IC, resistors  506 - 512  could be poly or transistor channel resistances placed, as shown, in series between the driver outputs  116  and  118  and core circuit terminals connected to signal paths  514  and  516 .  
         [0065]     The LVDS driver and series resistor arrangement could be as shown in  FIG. 5 , i.e. the driver and resistors are separate circuits connected together inside the device, or the driver and series resistors could be integrated to form a new driver circuit  518  applicable for use by the present disclosure. The circuitry and detail operation of the input circuits  504  will be described later in regard to  FIGS. 11, 12 , and  13 ,  
         [0066]     During operation of the devices in  FIG. 5 , source  1   202  of device  500  outputs data to driver  100  which transmits differential signals over the signal paths  514 - 516  to input circuit  504  of device  502  to be input to destination  1   206  of device  502 . Simultaneously, source  2   202  of device  502  outputs data to driver  100  which transmits differential signals over the signal paths  514 - 516  to input circuit  504  of device  500  to be input to destination  2   206  of device  500 .  
         [0067]     Resistors  506 - 512  should be equal in value or as near equal in value as possible to each other. The value of each resistor  506 - 512  is preferable less than the value of the termination resistor  112 . In the following description of the examples shown in  FIGS. 6A-6B ,  7 A- 7 B,  8 A- 8 B,  9 A- 9 B, and  10  it will be assumed for simplification that the termination resistors  112  are 100 ohms, resistors  506 - 512  are each 25 ohms, and the drivers  100  are 5 milliamp LVDS drivers.  
         [0068]     With 100 ohm termination resistors  112 , the parallel termination resistance (PRT)  412  across the signal paths  514 - 516 , due to the termination resistors  112 , is equal to 50 ohms. While these resistor and current values are used in the description, the present disclosure is not limited to use of only these values. Indeed, other resistance and current values can be used without departing from the spirit and scope of the present disclosure.  
         [0069]     In  FIG. 6A , it is seen that if source  1  of device  500  and source  2  of device  502  both output a logic high to drivers  100 , input circuit  504  of device  500  will input a logic high to destination  2  of device  500  and input circuit  504  of device  502  will input a logic high to destination  1  of device  502 .  
         [0070]     In  FIG. 6B , the electrical model of the  FIG. 6A  signal transfer operation is shown. As seen, driver  100  of device  500  sources current (I 1 ) into signal path  514  from terminal  116  and returns current (I 2 ) from signal path  516  at terminal  118 . Also as seen, driver  100  of device  502  sources current (I 3 ) into signal paths  514  from terminal  116  and returns current (I 4 ) from signal path  516  at terminal  118 . The sum of the source currents (I 1  and I 3 ) pass through PRT  412  (the parallel resistance of terminal resistors  112 ) and develop a voltage across PRT with the polarity shown. The voltage developed across PRT is input to the voltage input (Vin) of the input circuits  504  of  FIG. 6A . In response to Vin the input circuits  504  output logic high to destinations  1  and  2   206 .  
         [0071]     In  FIG. 6B , if the drivers  100  each provide a source current of 5 milliamps, the voltage across each resistor  506 - 512  will be 125 millivolts (i.e. 25 ohms×5 ma) and the voltage across PRT  412  will be 500 millivolts (i.e. 50 ohms×10 ma). A Vin of 500 millivolts with the polarity shown provides a differential LVDS input signal to the input circuits  504  that the input circuits  504  can easily recognize as a logic high. The 500 millivolts differential input signal also provides excellent noise immunity in applications with high noise and long signal paths  514 - 516 .  
         [0072]     While a 500 millivolts differential signal was produced in this example using the assumed currents and resistances, other differential signal voltages could be produced using different assumptions on currents and resistances.  
         [0073]     In  FIG. 7A , it is seen that is source  1  of device  500  and source  2  of device  502  both output a logic low to drivers  100 , input circuit  504  of device  500  will input a logic low to destination  2  of device  500  and input circuit  504  of device  502  will input a logic low to destination  1  of device  502 .  
         [0074]     In  FIG. 7B , the electrical model of the  FIG. 7A  signal transfer operation is shown. As seen, driver  100  of device  500  sources current (I 2 ) into signal path  516  from terminal  118  and returns current (I 1 ) from signal path  514  at terminal  116 . Also as seen, driver  100  of device  502  sources current (I 4 ) into signal path  516  from terminal  118  and returns current (I 3 ) from signal path  514  at terminal  116 . The sum of the source currents (I 2  and I 4 ) pass through PRT  412  and develop a voltage across PRT with the polarity shown. The voltage developed across PRT is input to the voltage input (Vin) of the input circuits  504  of  FIG. 7A . In response to Vin the input circuits  504  output logic lows to destinations  1  and  2   206 .  
