Abstract:
A system for connecting multiple users to multiple computers uses a switch for making and breaking connections between individual computers and input/output devices such as display screens, keyboards and pointing devices at user locations. A supervisory computer system includes one or more helper computers which run programs to aid the user in selecting connections to be made. The switch is arranged to capture preselected input sequences as designating commands, such as a command to connect the helper computer, and to forward these to the supervisory computer system. The supervisory computer system actuates the switch in response to these commands so that the user can establish communication with the helper computer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims benefit of U.S. provisional patent application No. 60/063,695 filed Oct. 28, 1997 the disclosure of which is hereby incorporated by reference herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to computer systems, and more particularly relates to systems in which plural users can interface with plural computers.  
         BACKGROUND OF THE INVENTION  
         [0003]    Various computer systems allow a user to employ a computer at a remote location. For example, some mainframe computer systems support remote terminals connected to the mainframe computer by dedicated lines or by other communications links. As the computing power available in small computers referred to as personal computers or “PCs” has increased, many applications previously performed on mainframe systems has been transferred to PCs. In many cases, PCs are connected to one another, to larger computers or both through networks which allow the transfer of information among the various computers. Thus, a user at any location can run programs on his or her own computer using files taken from other computers on the network, and also can run programs on other computers.  
           [0004]    While this approach offers numerous advantages, it also suffers from certain disadvantages. Maintaining all of the individual computers at dispersed locations takes considerable time and effort. The dispersed computers and their components are vulnerable to theft and damage. Moreover, controlling software updates on all of the various computers and assuring that each individual computer has the appropriate software presents a significant challenge. This challenge is especially significant in environments such as software development laboratories where the software to be used is changing continually. Moreover, the requirement that a computer be present in the immediate vicinity of the user means that the user must put up with the noise and heat generated by the computer and means that the computer will occupy some of the space which would otherwise be available at the user&#39;s desk. This latter drawback is especially annoying to users who must employ several computers at once as, for example, some securities and commodities traders.  
           [0005]    Various proposals have been advanced to alleviate these problems. For example, as disclosed in U.S. Pat. No. 5,721,842, input devices such as a keyboard and mouse and output devices such as a display monitor and speaker may be provided at numerous user locations. These devices are connected to signal conditioning circuits or “pods”, which in turn connect to a crosspoint switch. Numerous computers are also provided. The computers are connected through other “pods” to other terminals of the crosspoint switch. The crosspoint switch is arranged to connect any of the pods associated with user locations to any of the pods associated with the computers, so that each user can be connected to any computer. These connections include both digital connections for passage of input signals such as keyboard and mouse data to the computer and analog connections for video signals sent by the computer. Thus, the user can operate the remote computer in much the same manner as a user having a keyboard, mouse and display screen directly linked to the computer. In order to allow the user to select different computers, the system provides a processor within each pod at each user location. This processor recognizes special command keystroke sequences entered by the user, formats these commands and transmits the commands over separate command data lines to the crosspoint switch. The pod contains a rudimentary program which actuates the monitor to display a menu of the available commands when the user enters the beginning of the command sequence on the keyboard.  
           [0006]    This approach suffers from several serious drawbacks. The pods can at most display rudimentary lists of commands. This offers little or no guidance to the user in selecting appropriate computers to connect with for specific purposes. Moreover, it is difficult to control access by specific users to specific computers. Further, the need for separate command channels extending through the user locations, as well as the need for even rudimentary programmability and display generation capabilities at the user locations significantly increase the cost and complexity of the system. Thus, despite significant efforts heretofore in the development of multi-user, multi-computer systems, there still remains a considerable need for improvement in such systems.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention addresses these needs. One aspect of the present invention includes a method of interfacing a plurality of server computers with input and output devices at a plurality of user locations. A method according to this aspect of the invention includes the step of conveying input signals from input devices such as keyboards which are included in sets of input and output (“I/O”) devices at user locations to server computers associated with the user locations and conveying output signals from such server computers to output devices as, for example, display monitors included in the sets at the user locations. A method according to this aspect of the invention desirably further includes the step of detecting one or more predetermined helper codes in the input signals and connecting a helper computer to the set of input and output devices at the user location in response to the helper code. Typically, the helper computer is connected to the particular set of I/O devices which sent the helper code. The method further includes the step of running a program in the helper computer which interacts with the set of I/O devices connected to the helper computer and allows the user at such set to select one or more of the server computers for connection or disconnection. Additionally, the method includes the step of actuating a switch to connect or disconnect server computers as selected during operation of the interactive program for the input and output devices at one or more of the user locations. Most commonly, the interactive program run by the helper computer will allow a user at a particular location connected to the helper computer to select one or more of the server computers for connection or disconnection to that particular user location. Thus, a user can select computers for connection to his own location. In a variant, the program on the helper computer can allow the user to select computers for connection to other user locations.  
           [0008]    Methods according to this aspect of the invention can greatly simplify the task of the user. Thus, the program on the helper computer can take advantage of all of the techniques used by modern programmers to provide a user-friendly environment. For example, the helper computer may run the program in an environment such as a Windows® operating system which provides a graphical user interface. The helper computer program can present information about the available servers in any desirable manner as, for example, by presenting the servers organized in groups according to the types of programs available on each server or other criteria which are meaningful to the user. Moreover, the helper computer can obtain this list of servers from a database at a central location which can be maintained using conventional database management techniques.  
           [0009]    The helper computer program desirably includes routines for determining user identities as, for example, password identification utilities which require the user to enter an indication of his identity coupled with a secret password associated with that identity. Thus, the database may include information defining access rights for particular users or groups of users and the helper computer program may control access to servers according to the access rights set forth in the database. These capabilities can be provided using conventional programming techniques in the helper computer or another computer connected to the helper computer, whereas they would be difficult or impossible to provide without the use of the helper computer. Moreover, because only one or a few helper computers are required, and because these helper computers can be located at a centralized location along with the server computers and the switch, it is relatively easy to maintain the helper computers and to assure physical security of the entire system.  
           [0010]    Most preferably, each helper computer is connected into the switch in much the same manner as a server computer, so that the switch can connect each helper computer to any of the user locations. The switch itself desirably is controlled by a supervisory computer system. Typically, but not necessarily, the supervisory computer system includes a separate switch control in addition to the helper computer or computers. The switch control computer may be connected to the helper computer by a connection independent of the switch as, for example, a local area network so that the helper computer can pass information to define desired connections or disconnections to the switch control computer. The helper computers can be managed dynamically as a resource, so that requests for use of the helper computers can be queued and passed to the next available helper computer.  
