Abstract:
A superconductor crossbar switch for connecting a plurality of inputs with a plurality of outputs, including a switching cell having an input, an output and a circuit for connecting the input with the output for bidirectionally transmitting data therebetween. The connection of the retaining and releasing circuitry of a plurality of cells enables the switch to simultaneously retain a selected cell or cells of a group of cells and disable the remaining cells of that group, whereby a subsequent query on a disabled cell is inoperative until the selected cell or cells is released. The crossbar switch is characterized by latency on the order of nanoseconds, a data rate per channel on the order of gigabits per second, essentially zero crosstalk, and detection of contention in nanoseconds or less and resolution of contention in nanoseconds or less.

Description:
[0001]    This application claims priority from U.S. Provisional Application Serial No. 60/306,880 filed Jul. 23, 2001. The entirety of that provisional application is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to superconductive switching devices, and in particular to a superconducting crossbar switch for bidirectionally connecting a plurality of inputs with a plurality of outputs.  
           [0004]    2. Background of the Technology  
           [0005]    Advances in high performance computing are being pursued in many different directions. The technology thrust has been directed toward very high speed, high circuit density chips which are of low power (to permit small volume packaging) and organized into a small number of processors. Another thrust involves the use of many processors, tens to perhaps thousands, working in concert to perform the computation. In this case, the stress on the individual elements is relieved and there is greater computational power, but interconnection problems that arise with the added software complexity must be solved.  
           [0006]    One of the configurations for a massively parallel computing system calls for a large number of processors to be connected to a large shared memory system on an equal access basis. The demands placed upon the interconnection switch are formidable, in terms of complexity, speed, and intelligence. For example, the switch must have a short latency time and must establish the requested connection very quickly, ideally within a small fraction of the processor clock time. The data rate per channel must also be very high. For example, for a 32 bit word machine with a 30 nanosecond clock, a data rate of 10 9  bits/second (i.e., gigabits/second) per processor is required. Once established, the data path must be immune to noise, and crosstalk must be kept to a minimum. The established link must be inviolate during the processor transaction time and releasable very quickly, ideally within a clock cycle.  
           [0007]    There is a need to inform the processor of successful connection. The time during which two or more processors contend for the same memory port needs to be minimized with fast resolution of these contentions. Finally, data needs to be transferred in both directions. Although there are a number of switch architecture solutions, it is generally accepted that the best solution is a crossbar, which is a switch that allows the requesters equal access at the same level to any output line.  
           [0008]    Computer systems also need high bandwidth and short access times to carry out data exchange between memory and processors, and among processors.  
           [0009]    3. Related Art  
           [0010]    Crossbar switches are well-known in the prior art, as evidenced by U.S. Pat. No. 3,539,730 to Imamura, which discloses a crossbar switch used in a two-stage link connection system. Each switch is divided into two parts, in accordance with vertical groups. The parts of the switch are assigned to primary and secondary lattices, respectively, with links between the lattices being formed by connecting the outgoing lines from the primary lattice of one switch with the secondary lattice of another switch.  
           [0011]    Also known in the art are polarity switching circuits which utilize Josephson junction devices (e.g., interferometers) and superconducting interconnections coupled to a utilization circuit, including one or more memory cells or logic circuits. Such circuits are disclosed, for example, in U.S. Pat. No. 4,210,921 to Faris.  
           [0012]    Prior switching circuits possess certain inherent drawbacks that render them unsuitable for use with large numbers of computing elements. As a result, they cannot meet all of the requirements set forth above for a massively parallel computing system.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention overcomes the above identified drawbacks of the prior switching circuits, as well as others, by providing a modular crossbar switch that is extendable in size, operates under low power with low latency, and detects and resolves conflicts that arise when two or more processors contend for the same memory port. The switch of the present invention is capable of interconnecting N computers or processors with M memories, or other processors or computers where N and M can be of the order of 1000 or more. One embodiment of the present invention is also modular, in that small crossbars can easily be extended to become very large ones, (e.g., 32×32 can grow into 1000×1000). In addition, if the computer data rate exceeds that of one channel, paralleling of channels is easily performed. The switch is also suitable for general communications network usage, as well.  