         [0075]     In  FIG. 7B , if the driver  100  each provide a source current of 5 milliamps, the voltage across each resistor  506 - 512  will be 125 millivolts (i.e. 25 ohms×5 ma) and the voltage across PRT  412  will be 500 millivolts (i.e. 50 ohms×10 ma). A Vin of 500 millivolts with the polarity shown provides a differential LVDS input signal to the input circuits  504  that the input circuits  504  can easily recognize as a logic low. Again, the 500 millivolts differential input signal provides excellent noise immunity in applications with high noise and long signal paths  514 - 516 .  
         [0076]     As seen in  FIGS. 6A-6B , while a 500 millivolts differential signal was produced in the  FIGS. 7A-7B  example using the assumed currents and resistances, other differential signal voltages could be produced using different assumptions on currents and resistances.  
         [0077]     In  FIG. 8A , it is seen that if source  1  of device  500  outputs a logic high to the driver  100  of device  500  and source  2  of device  502  outputs a logic low to driver  100  of device  502 , input circuit  504  of device  500  will input a logic low to destination  2  of device  500  and input circuit  504  of device  502  will input a logic high to destination  1  of device  502 .  
         [0078]     In  FIG. 8B , the electrical model of the  FIG. 8A  signal transfer operation is shown. As seen, driver  100  of device  500  sources current (I 1 ) into signal path  514  from terminal  116  and returns current (I 2 ) from signal path  516  at terminal  118 . Also as seen, driver  100  of device  502  sources current (I 4 ) into signal path  516  from terminal  118  and returns current (I 3 ) from signal path  514  at terminal  116 . In this electrical situation, the current (I 1 ) sourced from driver  100  of device  500  is the current (I 3 ) returned to driver  100  of device  502 , and the current (I 4 ) sourced from driver  100  of device  502  is the current (I 2 ) returned to driver  100  of device  500 .  
         [0079]     Since resistors  506 - 512  are assumed to be 25 ohms each and the source currents I 1  and I 4  are assumed to be 5 milliamps each, the voltages present on signal path  514  and signal path  516  are the same or very close to being the same. With the same voltage present on the terminals of PRT  412 , no current, or only a small leakage current, flows through PRT  412 . Thus the voltage drop across PRT (i.e. Vin) that is input to input circuits  504  is extremely small.  
         [0080]     In response to the small Vin voltage, the input circuits  504  of devices  500  and  502  are designed to input the opposite logic level that each device  500  and  502  was outputting. For example, since source  1   202  of device  500  in  FIG. 8A  is outputting a logic high, the input circuit  504  of device  500  will respond to the small Vin voltage by inputting a logic low to destination  2   206  of device  500 . Likewise, since source  2   202  of device  502  in  FIG. 8A  is outputting a logic low, the input circuit  504  of device  502  will respond to the small Vin voltage by inputting a logic high to destination  1   206  of device  502 .  
         [0081]     In  FIG. 9A , it is seen that if source  1  of device  500  outputs a logic low to the driver  100  of device  500  and source  2  of device  502  outputs a logic high to driver  100  of device  502 , input circuit  504  of device  500  will input a logic high to destination  2  of device  500  and input circuit  504  of device  502  will input a logic low to destination  1  of device  502 .  
         [0082]     In  FIG. 9B , the electrical model of the  FIG. 9A  signal transfer operation is shown. As seen, driver  100  of device  500  sources current (I 2 ) into signal path  516  from terminal  118  and returns current (I 1 ) from signal path  514  at terminal  116 . Also as seen, driver  100  of device  502  sources current (I 3 ) into signal path  514  from terminal  116  and returns current (I 4 ) from signal path  516  at terminal  118 . In this electrical situation, the current (I 2 ) sourced from driver  100  of device  500  is the current (I 4 ) returned to driver  100  of device  502 , and the current (I 3 ) sourced from driver  100  of device  502  is the current (I 1 ) returned to driver  100  of device  500 .  
         [0083]     Since resistors  506 - 512  are assumed to be 25 ohms each and the source currents I 2  and I 3  are assumed to be 5 milliamps each, the voltage present on signal path  514  and signal path  516  are the same or very close to being the same. With the same voltage present on the terminals of PRT  412 , no current, or only a small leakage current, flows through PRT  412 . Thus the voltage drop across PRT (i.e. Vin) that is input to input circuits  504  is extremely small.  
         [0084]     In response to the small Vin voltage, the input circuits  504  of devices  500  and  502  are designed to input the opposite logic level that each device  500  and  502  was outputting. For example, since source  1   202  of device  500  in  FIG. 9A  is outputting a logic low, the input circuit  504  of device  500  will respond to the small Vin voltage by inputting a logic high to destination  2   206  of device  500 . Likewise, since source  2   202  of device  502  in  FIG. 9A  is outputting a logic high, the input circuit  504  of device  502  will respond to the small Vin voltage by inputting a logic low to destination  1   206  of device  502 .  