           [0011]    Desirably, the method further includes the step of detecting action codes other than the helper codes in the input data supplied by the user and actuating the switch to make or break connections between servers and sets of input and output devices at user locations in response to the action codes without using the interactive program running on the helper computer. For example, the switch control computer can be arranged to respond to the action codes independently of the helper computer and can be arranged to recognize the helper codes and can actuate the switch to connect the helper computer in response to the helper codes. The use of the action codes minimizes the system overhead involved with simple tasks. For example, the system desirably maintains a running set of servers associated with each user location. This running set includes a few particular servers which have been selected through use of the interactive program on the helper computer. An action code may allow the user to move through this set one server at a time and thus allow the user to “toggle” between servers of the running set. Thus, the helper computer is not involved where it is not needed.  
           [0012]    The computers used to supervise operation of the system may be arranged to record information about usage of the various servers. For example, the system can record the identities of users who access particular servers and the times of such access. Moreover, the switch control computer may be arranged to monitor faults in components of the system and to disable defective parts so as to preserve security of the system.  
           [0013]    A further aspect of the present invention provides a system for interfacing a plurality of server computers with output and input devices at a plurality of user locations. This system includes a switch selectively operable to connect the server computers with output and input devices at user locations so that any one of the server computers can be associated with a set of devices at user locations and so that the associated servers and devices are connected to another through the switch. This allows users at the user location to interact with the associated server computers. The system according to this aspect of the invention also includes a supervisory computer system. The supervisory computer system is connected to the switch so that the supervisory computer system can control operation of the switch. In this aspect of the invention, the supervisory computer system desirably includes a helper computer operative to run a program as discussed above, for selecting one or more of the server computers for association with a set of input and output devices at a user location. The switch is selectively operable to associate the helper computer with a set of input and output devices at a user location and connect the input and output devices to the associated helper computer. The system according to this aspect of the invention also includes code recognition devices connected to receive the input signals sent by the input devices. The code recognition devices are operative to detect one or more predetermined helper codes in the input signals and to pass the helper codes to the supervisory computer system. The supervisory computer system is arranged to instruct the switch to connect the set of input and output devices which sent the helper code to the helper computer. Thus, users at the user locations can selectively interact with the helper computer to select server computers as discussed above in connection with the method.  
           [0014]    Apparatus according to a further aspect of the invention provides a system for connecting plural I/O devices at plural user locations to selected computers in a plurality of server computers. Apparatus according to this aspect of the invention includes a plurality of server ports adapted for connection to server computers and a plurality of user ports adapted for connection to input devices, together with a switch for selectively associating user ports and server ports so that input device signals supplied to a user port by an input device connected thereto can be conveyed to an associated server port and to a server connected to such server port. The apparatus further includes code recognition devices associated with the user ports. The code recognition devices are operated to detect one or more command codes in input signals supplied to the user ports and to provide a code output including code data representing such command codes and address data representing the identity of the user port carrying the input signals in which the command code was detected. Most preferably, the code recognition devices are disposed at a central location, such as within the switch itself. Thus, command codes may be sent along with the input data, in the same data stream, to the central location. There is no need for a separate set of conductors to carry the command data. The command codes may include the helper codes and action codes as discussed above in connection with other aspects of the invention, and the system may also include devices such as a supervisory computer system for controlling the switch in response to the command codes.  
           [0015]    The ability of the system to operate without extra communication lines to the user locations for interchange of the command codes simplifies construction of the system and allows installation at reduced cost.  
           [0016]    The one or more code recognition devices desirably includes a plurality of user interface processors each connected to a subset including one or more of the user ports. Most preferably, each user interface processor is associated with only one user port. The system may further include at least one control processor and, for each control processor, a control data channel connecting a set of the user interface processors with such control processor. This connection is arranged so that each user interface processor can send code data representing a command code to the control processor and so that the control processor can identify the particular user interface processor which sent such command code data. For example, each control processor can be connected to the associated user interface processors through a time division multiplex control data link, so that each user interface processor sends any command codes in a predetermined timed slot of the multiplexed transmission scheme. The control processor appends address data to each command code based upon the identity of the user interface processor. Where there is more than one control processor, the control processor may assign part of the address data based on the identity of the control processor itself. Thus, the address data accompanying each item of code data representing a command code identifies the particular user port where the command code was detected. This allows the supervisory computer system or switch to act on the command data depending upon the identity of the user port. For example, where the command data indicates that the user port which originated the command should be connected to the helper computer, the supervisory computer system can establish the appropriate connection of the helper computer to the correct user port.  
           [0017]    Yet a further aspect of the invention provides methods of operating a plurality of server computer from a plurality of user locations which includes the steps of sending input data in a data stream along with command codes from input devices at user locations to user ports at one or more central locations where data from several users is concentrated. The method further includes the step of detecting command codes at the central locations, forwarding the command codes to a supervisory computer; forwarding the input data from the user locations through a switch to one or more of the server computers; and actuating the switch in response to some or all of the command codes to change connections between the server computers and the devices at the user locations. Here again, because the command codes are detected at the central location where data from several user location is concentrated, there is no need for separate command data lines.  
           [0018]    In a particularly preferred arrangement, the connections between the central location or locations and the user locations can be made through eight conductors constituting three twisted pairs for red, green and blue video signals, and another pair of conductors for bi-directional communication of input signals and command data from the input devices at the user to the central location and reverse data as, for example, for controlling characteristics of the user interface devices. These eight conductors can be connected, for example, with standard eight position connectors such as RJ45 telephone-type connectors and conventional wiring of the types used for computer networks.  
           [0019]    These and other objects, features and advantages of the invention will be more readily apparent from the detailed description set forth below, taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a diagrammatic view depicting apparatus used in one embodiment of the invention.  
         [0021]    [0021]FIG. 2 is a further diagrammatic view illustrating certain components of the apparatus depicted in FIG. 1.  
         [0022]    Each of FIGS. 3, 4,  5 ,  6 ,  7  and  8  is a further diagrammatic view illustrating additional components of the apparatus depicted in FIG. 1.  
         [0023]    [0023]FIG. 9 is a representation of a screen display used in one program employed in operation of the apparatus in FIGS.  1 - 8 .  