           [0014]    An embodiment of the present invention includes a crossbar switch for connecting a plurality of input devices with a plurality of output devices, and a switching cell having an input, an output, and an apparatus for connecting the output for bi-directionally transmitting data therebetween. The connecting apparatus includes a superconductive device having zero resistance and negligible crosstalk, and a control device to control operation of the connecting apparatus. The connecting apparatus provides a connection for a plurality of processors or functional units to be connected to one another. For example, a configuration of adders, multipliers, and dividers can be switched, such that data can be routed sequentially from one function to another with arbitrary freedom.  
           [0015]    Another embodiment of the present invention includes a second superconductive device and a second control device to retain and release the operation of the first superconductive device.  
           [0016]    An additional embodiment of the present invention includes a plurality of inputs, a plurality of outputs, and a plurality of cells arranged in a matrix, with the inputs coupled to one plurality of cells and the outputs connected to another plurality of cells, so as to define a superconducting device matrix. In an embodiment of the present invention, the cells are connected in parallel with the inputs and outputs.  
           [0017]    In a further embodiment of the present invention, each output includes a summing device for summing output voltages or currents of the cells connected therewith, in order to accommodate the inputs and to render the matrix extendable in numbers of inputs and outputs. The summing device may include a summing amplifier or an additional superconductive device. In another embodiment of the present invention, the switching cells include a feedback mechanism connected to the outputs which feeds data to the outputs and acknowledges pulses back to a requester.  
           [0018]    In yet another embodiment of the present invention, retaining and releasing devices for the cells are connected to the outputs and are interconnected and operable to simultaneously retain a selected cell of the plurality of cells, and disable the remaining cells of the plurality of cells, whereby a subsequent query on a disabled cell is inoperative until the selected cell is released. The crossbar also allows multicast or broadcast operation wherein any one input may be connected simultaneously or in arbitrary order to more than one or all of the output ports.  
           [0019]    In a further embodiment of the invention, a sensing apparatus is connected with each of the outputs for detecting simultaneous queries to cells of the respective groups of cells and for generating to the processors via the cells an indication of conflict from the simultaneous queries as well as resolving these conflicts while preventing further interference.  
           [0020]    Additional advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become more apparent to those skilled in the art upon examination of the following or upon learning by practice of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0021]    In the drawings:  
         [0022]    [0022]FIG. 1 illustrates a prior art Josephson junction device;  
         [0023]    [0023]FIG. 2 is a graph illustrating the operation of the Josephson junction device of FIG. 1;  
         [0024]    [0024]FIG. 3 is a simplified perspective view of a prior art Josephson junction device with a magnetic field control line;  
         [0025]    [0025]FIG. 4 shows prior art operation of Josephson junctions in which a resistor is placed between the electrode and the counter-electrode for the device shown in FIG. 1;  
         [0026]    [0026]FIG. 5 is a schematic representation of a prior art Superconducting Quantum Interference Device (SQUID) device;  
         [0027]    [0027]FIG. 6 is a graph representing the operation of the SQUID device of FIG. 5;  
         [0028]    [0028]FIG. 7 illustrates a matrix of cells comprising a superconductive crossbar switch, in accordance with an embodiment of the present invention;  
         [0029]    [0029]FIGS. 8 and 9 illustrate use of the superconductive crossbar switch, in accordance with an embodiment of the present invention;  
         [0030]    [0030]FIGS. 10 and 11 illustrate use of the superconductive crossbar switch, connected to a summing device, in accordance with an embodiment of the present invention;  
         [0031]    [0031]FIG. 12 is a schematic representation of a switch illustrating a plurality of summing devices, and the clamping and crossbar cell memory circuit, in accordance with an embodiment of the present invention;  
         [0032]    [0032]FIG. 13 is a schematic representation of the cell circuits and clamp circuit and their operation in the situation of no contention, in accordance with an embodiment of the present invention;  
         [0033]    [0033]FIG. 14 is a flow diagram illustrating operation of the circuits of FIG. 12, in accordance with an embodiment of the present invention;  
         [0034]    [0034]FIG. 15 is a timing diagram illustrating the operation of the circuits of FIG. 13, and a situation of non-simultaneous request (no contention) for a memory line, in accordance with an embodiment of the present invention;  
         [0035]    [0035]FIG. 16 is a schematic representation of the cell circuits in the situation of two simultaneous requests for the same memory line, in accordance with an embodiment of the present invention;  
         [0036]    [0036]FIG. 17 is a timing diagram illustrating the operation of two processors contending for the same output line, in accordance with an embodiment of the present invention;  
         [0037]    [0037]FIG. 18 illustrates a representative physical layout of a 128×128 crossbar switch, in accordance with an embodiment of the present invention; and  
         [0038]    [0038]FIG. 19 illustrates a representation of a crossbar switch chip, including separate decoders and the switching matrix, in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0039]    [0039]FIG. 1 illustrates a Josephson tunnel junction device known in the prior art. The Josephson tunnel junction device includes top and bottom layers  20 ,  21  of superconductor material sandwiching a thin insulating film  22 . If a voltage V is applied between the top and bottom layers through a resistance R, there is a range of current in which zero resistance current up to I m , can be transported between the two elements.  