         [0085]      FIG. 10  is provided to indicate that in applications where noise is low and the signal paths are short, a single termination resistor (RT)  1002  may be used between the signal paths instead of the two separate termination resistors  112  previously shown on the input of the input circuits  504 . It is clear that use of a single termination resistor  1002 , of say 100 ohms, will advantageously increase the Vin voltage to the input circuits  504  using the assumed 5 milliamp LVDS drivers  100 . The operation of the present disclosure in the single termination resistor arrangement of  FIG. 10  is identical to that previously described in  FIGS. 6A-6B  through  9 A- 9 B.  
         [0086]     As seen from the above description of  FIGS. 6A-6B ,  7 A- 7 B,  8 A- 8 B,  9 A- 9 B, and  10 , the present disclosure uses a network of resistances (R 1 , R 2 , R 3 , R 4 , and PRT/RT) in an LVDS signal path  514 - 516  in combination with special input circuits  504  to advantageously enable simultaneous differential signal communication between two devices.  
         [0087]      FIG. 11A  illustrates a first example  1100  that could be used to perform the function of input circuit  504 . Circuit  1100  comprises an inverter  1102  with its input coupled to the output  1120  from source circuit  202 , a differential receiver  102  with its non-inverting input  1108  coupled to signal path  514  and its inverting input  1110  coupled to signal path  516 , a window comparator  1104  with its A and  1112  coupled to signal path  514  and its B input  1114  coupled to signal path  516 , and a multiplexer  1106  with a first input coupled to the output of inverter  1102 , a second input coupled to the output of differential receiver  102 , a control input coupled to the output window comparator  1104 , and an output  1118  coupled to the input to destination circuit  206 .  
         [0088]     The function of the window comparator  1104  is to output a logic high on the C output  1116  whenever the voltage on its A input  1112  is greater than the voltage on its B input  1114  plus as offset voltage (OSV) “or” whenever the voltage on its B input  1114  is greater than the voltage on its A input  1112  plus an offset voltage (OSV). Otherwise the window comparator outputs a logic low on the C output  1116 .  
         [0089]     The offset voltages (OSV) are set such that if a small differential voltage, as described in  FIGS. 8A-8B  and  9 A- 9 B, is present between signal paths  514  and  516 , the voltage differential at the A and B inputs of the window comparator  1104  will not be sufficiently large enough to cause the C output of the window comparator to be set to a logic high. Thus in response to small differential voltages, the window comparator  1104  will output a logic low to the control input of multiplexer  1106 , which causes the inverted Out (Out*) signal from the source  202  of a device to be input to the destination  206  of the same device, via multiplexer output  1118 .  
         [0090]     On the other hand, if an adequately large differential voltage is present between the signal paths  514  and  516 , the differential voltage at the A and B inputs of the window comparator  1104  will be sufficiently large enough to exceed the offset voltages (OSV) and cause the C output of the window comparator to be set to a logic high. For example, the 500 mv signal of polarity shown in  FIGS. 6A-6B  will cause output C of the window comparator to be set high. Likewise, the 500 mv signal of polarity shown in  FIGS. 7A-7B  will cause output C of the window comparator to be set high. If output C of the window comparator is high, multiplexer  1106  will output the output of receiver  102  to the destination  206  via multiplexer output  1118 . For example, receiver  102  will output a logic high to destination  206 , via multiplexer  1106 , in response to receiving the 500 mv signal of polarity shown in  FIGS. 6A-6B . Further, receiver  102  will output a logic low to destination  206 , via multiplexer  1106 , in response to receiving the 500 mv signal of polarity shown in  FIGS. 7A-7B .  
         [0091]     In summary, the input circuit  1100  of  FIG. 11A  outputs the inverted output of the source  202  of a device to the destination  206  of the same device if the differential voltage on signal paths  514  and  516  is small and within a voltage window established by offset voltage (OSV) settings. The input circuit  1100  of  FIG. 11A  outputs the output of the receiver  102  to the destination  206  of the device if the differential voltage on signal paths  514  and  516  is large and outside the voltage window established by the offset voltage (OSV) settings.  
         [0092]      FIG. 11B  illustrates on example circuit  1124  that could be used as window comparator  1104  of  FIG. 11B . The circuit  1124  comprises a first comparator  1126  with its non-inverting input  1132  coupled to input A  1112  and its inverting input  1134  coupled to input B  1114 , a second comparator  1128  with its non-inverting input  1136  coupled to input B  1114  and its inverting input  1138  coupled to input A  1112 , an OR gate  1130  with a first input coupled to the output of comparator  1126 , a second input coupled to the output of comparator  1128 , and an output coupled to output C  1116 .  