         [0024]    [0024]FIG. 10 is a diagrammatic view depicting a system in accordance with a further embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    A system in accordance with one embodiment of the invention includes a plurality of computers  20  referred to herein as servers. Although only a few server computers  20  are depicted in FIG. 1, it should be appreciated that a typical system typically will include tens or even hundreds of server computers. Each computer  20  includes all of the internal components normally found in a personal computer as, for example, central processing units, memory storage devices such as disk drives and all of the components used for connecting these elements with one another. These internal elements of the server computer may be of any conventional type. Also, some or all of these server computers may include optional components such as data communications cards, modems, and the like for connecting the server computers or devices outside of the system. Each computer  20  includes a conventional VGA video output connection  22  (FIG. 3), keyboard connection  24  and mouse connection  26 . The computer is arranged in the conventional manner to receive keyboard inputs through connection  24  and also to provide some keyboard control signals as, for example, signals which control the status of indicator lights for caps lock, number lock and scroll lock lamps on the keyboard. Similarly, the computer is arranged to receive mouse inputs at connection  26  and to send mouse control signals through the same connection. These connections are arranged in the conventional manner, normally used with standard keyboards and mice. The VGA output connection  22  is arranged to provide video output in the VGA format accepted by conventional monitors. The VGA format includes separate analog RGB (red, green, blue) color signals, each of 0.7 volts peak to peak with separate horizontal and vertical synchronization signals on different signal lines. The synchronization signal polarity differs depending on the screen resolution. In ordinary use of a personal computer, connections  22 ,  24  and  26  are connected to a monitor, keyboard, and mouse, respectively, in the immediate vicinity of the computer, typically within about a meter or less. The signals provided at these ports, and particularly the VGA video signal are not well suited to long distance transmission.  
         [0026]    Each server computer  20  is connected to a device  28  referred to herein as a transmitter. Each transmitter is located in close proximity to the associated computer, typically within a meter of the computer. Transmitters  28  may be housed within the computers themselves or else may be housed in a separate chassis holding a few transmitters connected to server computers in the immediate vicinity of such separate chassis. Transmitters  28  may be conventional devices used for adapting the VGA output from the computer to a format suitable for long-distance transmission. Each transmitter is also arranged to adapt the keyboard and mouse connections  24  and  26  to send and receive data in a format which is also suited for long-distance transmission. Devices of this nature are well known in the art. One such device is commercial available under the trademark FreeDesk Transmitter from CCC Group, PLC of Farnbrough, Hants, United Kingdom and from CCC USA, Inc. of Melville, N.Y., USA. The FreeDesk Transmitter includes a video conditioning circuit  30  and a set of three differential output amplifiers  32 . These elements cooperate to convert each of the red, green and blue color signals in the incoming VGA signal into a pair of output signals having opposite polarities. The pair of video signals representing each color is provided at the output of one output amplifier  32 . The video conditioning circuit also serves to impress the horizontal synchronization signal onto the pair of output signals representing the red video signal. A combined horizontal and vertical synchronization signal is applied on the pair of outputs representing the green video signal, whereas a signal representing the polarity of the original VGA synchronization signal is combined with the pair of signals representing the blue video component. Transmitter  28  also includes a microprocessor  32  connected to the keyboard and mouse connection  24  and  26  of the computer. The microprocessor is connected to an outgoing serial data connection  34  and incoming serial data connection  36  which provide a duplex serial data communication link. The microprocessor combines control signals sent by the computer through the keyboard and mouse connections  24  and  26  into an outgoing serial data stream provided on this duplex link. The microprocessor is arranged to extract keyboard and mouse data from an incoming data stream on the same duplex link and to route the incoming data to the keyboard and mouse ports  24  and  26 .  
         [0027]    The three pairs of analog color signals from amplifiers  32  and the serial data connections  34  and  36  are connected at eight pin positions in a standard RJ45 telephone-type jack, as shown in Table I.  
                                   PIN   USAGE                   1   Red video − HS, 2 volts p-p       2   Red video + HS, 2 volts p-p       3   Current serial multiplex send       4   Green video − CS, 2 volts p-p       5   Green video + CS, 2 volts p-p       6   Current serial multiplex receive       7   Blue video − composite polarity, 2 volts p-p       8   Blue video + composite polarity, 2 volts p-p                  
 
         [0028]    The system also includes a set of output devices, typically monitors  40  and input devices such as keyboards  42  and mice or other pointing devices  44 . These I/O devices are disposed at numerous user locations  46 . Although only a few user locations  46  are depicted in FIG. 1, it should be appreciated that a typical system may include tens or even hundreds of user locations. Typically, user locations  46  are dispersed as, for example, throughout a building. The set of input and output devices disposed at each user location is arranged in groups. Each group may be a full group, incorporating a single monitor  40 , a single keyboard  42  and a single mouse or other pointing device  44 , or else may be a partial group omitting one or more of these devices. Typically, at least one full group is provided at each user location. For example, user location  46 A has only a single full group, whereas user location  46 B has a full group and three partial groups, each such partial group including only a monitor  40 . Each group of input and/or output devices is associated with a protocol converter referred to herein as a receiver (RX)  48 . Receiver  48  has an RJ45 jack  50  with connections corresponding to the connections in the jack of the transmitter. The receiver further includes differential input amplifiers  54  connected to the pairs of inputs on jack  50 . These input amplifiers are connected to a video conditioning circuit  56 . The video conditioning circuit is arranged to reverse the transformation provided by the video conditioning circuit  30  of the transmitter and to provide a standard VGA output signal at an output connection  58 . Receiver  48  further includes microprocessor  60  connected to a keyboard input jack  62  and a mouse input jack  64 . The microprocessor is also connected to two pins  66  and  68  which serve as the serial input and serial output respectively of the receiver. The microprocessor is arranged to accept keyboard input data from a standard keyboard  42  and mouse input data such as movement and button click data from a standard mouse  44  and to provide such data in a serial format on the duplex communication link provided by connections  66  and  68 . The microprocessor is also arranged to accept keyboard and mouse control data on the duplex communication link and send such control data to keyboard  42  and mouse  44 . The features of the receiver  48  may be identical to those used in the well known and commercially available FreeDesk (trademark) Receiver. In the well-known FreeDesk system, a jack  38  of a particular computer can be connected to the jack  50  of a receiver  48  using wiring having four twisted pairs as, for example, high-grade wiring of the type commonly referred to category 5 568B cable extending over tens or of meters, typically up to about 200 meters. When the transmitter and receiver are connected in this manner, the user can operate the computer using the group of I/O devices connected to the receiver  48 . Operation of the FreeDesk system is transparent to the user; the user can interact with the computer in substantially the same way as he or she could interact with a computer at his or her location connected directly to the keyboard, mouse and monitor. The same type of connection can be used to provide a part of the interface. For example, where the output of a computer is to be displayed at the users location, but input from the user at that location is not desired, a monitor can be connected to the VGA output of the receiver  48 , whereas the keyboard  42  and mouse  44  may be omitted. The system further includes a switch  70  (FIGS. 1 and 2) in addition to the FreeDesk transmitters and receivers. Switch  70  includes a set of server interface chassis  72  and a set of user interface chassis  74 . Here again, only a few of these components are shown in the drawings for clarity of illustration; typically, a large number of server interface chassis and a large number of user interface chassis are provided. Each server interface chassis  72  includes a backplane  76  with a 64-channel video bus  76   a  providing connections for  64  red, green and blue video signals, i.e., 192 individual color signals arranged in triplets. Each backplane  76  also includes a data bus  76   b  with connections for four bi-directional time division multiplexed (“TDM”) data channels. These data channels are operated with sixteen time slots per channel in each direction, and hence the data bus can accommodate  64  bi-directional data channels. The backplane further includes a control data serial bus  76   c . The backplane further includes power lines (not shown) for powering the various cards held by chassis  72 , as well as slot address pins (not shown) which interact with each card in the chassis, to designate a unique card address for each card.  