         [0040]    [0040]FIG. 2 illustrates the behavior of the circuit current I as the input voltage V is increased for a representative resistance R, in the Josephson tunnel junction device shown in FIG. 1. The voltage V j  across the device will be zero until the device current exceeds I m , at which point the junction will switch to the voltage state consistent with the circuit load resistor R and the device&#39;s own voltage-current curve J, determined by the physics and manufacturing art.  
         [0041]    [0041]FIG. 3 illustrates a Josephson junction device  29  known in the prior art, in which switching occurs by imposing a magnetic field into a junction via a control line placed above it. The current I to be controlled is carried through a first layer of superconducting material  30  on a substrate  31 . A thin film of insulator  32  separates the first superconducting material  30  from a second layer of superconducting material  33 . An insulator layer  35  separates layer  33  from a third layer of superconducting material  36 . When a control current I c , passes through layer  36 , a magnetic field  37  is created at the junction, which reduces the maximum allowed zero resistance current. Thus, if the device&#39;s transport current I is greater than the new allowed value, the device will switch into the voltage state, similar to as described above with regard to FIGS. 1 and 2. The device can be fabricated to switch with picosecond rise times, with its final voltage state in the millivolts range for presently available materials. The currents that are switched are most often in the hundreds of microamperes range. The power dissipation per unit is in the microwatt range.  
         [0042]    The prior art also includes fabrication of Josephson Junctions in which the device has the current versus voltage curve represented by FIG. 4, as compared with FIG. 2. This behavior may be acquired by a resistor being placed between the electrode and the counter-electrode of the device in FIG. 1. Or, equivalently, one may achieve such a “weak link” behavior by fabricating the Josephson device with a conductor between the two electrodes of FIG. 1. It is also well known that such behavior is a standard property of so-called “high temperature” superconductive Josephson Junctions. The effect of such a device is to provide a voltage, when switched, which is dependent upon the resistors in the circuit. Nevertheless, the circuits required can still be made from such junctions.  
         [0043]    If one connects a Josephson junction in parallel with an inductance, the closed loop forms a Superconducting Quantum Interference Device (SQUID), which is also known in the prior art. Insertion of a second junction into this loop, as illustrated in FIG. 5, also produces a SQUID, but with device properties that are very advantageous in switching applications. In particular, if an input current I g  is inserted and divided between the two junctions, J 1 , and J 2 , that zero resistance transport current can be controlled by introducing magnetic flux into the closed loop via the control current, I c . FIG. 6 shows the curve of allowed zero resistance current, I g , as a function of the imposed control current, I c . I m  represents the maximum gate current I g  as a function of the control current I c . In particular, if an input current  I   g  is inserted, it will be divided into two paths according to the size of the inductors L 1 , and L 2  and the maximum critical currents of J 1 , and J 2 , as well as by the control line current I c , which is magnetically coupled to the loop. FIG. 6 represents the joint values of I g  and I c , for which current  I   g  can be transported through the loop with zero resistance. This region is represented by the shaded voltage transport of I g . A control line can thus be used to change the maximum zero resistance current of a two terminal Josephson junction, or SQUID. The detailed properties depend upon the inductance, critical currents of the device, and insertion point(s) of the currents. (A more detailed explanation of the structure and operation of the Josephson junction and the SQUID is found in the IBM Journal of Research and Development, vol. 24, No. 2 (March 1980), which is hereby incorporated by reference.)  