         [0093]     Comparator  1126  is designed such that the voltage on its non-inverting input  1132  must be greater than the voltage on its inverting input  1134  by an offset voltage (OSV) value (assumed to be 80 millivolts in this example) before the comparator output will go high. Comparator  1128  is designed such that the voltage on its non-inverting input  1136  must be greater than the voltage on its inverting input  1138  by an offset voltage (OSV) value (assumed to be 80 millivolts in this example) before the comparator output will go high. If the voltage difference on the A and B inputs is less than 80 millivolts, output C goes low. If the voltage difference on the A and B inputs is greater than 80 millivolts, output C goes high. While 80 millivolts was used as an example OSV, any desired value of OSV may be used as well.  
         [0094]      FIG. 12A  illustrates a second example circuit  1200  that could be used to perform the function of input circuit  504 . Circuit  1200  comprises an inverter  1202  with its input coupled to the output  1216  from source circuit  202 , a window comparator  1204  with its A input  1208  coupled to signal path  514  and its B inputs  1210  coupled to signal path  516 , and a multiplexer  1206  with a first input coupled to a fixed logic high, a second input coupled to a fixed logic low, a third input coupled to the output of inverter  1202 , a control input coupled to output D  1214  of window comparator  1204 , and an output  1218  coupled to the input to destination circuit  206 .  
         [0095]     The functions of the window comparator  1204  are: 
    (1) to output a logic high on output C and a logic low on output D whenever the voltage on its A input  1208  is greater than the voltage on its B input  1210  plus an offset voltage (OSV) “and” the voltage on its B input  1210  is less than the voltage on its A input plus an offset voltage (OSV),     (2) to output a logic low on output C and logic high on output D whenever the voltage on its A input  1208  is less than the voltage on its B inputs  1210  plus an offset voltage (OSV) “and” the voltage on its B input  1210  is greater than the voltage on its A input plus an offset voltage (OSV),     (3) to output a logic low on output C and output D whenever the voltage on its A input  1208  is less than the voltage on its B input  1210  plus an offset voltage (OSV) “and” the voltage on its B input  1210  is less than the voltage on its A input plus an offset voltage (OSV).    
 
         [0099]     The offset voltages (OSV) are set such that if a small differential voltage, as described in  FIGS. 8A-8B  and  9 A- 9 B, is present between signal paths  514  and  516 , the voltage differential at the A and B inputs of the window comparator  1204  will not be sufficiently large enough to cause both the C and D outputs of the window comparator to be set high. Thus in response to small differential voltages, the window comparator  1204  will output logic lows to the control inputs of multiplexer  1206 , which causes the inverted Out (Out*) signal from the source  202  of a device to be input to the destination  206  of the same device, via multiplexer output  1206 .  
         [0100]     On the other hand, if an adequately large differential voltage is present between the signal paths  514  and  516 , the differential voltage at the A and B inputs of the window comparator  1104  will be sufficiently large enough to exceed the offset voltages (OSV) and cause either the C or D output of the window comparator to be set high. For example, the 500 mv signal of polarity shown in  FIGS. 6A-6B  will cause output C to be set high and output D to be set low. Likewise, the 500 mv signal of polarity shown in  FIG. 7A-7B  will cause output D to be set high and output C to be set low.  
         [0101]     If output C is high and output D is low, multiplexer  1206  will output the fixed logic high input to destination  206  via multiplexer output  1218 . If output C is low and output D is high, multiplexer  1206  will output the fixed logic low input to destination  206 . And as mentioned, if both output C and D are low, multiplexer  1206  will output the inverted output (Out*) of the source  202  of a device to the destination  206  of the same device.  
         [0102]     In summary, the input circuit  1200  of  FIG. 12A  outputs the inverted output of the source  202  of a device to the destination  206  of the same device if the differential voltage on signal paths  514  and  516  is small and within a voltage window established by offset voltage (OSV) settings. The input circuit  1200  of  FIG. 12A  outputs the fixed logic high to the destination  206  of the device if the differential voltage on signal paths  514  and  516  is such that the voltage on input A is sufficiently larger than the voltage on input B plus the offset voltage (OSV). The input circuit  1200  of  FIG. 12A  outputs the fixed logic low to the destination  206  of the device if the differential voltage on signal paths  514  and  516  is such that the voltage on input B is sufficiently larger than the voltage on input A plus the offset voltage (OSV).  
         [0103]      FIG. 12B  illustrates one example circuit  1220  that could be used as window comparator  1204  of  FIG. 12B . The circuit  1220  comprises a first comparator  1222  with its non-inverting input  1226  coupled to A  1208 , its inverting input  1228  coupled to input B  1210 , and its output coupled to output C  1212 , and a second comparator  1224  with its non-inverting input  1230  coupled to input B  1210 , its inverting input  1232  coupled to input A  1208 , and its output coupled to output D  1214 .  