         [0029]    Each server interface chassis  72  has up to four server interface cards  78  mounted therein. Each server interface card is arranged to accept and send control signals and status information on control bus  76   c  through a control data connection  90 . Each of the server interface cards includes sixteen RJ45 type jacks or server input ports  80 , of which only one is shown in FIG. 5. In use server input ports  80  are connected to the output connectors  38  of transmitters  28  (FIG. 1). Each of the server input ports  80  is connected to a server port interface circuit  82 , which includes a microprocessor and video conditioning circuitry. Each server port  80 , and the associated circuit  82  have a unique address within the chassis  72 . Thus, the ports on the first card  78  are at intra-chassis server port addresses  1 - 16 ; those on the second card are at addresses  17 - 32 , and so on. The video conditioning circuit is adapted to convert each of the differential analog color signal pairs (red, green and blue) to a corresponding single signal, and to compensate for line losses and delays in the analog circuitry conveying the signal to the port. The microprocessor included in each server port interface circuit  82  is arranged to convert between serial and TDM data formats. The analog video signals of all sixteen interface circuits  82  are connected through a buffer and selective enabling circuit  83  to a sixteen channel RGB video backplane connector  84 , which in turn is connected to sixteen of the video channels  76   a  of the backplane. Circuit  83  is also arranged to enable or disable video transmission from each port  80  to the connector  84  in response to control signals addressed to that circuit from a control processor  87 . Control processor  87  in turn is connected to the control bus of the backplane through the control data connection  90  of the server interface card. As further discussed below, other components selectively make or break video connections with the video channels  76   a  of the backplane so as to connect the video from individual servers to the desired users. The video connection from each port  80  to the connector  84  and hence to the backplane video channels  76   a  could remain enabled at all times. However, the connection to the backplane of those video channels which are not connected to any user is disabled so as to save power and reduce noise on the backplane.  
         [0030]    The server interface card  78  further includes a multiplex circuit  86 . The multiplex circuit is connected to the processor in each of the interface circuits  82 . The processor in each of the interface circuits  82  captures serial data transmitted into the port  80  and sends the same to multiplex circuit  86 . The multiplex circuit is arranged to send the data captured from the various serial signals passing through the interface circuits in time division multiplex format, in a single bi-directional TDM channel with 16 bi-directional TDM slots, through connection  88 , such that the data from each server port  80  is sent in a particular time slot of the TDM transmission scheme. Similarly, the time division multiplex circuit  86  is arranged to accept time division multiplex data on connection  88  and to send data in a particular time slot of the TDM transmission scheme to a particular interface circuit  82  and server port  80  associated with that particular time slot.  
         [0031]    The server interface card  78  is connected to the backplane  76  of a server interface chassis  72  so that the sixteen channel video connector  84  of each card  78  is connected to sixteen of the sixty-four video channels on the backplane. Thus, each port  80  is coupled to the video channel corresponding to the intra-chassis address of that port. Also, the TDM data connection  88  of each card  78  is connected into one of the four TDM data channels of the backplane so that the serial data connections of each server port  80  are connected to the corresponding TDM slot on the backplane. For example, the port with intra-chassis address  17  is coupled to that 17th video channel of bus  76 D and to the 17th TDM slot of TDM data channels  76   b.    
         [0032]    Each server interface card  78  further includes an expansion connection  92 . The expansion connection carries buffered replicates of the incoming video signals and also carries a TDM data channel including the 16 bi-directional TDM slots as carried by data connection  88 . As discussed below, the expansion connection can be used to connect additional server interface chassis into the system. The video signals to expansion connection  92  are always enabled.  
         [0033]    Each server interface chassis  72  also includes up to 16 matrix cards  94 . Each matrix card  94  includes an analog switching circuit  96 ; a digital switching circuit  98  and a controller  100 . Controller  100  may include one or more microprocessors connected through a control port  102  to receive control signals on the control bus  76   c  of backplane  76 . Each matrix card  94  includes a 32 channel video input connector  104  and a 32 channel video output connector  106 . These connectors are arranged to handle 32 channels of RGB video signals. The matrix card further includes a 64 channel video backplane connector  108  adapted to connect the 64 channel RGB video bus  76   a  of the backplane with the analog switching circuit  96 . Appropriate buffer amplifiers (not shown) are also associated with the input connectors  108  and  104  and with the output connector  106 . The matrix card  94  further includes a four channel TDM backplane connection  110  adapted to mate with the data TDM  76   b  of the backplane in the server chassis  72 ; a dual 16 slot TDM input connection  112  providing  32  TDM data slots in all; and a similar, dual 16 slot output connection  114 . The TDM connections  110 ,  112  and  114  are linked to digital crosspoint controller  98 .  
         [0034]    The analog switching circuit  96  may include a set of crosspoint switching devices such as twenty-four AD8116 16×16 Crosspoint Video Switches having inputs connected to backplane connector  108  and having outputs connected to the channels of output connector  106 , together with additional switches for selectively connecting each channel of input connector  104  to the corresponding channel of output connector  106 . As further discussed below, the analog switching circuit  96  can connect any of the 32 output video channels on output connector  106  with any of the 64 video channels from the backplane at connector  108 , or with any of the thirty-two video channels from the video input connector  104 . Similarly, the digital switching circuit includes a set of switching devices which may incorporate a pair of I-Cube 96 Way Keyboard and Mouse Switches connected to TDM backplane connection  110  and to the TDM output connection  116 . Circuit  98  can associate any of the 32 bi-directional slots provided by the two outgoing TDM channels at output  114  with any of the sixty-four TDM slots available on the backplane or with any of the thirty-two TDM slots available at the input connection  112 .  
         [0035]    Each server interface chassis  72  also includes a controller card  120 . Each controller card includes two identical halves  122 . Each half of the controller card incorporates a controller  124 , desirably an 80×86 microprocessor such as an 80186 microprocessor of the type available from the Intel Corporation. The two microprocessors  124  are connected to one another so that each can monitor the status of the other. When power is first applied to the controller card, the two microprocessors compete for control of the system; the first one of the two microprocessors which completes its power-on sequence wins control. The half  112  of the card having the winning microprocessor  124  remains active, whereas the other half remains inactive but continues to perform a check on the first half. However, upon a failure or fault condition, in the active half of the card, the inactive half takes over, and the other half enters the fault-checking mode.  