         [0044]    Superconductive Crossbar Switch  
         [0045]    [0045]FIG. 7 illustrates a superconductive crossbar switch, in accordance with an embodiment of the present invention. The superconductive crossbar switch  39  includes at least one cell  41  in a matrix, which are arranged in rows and columns in accordance with the number of input lines I 1 ,  40  and output lines O 1    43 . For example, there are N inputs and M outputs for coupling, such as via or including wired, wireless, or fiberoptic connections, N processors with M memories. The number of inputs and outputs need not be equal. In one embodiment of the present invention, the superconductive crossbar switch  39  is extendable to accommodate large numbers of processors and memories. Thus, for example, the module can easily be extended from a 32 input×32 output to a 1024 input×1024 output configuration, as is further described below.  
         [0046]    Each input port connects to a row of cells  41  via an input line I 1 ,  40 . Each cell  41  includes a connecting circuit  44 , which connects the input line I 1 , to a selected output line O 1  for bidirectionally transmitting data therebetween. The connecting circuit includes a first superconductive device  42 , which has zero-resistance. A first control signal applied to a first terminal  45  controls the first superconductive device  42  externally on command, for controlling operation of the superconductive crossbar switch  39 . In one embodiment, the first control signal comprises an electrical current.  
         [0047]    Each cell  41  also includes a retaining and releasing circuit for retaining (i.e., clamping) and releasing the operation of the first superconductive device  42 . The retaining and releasing circuit includes a second superconductive device  46  a second control signal, delivered through a clamp line  49  at a second terminal  47 , for controlling the second superconductive device  46  and the devices  46  of the cells  41  in the same column of cells  41 , as shown in FIG. 7.  
         [0048]    The first and second superconductive devices  42  and  46  can also be addressed by optical illumination, in another embodiment of the present invention. For example, if the first superconductive device  42  is optically illuminated, the switch cell connection from input to output will be maintained for the duration of the optical signal. In effect, the optical beam has “enabled” the desired connection. If the second superconductive device  46  is addressed by the optical beam, the current will be steered into the control line for the first superconductive device  42 . Alternatively, an electron beam could be used instead of an optical beam.  
         [0049]    When the cells  41  are arranged in a matrix as shown in FIG. 7, each input line I i    40  is coupled to a cell  41  (e.g., a row of cells), thereby to define a matrix of cells.  
         [0050]    Method of Using the Superconductive Crossbar Switch  
         [0051]    [0051]FIGS. 8 and 9 illustrate use of the superconductive crossbar switch  39 , in accordance with an embodiment of the present invention. Referring to FIG. 8, the processors and memories coupled to the input lines  40  (I i =I 4 , I 5 , I 6 ) and output lines  43  (O i =O 8 , O 9 , O 10 ), respectively, need not be synchronously clocked, but instead may be run independently. The operation will be described for the example of a 32 input×32 output crossbar chip organized as shown in FIG. 8, but it should be understood that any number of inputs  40  and outputs  43  may be provided. In this example, each of the  32  input lines from the  32  processors transmits a serial bit stream. The first serial bit word, or part of it, from a processor (or other source), contains the address of the specific memory line which the processor is attempting to acquire. In addition, the address bits are followed by a “FLAG” bit, a “one.” This first word carrying the destination address and the FLAG bit is input to the requesting processor&#39;s data line. The decoder selects the appropriate  1   st  control line ( 45   a ) and powers it, thereby permitting the FLAG bit to proceed to the output line.  
         [0052]    The initial state of each cell is a zero current condition in the first address terminals  45   a ,  45   b ,  45   c , and  45   d , corresponding to the first terminal  45  in FIG. 7. As there is no current in the address lines, all the devices  42   a ,  42   b ,  42   c , and  42   d  will short the processor pulses on input lines  40  to ground and therefore no output is observed at output lines  43 .  