         [0104]     Comparator  1222  is designed such that the voltage on its non-inverting input  1226  must be greater than the voltage on its inverting input  1228  by an offset voltage (OSV) value (80 millivolts in this example) before the comparator output C will go high. Comparator  1224  is designed such that the voltage on its non-inverting input  1230  must be greater than the voltage on its inverting input  1232  by an offset voltage (OSV) value (80 millivolts in this example) before the comparator D will go high. If the voltage difference on the A and B inputs is less than 80 millivolts, comparator outputs C and D go low. While 80 millivolts was used as an example OSV, any desired value of OSV may be used as well.  
         [0105]      FIGS. 13A-13D  show another circuit  1300  than could be used to realize window comparator  1104  of  FIG. 11A . Circuit  1300  comprises a first comparator  1302  with its non-inverting input coupled to the A input and its inverting input coupled to a reference voltage (assumed to be 250 mv in the  FIG. 13A-13D  examples), a second comparator  1304  with its non-inverting input coupled to the B input and its inverting input coupled to the reference voltage, an OR gate  1306  with a first input coupled to the output of comparator  1302 , a second input coupled to the output of comparator  1304 , and an output coupled to the C output. To simply the description, circuit  1300  will be shown used in the signaling arrangements previously described in  FIGS. 6B, 7B ,  8 B, and  9 B, and with the previously assumed resistance and current values stated for said Figures.  
         [0106]      FIG. 13A  illustrates that in the previously described signaling arrangement of  FIG. 6B , the voltage (625 mv) on the A input of circuit  1300 , coupled to signal path  514 , will be greater than the reference voltage (250 mv) and the voltage (125 mv) on the B input, coupled to signal path  516 , will be less than the reference voltage (250 mv). Thus comparator  1302  will output a logic high to OR gate  1306  and comparator  1304  will output a logic low to OR gate  1306 . In response, the OR gate will output a logic high on the output C, causing multiplexer  1106  of  FIG. 11A  to output the output of receiver  102  to destination  206  as previously described.  
         [0107]      FIG. 13B  illustrates that in the previously described signaling arrangement of  FIG. 7B , the voltage (125 mv) on the A input of circuit  1300 , coupled to signal path  514 , will be less than the reference voltage (250 mv) and the voltage (625 mv) on the B input, coupled to signal path  516 , will be greater than the reference voltage (250 mv). Thus comparator  1302  will output a logic low to OR gate  1306  and comparator  1304  will output a logic high to OR gate  1306 . In response, the OR gate will output a logic high on the output C, causing multiplexer  1106  of  FIG. 11A  to output the output of receiver  102  to destination  206  as previously described.  
         [0108]      FIG. 13C  illustrates that in the previously described signaling arrangement of  FIG. 8B , the voltage (125 mv) on the A input of circuit  1300 , coupled to signal path  514 , will be less than the reference voltage (250 mv) and the voltage (125 mv) on the B input, coupled to signal path  516 , will be less than the reference voltage (250 mv). Thus comparator  1302  will output a logic low to OR gate  1306  and comparator  1304  will output a logic low to OR gate  1306 . In response, the OR gate will output a logic low on the output C, causing multiplexer  1106  in  FIG. 11A  to output the output (Out*) of inverter  1102  to destination  206  as previously described.  
         [0109]      FIG. 13D  illustrates that in the previously described signaling arrangement of  FIG. 9B , the voltage (125 mv) on the A input of circuit  1300 , coupled to signal path  514 , will be less than the reference voltage (250 mv) and voltage (125 mv) on the B input, coupled to signal path  516 , will be less than the reference voltage (250 mv). Thus comparator  1302  will output a logic low to OR gate  1306  and comparator  1304  will output a logic low to OR gate  1306 . In response, the OR gate will output a logic low on the output C, causing multiplexer  1106  of  FIG. 11A  to output the output (Out*) of inverter  1102  to destination  206  as previously described.  
         [0110]      FIGS. 13E-13H  depicts another circuit  1308  that could be used to realize window comparator  1204  of  FIG. 12A . Circuit  1308  comprises a first comparator  1310  with its non-inverting input coupled to the A input, its inverting input coupled to a reference voltage (assumed to be 250 mv in the  FIG. 13E-13H  examples), and an output coupled to the C output, a second comparator  1312  with its non-inverting input coupled to the B input, its inverting input coupled to the reference voltage, and an output coupled to the D output. To simply the description, circuit  1308  will be shown used in the signaling arrangements previously described in  FIGS. 6B, 7B ,  8 B, and  9 B, and with the previously assumed resistance and current values states for said Figures.  