         [0036]    Each half of the card includes a clock or timing circuit  126  arranged to produce clock and framing signals as needed for time division multiplex communications. Timing circuits  126  are connected to timing ports  127 . Each half of the card also includes appropriate communications interfaces (not shown) adapted to connect the microprocessor or controller  124  with the control data bus  76 C (FIG. 2) of the backplane through a control data port  125 . Each half also includes a keystroke TDM data port  128  and appropriate interface devices (not shown) for routing TDM data received at port  128  to the microprocessor  124 . The keystroke TDM data port is not used in those control cards which are installed in server interface chassis. Each half of the card also includes an external communications interface or serial interface  130  connected to an RS422 serial data port  132  and an RS232 standard serial data port  134 . As further discussed below, the controller card associated with each server interface chassis  72  is arranged to receive command signals from a switch control computer through the RS422 serial port and is further adapted to interpret these commands and use the interpreted commands to control the server interface cards and matrix cards in the chassis.  
         [0037]    Each user interface chassis  74  includes a backplane  140  having power connectors (not shown), a control bus  143  and a 16-slot TDM channel  145  referred to as the keystroke TDM channel. (FIG. 2). Each backplane  140  also provides slot address pins (not shown) for providing the various cards inserted in the backplane with hard-coded slot addresses. Each user interface chassis is provided with a control card  142  identical to the control cards  120  discussed above with reference to the server interface chassis. The control data port  125  of the control card is connected to the control data bus  143  of the backplane, whereas the keystroke data port  128  of the control card is connected to the keystroke data bus  145 .  
         [0038]    Each user interface chassis also accommodates up to twenty user interface cards  144 . Each user interface card  144  incorporates sixteen channel circuits  146 , of which only two are shown in FIG. 8 for clarity of illustration. Each channel circuit includes video conditioning circuitry  148  and output amplifiers  150 . The video conditioning circuitry is arranged to accept RGB video in the format provided by matrix cards  94  and to realign the red, green and blue components with one another based upon the synchronization signals in these components. The video conditioning circuit is also arranged to adjust the gains of these various components based upon the size of the sync pulses in the various components. Thus, the video conditioning circuit restores the timing and relative intensities of the red, green and blue signals, thereby compensating for any differences in signal propagation times and amplitude gain in other components of the switching system. The video conditioning circuit and amplifiers  150  provide the outgoing video signal as a set of three opposite polarity signal pairs on terminals of a user port  157  having the same pin assignments as indicated in Table I, above. Each channel  146  also includes a microprocessor connected to buffer amplifiers  154  to a serial send terminal  156  and a serial receive terminal  158  of user port  157 . The microprocessor  152  of each channel desirably is a PIC model 16C622, made by Microchip Technology, of Chandler, Arizona.  
         [0039]    The processors  152  of the various channels  146  are connected to a time division multiplex interface  160 . Interface  160  is arranged to receive a sixteen channel time division multiplex data stream from one of the matrix cards  94  and to divert signals in each of the sixteen channels to a particular processor  152  in a particular data channel  146  associated with that slot. Interface  160  is also arranged to accept data from the processor  152  of each channel and send that data in the appropriate slot of the time division multiplex transmission scheme.  
         [0040]    The user interface card includes a further time division multiplex interface  162  having a connection  164  that mates with the control lines  143  of the user interface chassis  74  (FIG. 2). The processor  152  of each channel  146  is arranged to examine data arriving on serial input  158 , and to recognize preselected command codes appearing in that data. Preferably, processor  152  is arranged to recognize a predetermined attention sequence such as the key press code for alt-break and to treat a preselected number of characters following the attention sequence as a command data. For example, the processor may be arranged to treat the key press appearing immediately after the attention sequence as the command data. Each processor  153  is arranged to strip the command code (attention sequence and command data) out of the data before forwarding the data to TDM interface  160 . Each processor  152  diverts the command data to interface  162 . As discussed in greater detail below, the data arriving at each processor  152  on the serial input line  158  is raw scan code data encoded by the keyboard itself. Keyboard scan codes for standard keyboards are widely known in the art and are available in standard keyboard technical reference manuals. For example, using a PS/2 keyboard, each key press generates a particular keyboard scan code when the key is pressed down and generates the same scan code preceded by a delimiter byte ( 0 XF 0 ) when the key is released. Thus, the processor  152  of each channel is arranged to recognize the key press sequence corresponding to Alt-break as an attention sequence, and to divert a preselected number of key press sequences following the attention sequence to TDM interface  162 .  
         [0041]    Each channel  146  and each port  157  has a unique address within the chassis. The TDM interface is arranged to send the raw keystroke data diverted by each processor  152  as command data in a TDM slot corresponding to the intra-chassis address of the channel  146  where the data was captured, i.e., the intra-channel address of the processor  152  and user port  157  where the command code was captured.  
         [0042]    The various server interface chassis  72  and user interface chassis  74  are interconnected with one another as shown in FIGS. 1 and 2. The timing devices of the various control cards on the server interface chassis and user interface chassis are interconnected with one another by timing patch cords  160 . As noted above, the various control cards have two halves with one clock in each half. The interconnections between the various control cards connect one clock on each card in one set of clocks (“clock A”) and another clock on each card in a separate set (“clock B”). Within each set, one clock is set by internal jumpers to be a master and the other clocks are slaved to this master. The cards all use one clock set and ignore the other unless the first set fails. The clock signals are used to provide synchronization of the various time division multiplex signals on the various chassis. Stated another way, all of the transmission and reception times associated with the various time slots in the various time division multiplexing schemes are set with reference to a common clock, so that TDM signals sent by a circuit on one chassis can be received and understood by circuits on another chassis. The RS422 serial communication ports of the control cards  120  in the server interface chassis  72  are connected to one another so as to provide a common server interface serial link  172 . The RS422 serial ports of each control card  142  in each user interface chassis  74  are connected as a separate user interface serial line  174 . The control card  120  in each server interface chassis is programmed with a designation indicating that the card is part of a server interface chassis rather than part of a user interface chassis, and with a chassis address designating the particular server interface chassis. Likewise, the control card  142  of each user interface chassis  74  is programmed with a designation indicating that the card is incorporated in a user interface chassis and with a user interface chassis address number. The matrix cards  94  in the various server interface chassis are connected to one another patch cords so as to form columns of matrix cards extending across all of the server interface chassis. For example, matrix  94 A and  94 B form one such column whereas cards  94 C and  94 D form another column. Within each vertically extensive column, the video input  104  of each matrix card is connected to the video output  106  of the next higher card in the column. Likewise, the bi-directional TDM channel inputs  112  of each matrix card in the column is connected to the TDM output  114  of the next higher matrix card in the column. The interconnections between the cards in the column thus provide thirty-two user video channels extending vertically across all of the server interface chassis and  32  user slots of bi-directional TDM communication (two channels, each 16 slots) also extending across all of the server interface chassis. Each of the matrix cards can configure any particular user video channel or data slot either as a feed through from the input  104  of that matrix card, in which case the channel will connected to the next higher matrix card in the column. Alternatively, each matrix card can connect a particular video channel to a video channel on the backplane of that particular chassis. Thus, any of the thirty-two user video channels provided by each column of matrix cards can be connected to any of the video channels  76   a  on the backplane of any of server interface chassis  72 . In the same manner, any of the user TDM slots can be connected to any of the TDM slots  76   b  provided on the backplane of any server interface chassis. The user video channels and user TDM slots defined by the columns of matrix cards are arranged in order. Thus, the first column of matrix cards defines user video channels  1 - 32  and user TDM slots  1 - 32 ; the second column defines video channels  33 - 64  and user TDM slots  33 - 64  and so on.  