         [0053]    If now the processor coupled to input line  14  attempts to access the memory coupled to output line O 8 , the process of FIG. 9 is followed. The processor decoder selects the address line for the output line O 8 , contained in the processor&#39;s request word (step  905  of FIG. 9). After the address line is found (decoded), it is determined if there is a control current for the address line (step  910  of FIG. 9).  
         [0054]    If there is a control current (step  910  of FIG. 9), a decoder current is impressed at terminal  45   a , which depresses the zero resistance current threshold of superconductive device  42   a , thereby allowing input pulses to be transferred across the superconductive device  42   a  (step  915  of FIG. 9). Subsequent pulses from the processor or input line I 4  are then fed into the output line O 8 , and thus, for example, into a summing circuit  50 .  
         [0055]    If there is no control current for the address line (step  910  of FIG. 9), the input pulses on input line I 5  are not transferred to output line O 8  (step  920  of FIG. 9), but are shorted to ground by the superconductive device  42   b , as shown in FIG. 8. Thus, the input pulses from another processor do not interfere with the data pulses from input line I 4  on output line O 8 .  
         [0056]    Correspondingly, if, for example, input line I 5  seeks to send data to output line O 9 , then a current is impressed at terminal  45   d  by the processor decoder and the input data pulse stream is then imposed upon output line O 9 , with no interference from the processor coupled to input line I 4  because its control line  45   b  is not driven.  
         [0057]    Superconductive Crossbar Switch Coupled to Summing Device  
         [0058]    [0058]FIGS. 10 and 11 illustrate use of an example superconductive crossbar switch  39  coupled to a summing device  68 , in accordance with an embodiment of the present invention. Referring to FIG. 10, each cell  41  is similar to the cells  41  shown in FIG. 8. In addition, each output line  43  is coupled to a summing device  68  containing a third superconductive device  51  controlled by the control line  65 . An input driver circuit  52  couples each input line to its corresponding processor.  
         [0059]    [0059]FIG. 11 illustrates an exemplary process for using the superconductive crossbar switch  39 , coupled to a summing device  68 , as shown in FIG. 10. In FIG. 10, pulses a from the processor drive additional superconductive devices  55  into voltage state and thereby impress a voltage on the input line  40  (step  1105  of FIG. 11), which is coupled to all the row cells accessed by that processor. The impressed voltage causes a current to flow through the resistor  57  to be shorted to ground via the first superconductive device  42  (step  1110  of FIG. 11).  
         [0060]    It is determined if the control signal provided at terminal  45  is powered (step  1115  of FIG. 11). If yes, that control signal can be made sufficient to reduce the critical current through first superconductive device  42 , such that it exhibits a “gap” voltage (step  1120  of FIG. 11). The pulse current passing through the first superconductive device  42  will exceed the maximum zero resistance current and the first superconductive device  42  will switch into the voltage state, thereby impressing its “gap” voltage upon the cell tie point between resistors  57  and  62 .  
         [0061]    If the control signal is not powered (step  1115  of FIG. 11), the superconductive device  42  may be operated such that it transfers to a resistive state, or the superconductive device  42  itself may be fabricated such that it does not exhibit a “gap” voltage (step  1125  of FIG. 11). This voltage will cause a current to flow through resistor  62  down to the output line  63  through control  65  and additional superconductive devices  66 . This current will be insufficient to switch additional superconductive devices  66 , and therefore, since control  65  is of very low inductance, the voltage across control  65  and additional superconductive devices  66  will be very small and will decay very rapidly, such that the current through resistor  62  will predominately go through control  65  and only a negligible amount will pass through resistor  67 , and eventually all current will pass through control  65  and additional superconductive devices  66 .  
         [0062]    The current through control  65  depresses the maximum allowed zero resistance current of superconductive device  51 , which then triggers and produces a signal for transfer to the memory circuits (step  1130  of FIG. 11). In an alternative embodiment, other equivalent sensing circuits may be used instead. The pulse sensed by the memory circuit is inverted, amplified, and fed back via terminal  70  (step  1135  of FIG. 11) after an appropriate delay, with sufficient current to exceed the allowed maximum current through superconductive devices  66  and thereby impose a voltage on the output line  43 , which will cause current to flow through resistors  62  and  67 . However, only superconductive device  42  has a suppressed maximum current, and therefore only line  56  will experience a current back into control  71  via resistor  57 ; input line  72  will not. As before, control  71  will control superconductive device  75  and the current through superconductive devices  55  will be too small to switch superconductive devices  55  into the voltage state. The input pulse a will be returned as an ‘acknowledge’ pulse t only to the processor  61 , which generated pulse a (step  1140  of FIG. 11), and to no other, provided that the selected cell  41  is the only one energized. This return path is also valid for transfers of data from memory back to the processor  61 .  