         [0111]      FIG. 13E  illustrates that in the previously described signaling arrangement of  FIG. 6B , the voltage (625 mv) on the A input of circuit  1308 , coupled to signal path  514 , will be greater than the reference voltage (250 mv) and the voltage (125 mv) the on B input, coupled to signal path  516 , will be less than the reference voltage (250 mv). Thus comparator  1310  will output a logic high on the C output and comparator  1312  will output a logic low on the D output. In response to C being high and D being low, multiplexer  1206  of  FIG. 12A  will output the fixed logic high input to destination  206  as previously described.  
         [0112]      FIG. 13F  illustrates that in the previously described signaling arrangement of  FIG. 7B , the voltage (125 mv) on the A input of circuit  1308 , coupled to signal path  514 , will be less than the reference voltage (250 mv) and the voltage (625 mv) the on B input, coupled to signal path  516 , will be greater than the reference voltage (250 mv). Thus comparator  1310  will output a logic low on the C output and comparator  1312  will output a logic high on the D output. In response to C being low and D being high, multiplexer  1206  on  FIG. 12A  will output the fixed logic low input to destination  206  as previously described.  
         [0113]      FIG. 13G  illustrates that in the previously described signaling arrangement of  FIG. 8B , the voltage (125 mv) on the A input of circuit  1308 , coupled to signal path  514 , will be less than the reference voltage (250 mv) and the voltage (125 mv) the on B input, coupled to signal path  516 , will be less than the reference voltage (250 mv). Thus comparator  1310  will output a logic low on the C output and comparator  1312  will output a logic low on the D output. In response to C being low and D being low, multiplexer  1206  of  FIG. 12A  will output the output (Out*) of inverter  1202  to destination  206  as previously described.  
         [0114]      FIG. 13H  illustrates that in the previously described signaling arrangement of  FIG. 9B , the voltage (125 mv) on the A input of circuit  1308 , coupled to signal path  514 , will be less than the reference voltage (250 mv) and the voltage (125 mv) the on B input, coupled to signal path  516 , will be less than the reference voltage (250 mv). Thus comparator  1310  will output a logic low on the C output and comparator  1312  will output a logic low on the D output. In response to C being low and D being low, multiplexer  1206  of  FIG. 12A  will output the output (Out*) of inverter  1202  to destination  206  as previously described.  
         [0115]     While  FIGS. 11A-11B ,  12 A- 12 B, and  13 A- 13 G have shown various examples of how to design input circuits  504  for use by the present disclosure, it is anticipated that other ways of designing input circuits  504  will be conceived by those skilled in the art. Thus the present disclosure is not limited to only using the example input circuit designs shown and described herein.  
         [0116]      FIG. 14  illustrates two devices  1400  and  1402  coupled together using an LVDS signal path  514 - 516  for transferring data signals and an LVDS signal path  1424 - 1426  for transferring clock signals. The devices communicate data simultaneously between each other using input circuit  504 , driver  100 , and signaling path resistor network as previously described. The data being communicated could be of any data type, including but not limited to; functional data, test data, debug data, trace data, and emulation data.  
         [0117]     Device  1400  comprises a deserializer  1404  for inputting serial data from input circuit  504 , data receiving circuitry  1406  for inputting parallel data from the deserializer  1404 , a serializer  1408  for inputting serial data to driver  100 , and data providing circuitry  1410  for inputting parallel data to serializer  1408 . The combination of the data receiving circuitry  1406  and deserializer  1404  represent one example design for a destination circuit  206 . The combination of the data providing circuitry  1410  and serializer  1408  represent one example design for a source circuit  202 . Device  1400  also comprises clock output circuitry  1412  and an LVDS clock driver  1428 . The clock output circuitry  1412  provides a clock output to driver  1428  and outputs control (CTL) signals to operate the providing circuitry  1410 , serializer  1408 , deserializer  1404 , and receiving circuitry  1406 . The control (CTL) signals output to the serializer and deserializer from the clock output circuit will operate faster than the control signals to the receiving and providing circuits since they will be controlling the higher speed serial input and output operations occurring over signal paths  514  and  516 . The clock output circuit  1412  may employ use of clock and control signal modification circuits such as but not limited to; a phase lock loop, a phase shifter, a frequency divider, or a frequency multiplier. Driver  1428  is similar to driver  100  and drives LVDS clock outputs from device  1400  on signal paths  1424  and  1426 . LVDS clocking is shown being used to provide high speed clock signals from device  1400  to device  1402 . If desired, single ended clocking could be used instead of the differential clocking shown, but the clocking frequency would be reduced between device  1400  and  1402 . Device  1400  is assumed to be a master device since it outputs the LVDS clock on signal paths  1424 - 1426 .  