         [0043]    At the bottom of each column, the thirty-two user video channels and thirty-two user TDM slots are split into two paths, each including sixteen user video channels and sixteen bi-directional user TDM slots. Each path is connected to one user interface card  144 . Each of the sixteen user video channels is connected to the video input of one channel  146  on the user interface card and hence is connected to the video output of one user port  157 . Also, the sixteen user TDM slots are interfaced through the TDM processor  160  of the user interface card so that each such user TDM slot is connected to the serial inputs and outputs of a particular user interface port. Thus, each of the thirty-two user video channels and thirty-two TDM slots defined by a vertically extensive column of matrix cards is connected to a single user port. As discussed above, each of the user ports  157  has an intra-chassis address. Each user port also has an overall address Thus, a user port  157  having intra-chassis address IUA in the i th  user chassis has overall user address OUA=IUA+(Q×(i−1)), where Q is the number of user ports per user interface chassis, i.e., the highest intra-chassis address for a fully-configured chassis. In the system depicted in the drawings, which includes up to 20 user interface cards, each with 16 user ports, in each user interface chassis, Q=320. Each of the user video channels and user TDM slots is connected to the output port having an overall address corresponding to the channel number and slot number, i.e., the n th  video channel and n th  user TDM slot are connected to the user port having overall address OUA=n.  
         [0044]    Similarly, each of the server ports  80  has an overall server port address OSA based on the intra-chassis server port address ISA. That is, for a server port in the i th  server interface chassis, OSA=ISA+(Z×(i−1)) where Z is the maximum number of server interface ports per server interface chassis. For example, in the system shown in the drawings, each server interface chassis can accommodate up to 64 server interface ports, and hence Z=64. A server port  80  having intra-chassis address 10 in the second of the interface chassis has overall address 10+(64×(2−1)) or 74. As discussed in greater detail below, the matrix cards can connect the serial data connections  156  and  158  of any user port  157  to the serial data connections of any server input port  80  on any of the server interface chassis. Similarly, the matrix cards can connect the video outputs in any user output port  157  to the video inputs of any server port  80 . Such a connection can be specified completely simply by designating whether the connection is to be a video connection or a data connection, and by designating the overall addresses of the server port and user port which are to be connected.  
         [0045]    The system further includes a supervisory computer system which includes a switch control computer  200 ; one or more helper computers  202  and one or more administration computers  204 . The helper computers  202  and administration computers  204  most preferably are separate computers, distinct from the switch control computer  200 . The helper computers, administration computers and switch control computers are interconnected to one another in a local area network  206  separate from the switch  70 . Desirably, the various computers run under an operating system which is readily integrated with a local area network such as Microsoft® Windows NT Server, version 4.0 or higher. All the various computers desirably are part of a single Windows NT domain. Each helper computer  202  is also connected to a server input port  80  of switch  70  through a transmitter  28  in exactly the same way as server computers  20 . The switch control computer is equipped with a server control serial interface  208  connected to the server interface chassis serial line  172 . The switch control computer  200  is also provided with individual serial interfaces  210  connected to the individual serial communications lines  174  associated with the control cards of user interface chassis  74 . As further discussed below, the switch control computer  200  acts as the server in LAN  206 . For that reason, the switch control computer is sometimes referred to as a “switch server”. A database  212  is also provided on local area network  206 . Because database  212  is utilized by the switch control computer or switch server  200 , the database is depicted in FIG. 1 as physically associated with the switch control computer. However, the database may be maintained either on the switch control computer itself or on another computer connected to LAN  206 . The database may be maintained using a standard database administration program such as Microsoft® SQL Server, version 6.5. The switch control computer or “switch server” also runs a main program which performs the various operations discussed below. This program consists of various modules each of which are responsible for their own tasks. For instance, one module listens on the communications ports  210  connected to the user interface cards. The program uses multi-threading; one thread is maintained for each user location. When an incoming command from a user location is detected, it is handed to the relevant thread which represents each user location. The work area thread then calls the functions corresponding to the required action. Each thread can also request resources such as a Helper PC or an open database connectivity or “ODBC” link to the database and these are managed by other modules such as the Helper PC manager and ODBC manager which will allocate and de-allocate the required resources as necessary.  
         [0046]    The database includes data defining identities for particular servers  20  and helper computers  202  connected to the system as, for example, names for such servers and helper computers. The database also includes data defining associations between particular servers and helper computers and particular server ports  80  on the server interface chassis. The database further includes information about particular user locations or work areas  46 , such as the number of receivers  48  at each such location; whether the group of I/O devices associated with each such receiver includes input devices such as keyboard  42  and mouse  44 ; output devices such display monitors  40 ; or both and a user port address for each receiver.  
         [0047]    The database desirably further includes information about each authorized user of the system such as a user name; a password and data defining access rights to particular servers  20  for each user. This data may be provided as an individual list of particular servers authorized for each user. Alternatively or additionally, each user may be defined as belonging to one or more user groups, whereas each server may be defined as belonging to one or more server groups, and rights may be allocated on a group-wide basis. The database may also include a temporary list defining a running set of servers for each user location.  