         [0063]    Superconductive Crossbar Switch with Summing Devices  
         [0064]    With reference to FIG. 12, there is shown a crossbar switch  39  having a number of summing devices  78 , represented in this embodiment by amplifiers M 1 , coupled to each output line  43  (e.g., O 3 , O 9 , O 10 ). The summing amplifier M 9  is coupled to output line O 9 , and so on. The summing devices  78  are operable for summing the output voltages of the cells coupled to the respective output line O 1 . Summing the input voltages in this manner enables the crossbar switch to accommodate a plurality of inputs and thus renders the matrix extendable in numbers of inputs and outputs. This comes about because the input terminal of the amplifier is summed to zero voltage, thereby producing no crosstalk from the selected processor to the other processors.  
         [0065]    Superconductive Crossbar Switch with Additional Junction and Control Line  
         [0066]    In order to prevent interference by other processors after an output line has been acquired, an additional junction and control line is provided as is illustrated in FIG. 13. The operation of this device will also be described below with reference to FIGS. 14 and 15. In the crossbar&#39;s initial state, current is applied to clamp line C 8 , thereby depressing the maximum zero resistance current of superconductive device  46   a  (step  1405  of FIG. 14). When terminal  45   a  is activated, current will be caused to flow through inductor  106  and control line  107 , because the critical current of superconductive device  46   a  has been reduced to below the imposed decoder current level. This will occur because the clamp current C suppresses the maximum zero resistance current of device  46   a .  
         [0067]    With respect to cell  41   b , current at terminal  45   b  will initially flow into inductor  108  because the inductance of inductor  108  is required to be lower than the inductance of inductor  106  (step  1410  of FIG. 14). However, since superconductive device  46   a  has its maximum zero resistance current reduced because of the signal imposed at C 8 , superconductive device  46   a  will switch to the voltage state, and all the current imposed on terminal  45   b  will be directed through inductor  106  and control line  107 .  
         [0068]    After the decoder has applied its current at terminal  45   b,  a flag pulse or set of pulses is inserted into the processor datastream at I 4  (step  1415  of FIG. 14). These pulses would normally immediately follow those that select the address. When these flag pulses are detected on the output line O 8  for the cell, the CLAMP current on C 8  will be dropped (step  1420  of FIG. 14). Now, if decoder power to terminal  45   b  is removed, the flux stored in inductor  106  will be maintained by a circulating current in the loop comprising inductors  106 ,  108  and device  46   a.  This action of dropping the clamp signal succeeds in not only retaining the usage of the output line in cell  41   a  after the decoder is powered down, but it also prohibits interference by other requesters for the same output line (e.g., in cell  41   c ).  
         [0069]    With reference to cell  45   c  of FIG. 13, the initial state has the CLAMP line C 8  energized, similar to as described above with regard to FIG. 7. Removal of the CLAMP current at C 8  causes the critical current of device  46   a  to no longer be depressed (step  1425  of FIG. 14). Decoder power applied to terminal  45   c,  will then flow predominately through inductor  111 , which is required to have much smaller inductance than the inductance of inductor  112 .  
         [0070]    The resulting current through inductor  112 , and thus control line  113 , will be insufficient to depress the critical current of device  116  enough for it to switch when data current flows through resistor  117 . This, in effect, prevents interference by the processor (step  1430  of FIG. 14) coupled to line I 5 , with the output line O 8  already in use. By extension, this operation will hold for all late requesters for an output line.  
         [0071]    [0071]FIG. 15 summarizes the above described behavior. Processor  1  is shown having powered its decoder output, thereby permitting its flag bit to be sent to the SENSE circuits. At a later time, this causes the CLAMP to be dropped at time C from OPEN to CLAMPED at the cell location. Processor  2  thus is unable to insert its flag pulse onto the output line. Finally, the “acknowledge” return pulse is received by only processor  1 , as processor  2  connection is not enabled.  