         [0118]     Device  1402  comprises a deserializer  1418  for inputting serial data from input circuit  504 , data receiving circuitry  1420  for inputting parallel data from the deserializer  1418 , a serializer  1414  for inputting serial data to driver  100 , and data providing circuitry  1416  for inputting parallel data to serializer  1414 . As in device  1400 , the combination of the data receiving circuitry  1420  and deserializer  1418  represent one example design for a destination circuit  206 , and the combination of the data providing circuitry  1416  and serializer  1414  represent one example design for a source circuit  202 . Device  1402  also comprises clock input circuitry  1422  and an LVDS clock receiver  1430 . The clock input circuitry  1422  receives the clock output from receiver  1430  and outputs control (CTL) to operate the providing circuitry  1416 , serializer  1414 , deserializer  1418 , and receiving circuitry  1420 . The control (CTL) signals output to the serializer and deserializer from the clock input circuit will operate faster than the control signals to the receiving and providing circuits since they will be controlling the higher speed serial input and output operations occurring over signal paths  514  and  516 . The clock input circuit  1422  may employ use of clock and control signal modification circuits such as but not limited to; a phase lock loop, a phase shifter, a frequency divider, or a frequency multiplier. Receiver  1430  is similar to receiver  102  and inputs the LVDS clock outputs from device  1400  on signal paths  1424  and  1426 . Device  1402  is assumed to be a slave device since it inputs the LVDS clock on signal paths  1424 - 1426 .  
         [0119]     During operation data is transmitted from the providing circuitry  1410  and serializer  1408  of device  1400  to the deserializer  1418  and receiving circuitry  1420  of device  1402 . Simultaneous with data transmitted from device  1400  to device  1402 , data is transmitted from the providing circuitry  1416  and serializer  1414  of device  1402  to the deserializer  1404  and receiving circuitry  1406  of device  1400 . The simultaneous data transfers between devices  1400  and  1402  are controlled by clock output circuitry  1412  of device  1400  and the clock input circuitry  1422  of device  1402 . As mentioned, internal to device  1400 , clock output circuitry  1412  provides the control (CTL) inputs to operate the providing  1410 , serializer  1408 , deserializer  1404 , and receiving  1406  circuits. External to the device, Clock output circuitry  1412  provides the LVDS clock input to device  1402 . Internal to device  1402 , and in response to the LVDS clock input from device  1400 , the clock input circuitry  1422  provides the control (CTL) inputs to operate the providing  1416 , serializer  1414 , deserializer  1418 , and receiving  1420  circuits.  
         [0120]      FIG. 15  is provided to indicate that a plurality of the providing ( 1410 , 1416 ), serializer ( 1408 , 1414 ), deserializer ( 1404 , 1418 ), and receiver ( 1406 , 1420 ) circuit arrangements  1504 - 1510  of  FIG. 14  could exist in devices  1500  and  1502 . Each arrangement  1504 - 1506  in device  1500  operable, in response to the clock output circuitry  1412  to communicate data simultaneously with an associated arrangement  1508 - 1510  in device  1502  via an input circuit  504 , driver  100 , resistors, and LVDS signal path  1512 / 1514 .  
         [0121]      FIG. 16  illustrates a device  1600  coupled to a debug, trace, or emulation controller  1610  via an LVDS signal path  1606  and LVDS clock path  1608  according to the present disclosure. The debug, trace, or emulation controller  1610  is similar in design to the master device  1400  of  FIG. 14  with the exception that its specific function is to control a debug, trace, or emulation operation in device  1600  via the data and clock signal paths  1606  and  1608 . Device  1600  is similar to the slave device  1402  of  FIG. 14  with the exception that the providing circuit  1416  of  FIG. 14  is indicated to be a memory or other circuit  1602  that needs to be controlled by device  1610  to output data during a debug, trace, or emulation operation, and the receiving circuit  1420  of  FIG. 14  is indicated as being a memory or other circuit  1604  that needs to be controlled by device  1610  to input data during a debug, trace, or emulation operation.  
         [0122]     Using the LVDS signaling approach of the present disclosure, much higher debug, trace, and/or emulation communication can occur between master device  1610  and slave device  1600 , as opposed to other approaches used in the industry today. For example, it is well known to use the IEEE 1149.1 standard interface (i.e. JTAG) for debug, trace, and/or emulation operations. However, standard JTAG communication rates between a master and slave device is limited to around 50-100 MHz. Since the present disclosure uses LVDS signaling, the communication rates between a master  1610  and slave  1600  during debug, trace, and/or emulation operations can be greater than 400 MHz. Indeed, using the LVDS signaling approach of the present disclosure, communication for debug, trace, and/or emulation operations may well extend into the gigahertz range. Device  1600  can be extended, as shown in device  1502  of  FIG. 15 , to include a plurality of LVDS signal paths and associated driver  100 , input circuits  504 , serializer  1414  and deserializer  1418  such that high speed communication to greater number of debug, trace, and/or emulation circuits  1602  and  1604  is possible.  