         [0048]    In operation, when the system is started, all of the I/O devices at user locations  46  are initially disconnected from the server computers  20  and from helper computer  202 . A user at a particular location as, for example, at location  46 A may enter a startup helper code (alt-break followed by enter). This code is passed through the serial output  68  (FIG. 4) of the receiver  48  associated with the user&#39;s keyboard and is passed into the serial input  158  of the particular user port  157  connected to that receiver  48 . The processor  152  associated with that port  157  (FIG. 8) responds to the attention sequence (alt break) by trapping the next key stroke indication (enter) and forwarding that keystroke indication to the command TDM module  162 . The TDM module sends the command data indicating depression of the enter key in a particular slot of the TDM transmission from the particular card associated with intra-chassis address of that channel. Thus, if the processor which trapped the helper code was the processor for the has an intrachassis address IUA, the, the signal sent along the keystroke bus  145  to the control card  142  (FIG. 2) of the user interface chassis will appear in a transmission uniquely identified with intra-chassis address IUA. The processor  124  of the control card  142  adds the intra-chassis address within the chassis to the starting address of the chassis to compute the overall user port address OUA. The controller formulates a message including the overall user port address and the particular key stroke included in the data (in this case, the enter key) and transmits that message along the serial communications link  174  to one of the serial interfaces  210  of switch control computer  200 . Communications between the control cards and the serial ports of switch control computer use an ANSI X3.28 compliant packet communications protocol. As is well known in the art, communications of this nature include features such as message acknowledgment and, in some cases, a check sum for error correction, so as to provide a robust communications link with good assurance that errors in communication will be detected. The switch control computer  200  interprets the message conveying the overall user port address together with a enter key stroke value as a request to connect the input devices (keyboard and mouse) and output device (monitor) associated with that user port to a helper computer in a sign-on mode. The switch control computer checks the database for helper computers, finds a helper computer which is not currently occupied, and finds the server port address for that helper computer.  
         [0049]    The switch control computer then broadcasts a signal on the server interface chassis serial control line  172  through communications port  208 , again using the ANSI X3.28 protocol. The command includes a video connect signal including a code predesignated as meaning “Connect video” together with two integers (X and Y), so that the overall command has the meaning “CONNECT VIDEO X Y” where X is the overall server port address of the helper computer and Y is the overall user port address which sent the helper command. The computer also sends a command of the meaning “CONNECT KEY X Y” where X and Y have the same meanings. The control cards  120  of the various serial interface chassis  72  all receive these commands. Each controller will compare the server port address within each command to the range of overall server port addresses included in that server interface chassis. The controller card in a server interface chassis having a range of overall server port addresses including X will acknowledge the command, whereas the other controller cards will ignore it. The controller card which acknowledges a CONNECT VIDEO command converts the overall server port address to an intra-chassis server port address based on the starting port address of the chassis. For example, the second server interface chassis starts with overall server port address  65 . Therefore, if the overall server port address “70” is indicated by the X value in a CONNECT VIDEO command, the control card in the second server interface chassis will respond to the command and will select the sixth server port  80  of that chassis. Thus, the control card will select the sixth of the 64 video channels on the backplane of the chassis and will actuate the sixth of the 64 video server interface circuits  82  to route video from its port unto the video channel of the backplane. The control card will also select the particular matrix card encompassing the user video channel corresponding to the user port address designated by the value of Y in the command. The control card will send a message to the matrix card including that user video channel instructing it to connect the particular user channel or slot to the particular video channel on the backplane. For example, in response to the response to the command “CONNECT VIDEO 70 35”, the control card associated with the second server interface chassis will cause the second matrix card to connect the third one of its video outputs (the video output associated with the 35th user video channel) onto the sixth video channel of the backplane in the server interface chassis (the video channel associated with overall server port address “70”). The matrix cards maintain continuity between video inputs  104  and video outputs  106  in the absence of specific instructions. Thus, the video coupled onto any user video channel or slot at a particular server interface chassis is transmitted through the matrix cards disposed beneath it on other server interface chassis. Video coupled onto a user video channel is transmitted down the channel to the particular channel  146  of the user interface card and to a particular user port  157  associated with the user video channel. Thus, the video from a particular server port X is coupled to the video output of a particular user port Y. In a directly analogous manner, the control cards  120  and the matrix cards respond to the command “CONNECT KEY X Y” by connecting a particular time division multiplex slot in a backplane data channel  76 B associated with a particular server port  80  designated by the server port address X with the serial input and output connections  156  and  158  of the particular user port designated by the user port address Y.  
         [0050]    As pointed out above, the message sent by the control card of the user interface chassis  74  to switch control computer  200  tells the switch control computer which user port originated the helper command. The switch control computer uses that port as the user port address in the CONNECT VIDEO and CONNECT KEY commands and thus connects the helper computer to the user port which originated the helper command. The switch control computer  200  also sends a message over the LAN to the helper computer advising the helper computer of the identity of the user port, and advising the helper computer that the user at such port wishes to connect to the helper computer in the sign-on mode.  
         [0051]    At this point, the user is connected to the helper PC  202  through a receiver  48 ; through a user port  157  and switch  70  to a server port  80  associated with the helper PC  202  and through the transmitter  28  associated with the helper PC. The monitor  40  at the user&#39;s location shows output from the helper PC, whereas the keyboard and mouse are connected to the keyboard and mouse connections of the helper PC, so that the user can interchange data with the helper PC in exactly the same manner as if the monitor, keyboard and mouse were directly connected to the corresponding connections of the helper PC  202 .  
         [0052]    In the sign-on mode, the helper computer enters an initial access routine in which it generates a screen display calling for the user to enter his or her user ID and password.  
         [0053]    Upon authentication of the user&#39;s identity and password, the helper PC accesses the database to obtain the list of servers authorized for access by that user. The helper PC also and also accesses information in the database setting forth a correlation between user port addresses and user locations to find the configuration of the input and output devices at the user&#39;s particular location, and the user port addresses associated with various groups of devices. The helper PC displays a list of authorized servers and the input output devices. The helper PC accepts input from the user defining particular servers to be connected to particular I/O devices. One example of a display which may be generated by the helper PC is shown in FIG. 9. The list of servers is presented under the heading “Server Neighborhood”. This list includes only those servers which are authorized for access by the particular user. Note that the servers are identified by names rather than by port number. Also, only those servers authorized for access by the particular user are displayed. Where the servers are arranged in groups, the display of available servers may also be grouped. Also, the available groups of input and output devices at the user location are displayed under the heading “Work Area”. Here again, the available I/O devices are designated on the display by names which are intelligible to the user, rather than by user port numbers associated with the devices. Each group of input and output devices at the user&#39;s location is shown by a separate designation such as “screen 1”, “screen 2”, etc. Each such group has one receiver  48  and is associated with one user port  157  of switch  70 . The user can designate one or more of the available servers for association each such complement of devices. For example, as shown in FIG. 9, servers HPC2 and R2D2 have been designated for association with one group of devices (“Screen 1”) whereas servers HPC1 and NOSTROMON have been designated for association with another group of devices (“Screen 2”) and so on. The helper PC program thus establishes a running set of servers associated with the user&#39;s location. Within such running set, there may be none, one or more than one server associated with a particular group of I/O devices and hence with a particular user port. If there is more than one server associated with a particular group of I/O devices (e.g., Screen 1), the server at the top of list for that group (HPC2 in FIG. 9) is active, whereas all other servers are inactive. Also, the connection to any port may be designated either as view-only, where only the video output is connected to the user location, or as bi-directional, with connections for video output and user input (keyboard and mouse). A choice between view-only and bi-directional connection may be made by the user. Also, the table of rights stored in the database may give the particular user only the right to establish a view-only connection to a particular server and not the right to establish a bi-directional connection.  