         [0072]    Contention Situation  
         [0073]    If two processors request the same memory line at the “same time,” a contention situation occurs. For example, in FIG. 16, if the address lines  45   b  and  45   d  are “simultaneously” powered, contention will occur between the processors coupled via inputs I 4  and I 5  for the memory coupled to output line O 9.  This will cause the memory acquisition to “flag” bits from both the processors coupled to I 4  and I 5 , and thus to drive the output line at the same time. This will produce two units of current in the control line  91 , which triggers the “contention” sensor  92 . Detection of this event will cause the support electronics to ignore the SENSE signal and to keep the CLAMP line on current HIGH. This function may also be provided by cryogenic circuitry. No return “acknowledge” pulse is sent, thereby, by its absence, informing the requesting sources of their failure to acquire the requested output line.  
         [0074]    Situation where Two Processors Have Requested Memory  
         [0075]    [0075]FIG. 17 depicts the situation wherein processors  1  and  2  have requested the memory at the same time. In that event, the clamp line is not dropped at C, the crossbar cells on that memory line stay available, and no “acknowledge” pulse is returned to the requesters. This silence advises them to retry. If processor  2  requests the memory line at a time between a and C, the electronics can still keep CLAMP high, withhold “acknowledge,” and thereby maintain availability to other requesters. This may be done at cryogenic temperature or at room temperature.  
         [0076]    Example of 128×128 Switch  
         [0077]    [0077]FIG. 18 shows an example of a 128 input×128 output crossbar switch embodying the features of the present invention. In this example, 64 processors  126 - 126  are coupled to a processor glue chip  127  and 64 memories  128 - 128  are each coupled to a memory glue chip  131 . There are 64 more processors  136 - 136  coupled to a second processor glue chip  137  and an additional 64 memories  132 - 132  coupled to a second memory glue chip  133 . Connected between these glue chips is a crossbar switch  138  essentially comprising a plurality of interconnecting matrices of cells S 1 -S 16 , each of which is a 32×32 crossbar matrix. Each of the 64 processors  126 - 126  is coupled via an input data line  141  to processor glue chip  127 , to which each of the 64 processors  126 - 126  transmits serial bit data. Processor glue chip  127  outputs and receives that data into chips S 1 , S 2 , S 5 , S 6  for transactions to and from memories  128 - 128  by the 64 processors  126 - 126 . It also outputs and receives the data into chips S 9 , S 10 , S 13 , S 14  for transactions to and from memories  132 - 132  by the same 64 processors  126 - 126 . Likewise, chips S 3 , S 4 , S 7 , S 8  connect processors  136 - 136  to memories  128 - 128  while chips S 11 , S 12 , S 15 , S 16  connect processors  136 - 136  to memories  132 - 132 .  
         [0078]    The selection of a memory line by a given processor is accomplished by including a destination memory address in that processor&#39;s submitted data word and clocking it via the appropriate input clock line on the proper crossbar chip (i.e., the required input processor and sought-for-output memory line). The destination address may also be introduced by an external controller and may also be decoded by an external decoder.  
         [0079]    The return data from the interrogated memory line is fed into the corresponding memory glue chip as DRIVE, returned in parallel to the crossbar bank and is transferred to only the activated and locked processor line. From there, it continues to the corresponding processor glue chip and on to the originating processor. Clamping is accomplished by controlling a separate line (not shown), which disables access of all the unselected processors to the activated memory line. Contention is separately detected on the memory glue chip.  
         [0080]    Switch Chip  
         [0081]    [0081]FIG. 19 illustrates an embodiment of a switch chip that interconnects 32 input lines to 32 output lines via the previously described matrix of cells. In this example, each processor is assigned and coupled to its own decoder, which decodes the destination address that was requested by that processor and activates the address line of the proper cell in the matrix, as previously described. Such a chip may be replicated to populate the 128×128 matrix described in FIG. 17.  
         [0082]    Example embodiments of the present invention have been described in accordance with the above advantages. It will be appreciated that these examples are merely illustrative of the invention. Many variations and modifications will be apparent to those skilled in the art.