         [0123]      FIG. 17  is provided to indicate that a slave device  1700  may use a shift register  1702  during debug, trace, and/or emulation operations instead of a separate serializer  1414  (i.e. a serial in/parallel our circuit) and a separate deserializer  1418  (i.e. a parallel in/serial our circuit) if desired. In operation the shift register  1702  loads parallel debug, trace, and/or emulation data from circuit  1602  and shifts the data out to driver  100  as debug, trace, and/or emulation data is shifted in from input circuit  504  to be loaded in parallel to debug, trace, and/or emulation circuit  1604 .  
         [0124]      FIG. 18  illustrates a device  1800  coupled to an IC or Die tester  1810  via an LVDS signal path  1806  and LVDS clock path  1808  according to the present disclosure. The tester  1810  is similar in design to the master device  1400  of  FIG. 14  with the exception that its specific function is to control a test operation in device  1800  via the data and clock signal paths  1806  and  1808 . Device  1800  is similar to the slave device  1402  of  FIG. 14  with the exception that a scan path  1802  is coupled between the output of the input circuit  504  and the input of driver  100 , and a circuit under test  1804  is shown coupled to the scan path  1802  to be the receiving  1420  and providing  1416  circuits during test operations. Device  1800  can be a packaged IC, an unpackaged IC die, or a die on wafer. The circuit under test  1804  is typically, but not limited to being, combinational logic. The serial data input to scan path  1802  from input circuit  504  is stimulus test data to be applied in parallel  1812  to the inputs of circuit under test  1804 . The serial data output from scan path  1802  to driver  100  is response test data loaded in parallel  1814  to the scan register from the circuit under test outputs. Scan testing is well known. What is new is performing scan testing using the LVDS signaling approach of the present disclosure.  
         [0125]     Using the LVDS signaling approach of the present disclosure, much higher test input and output communication can occur between master device  1810  and slave device  1800 , as opposed to other approaches used in the industry today. For example, known scan interface used in the industry today (IEEE standards 1149.1 and 1500) are limited to scan test communication rates/frequencies of around 50-100 MHz. Since the present disclosure uses LVDS signaling, the communication rates between a master  1810  and slave  1800  during scan testing can be greater than 400 MHz. Indeed, using the LVDS signaling approach of the present disclosure, communication for scan test operations may well extend into the gigahertz range.  
         [0126]      FIG. 19  illustrates a device  1900  coupled to an IC or Die tester  1912  via a plurality of LVDS signal paths  1906 - 1908  and an LVDS clock path  1910  according to the present disclosure. Each LVDS signal path  1906 - 1908  is coupled to an arrangement  1902 - 1904  of drivers  100 , input circuits  504 , scan paths  1802 , and circuits under test  1804 . The tester  11912  is similar to tester  1810  with the exception that it can communicate to the device  1900  over the plurality of LVDS signal paths  1906 - 1908 , instead of the single LVDS signal path of  FIG. 18 . By increasing the number of LVDS signal paths and arrangement  1902 - 1904  a large number of circuits  1804  can be tested in parallel, which decreases test time of device  1900 .  
         [0127]      FIG. 20  illustrates either a plurality or ICs  2018 - 2030  in a fixture  2000  or a plurality of die  2018 - 2030  on a wafer  2000  interfaced to a plural IC or die tester  2002  via LVDS data and clock signal paths  2004 - 2016 . If the IC or die  2018 - 2030  are the type shown in  FIG. 18 , there will be one LVDS data signal path pair and one LVDS clock signal path pair between the tester  2002  and each IC or die  2018 - 2030 . If the IC or die  2018 - 2030  are the type shown in  FIG. 19 , there will be one LVDS clock signal path pair and a plurality of LVDS data signal path pairs (indicated by increased line width) between the tester  2002  and each IC or die  2018 .  FIG. 20  illustrates how a plurality of ICs  2018 - 2030  in a fixture  2000  or a plurality of die  2018 - 2030  on a wafer  2000  may be scan tested in parallel (i.e. at the same time) using the LVDS signaling approach of the present disclosure.  
         [0128]     While  FIGS. 18-20  have illustrates the LVDS signaling approach of the present disclosure for testing ICs or die using a scan test approach, other test approaches may be interfaced to the LVDS signaling approach of the present disclosure as well. Other test approaches that may be interfaced to the LVDS signaling interface of the present disclosure may include but are not limited to, (1) a test approach based on IEEE standard 1149.1, (2) a test approach based on IEEE standard 1149.4, (3) a test approach based on IEEE standard 1149.6, (4) a test approach based on IEEE standard 1500, (5) a test approach based on built in self test, and (6) a test approach based on functional testing.  
         [0129]     Although the present disclosure has been described in detail, it should be understood that various changes, substitutions and alterations may be made without departing from the spirit and scope of the disclosure as defined by the appended claims.