         [0054]    Once the user has input a running list of servers, the user inputs a command to the helper PC to implement the connection. Helper PC  202  communicates this command through LAN  206  to switch control computer  200  and enters the appropriate information representing the new running list into database  202 . Switch control computer  200  issues disconnect commands similar to the CONNECT VIDEO and CONNECT KEY commands discussed above which cause the switch  70  to disconnect helper PC  202  from the particular user port which was connected to the helper PC. The switch control computer then issues the CONNECT VIDEO commands and CONNECT KEY commands as appropriate to connect the active devices specified in the running list for a particular user to input and output devices at the user&#39;s location. For example, where the running list input through the helper PC specifies a video-only link between computer  20 A and screen  40 B (user port  157 B) and also specifies a bi-directional link between server  206 , the group of devices (monitor  40 C, keyboard  42 C and mouse  44 C) associated with receiver  48 C and user port  157 C, the switch control computer will issue a CONNECT VIDEO command to connect server  20 A with user port  157 B and will issue both CONNECT VIDEO and CONNECT KEY commands to connect server  20 C with user port  157 C.  
         [0055]    At this juncture, the user is in communication with one or more server computers. While the user is interacting with server computers, the user can send a further helper command, referred to herein as an intra-session helper command, by entering alt-break followed by a tab key. The helper command is handled in the same manner as the startup helper command discussed above. The intra-session helper command includes the attention sequence (Alt-break) followed by depression of the TAB key. Once again, the user interface processor associated with the user port receiving the command responds to the attention sequence by trapping the following keystroke signals associated with the tab key press and forwarding a the command to the control card, which sends the intrasession helper command signal to the switch control computer. In response to the intra-session helper command, the switch control computer  200  disconnects the group of I/O devices at the user&#39;s location which originated the command from a server and connects the user port associated with that group of devices to the helper PC in the substantially same way as discussed above. However, the switch control computer signals the helper computer that the connection is an intra-session request rather than a startup request. Therefore, the helper computer does not enter the user identification routine discussed above. Instead, the helper computer retrieves the identity of the user who is currently signed on at the user location associated with the user port where the command originated.  
         [0056]    Also, while the user is interacting with the servers, the user can enter action codes different from the helper codes. The action codes include the alt-break attention sequence followed by one or more keystrokes other than the tab key or enter key. These action codes are captured by the processor  152  associated with the user port  157  in exactly the same manner as discussed above with reference to the helper codes. Here again, the processor passes the keystroke following the attention (alt-break) sequence to the command TDM module of the user interface card which forwards the keystroke signal to the control card  142  of the user interface chassis. The control card passes a signal along the serial connection  174  giving the overall user port address. The action codes are as follows:  
         [0057]    Alt-break plus up arrow or alt-break plus down arrow: Shift among servers associated with the particular user port. This causes the switch control computer to disconnect the currently active server from the particular user port where the action code was entered and to connect an inactive server designated in the running list for the same user port. In this regard, where more than one server is designated for a particular port in the running set of servers, the database treats these servers as an ordered but circular stack, so that the first server in the stack follows after the last server in the stack. For example, if servers designated as A, B , C and D are in the stack in that order, and server B is currently active, Alt-Break with up-arrow will make server A active, whereas Alt-Break with down-arrow will make server C active. If server D is active, Alt-Break with down arrow will make server A active, whereas Alt-Break with up arrow will make server C active.  
         [0058]    Alt-break plus left arrow or right arrow—This causes the switch control computer to shift the keyboard and mouse connection to a different server port among the currently active server ports associated with the same user location. For example, assume that server  20   a  is connected to the keyboard and video display of user port  157 C, whereas server  20   b  has a video-only connection to user port  157 B, both user ports being associated with user location  46 B. If an alt-break left arrow sequence is received on user port  157 C, the keyboard and mouse of port  157 C would be disconnected from server  20   a  and reconnected to server  20   b . The Alt-break right arrow sequence would be processed in the same manner, to connect the keyboard to a different server. These command effectively shifts the effects of the control input to a server associated with a different screen. Because these commands are analogous to the change-focus command in a windowed programming environment, they are referred to herein as change-focus commands.  
         [0059]    Stated another way, for each user location having more than one user port, and hence more than one output device, the running set of servers is maintained as a two-dimensional matrix, with a number of columns equal to the number of user ports and with the stack of servers for each user port constituting one column. This matrix is established by the helper computer when the users selects the set of servers during operation of the helper computer. The particular column where the keyboard is connected is the current column. The up arrow or down arrow sequence moves both the keyboard and video connections up or down the current column, so that the user can pick a new active server for interaction with the keyboard and video. The left arrow and right arrow sequences moves the keyboard and mouse connection to a different column, and thus select a new current column and shifts the effects or focus of control inputs applied through the keyboard and mouse into the active server in the new column.  
         [0060]    The server computer also takes account of the user&#39;s authorized level of access to particular servers, to preclude a user from gaining unauthorized access by use of action commands. The switch control computer may access the database to determine the identity of the user connected to a particular port, and retrieve that user&#39;s access level for a particular server denoted by an action command, and grant or deny access based on such comparison. Preferably, however, when the helper computer establishes the matrix of servers for a particular user location and hence for the particular user at that location, the helper computer may denote each server as either full-access or video-only access. The server computer can use these indications in the matrix defining the running set, and hence need not refer to the data defining the individual user or to the privileges tables for individual users in the larger database.  
         [0061]    These action codes allow the user to switch among the various servers and to move the outputs of the various servers to convenient locations at his work location without invoking the helper PC. The system thus provides simple commands for performing simple, intuitive operations such as shifting among a few servers on the active list or shifting servers around among different display monitors. However, the user always the option of accessing the helper PC. The user need not rely on his own memory to find appropriate servers. Because all of the helper codes and action codes are captured at the user interface chassis, the same cabling which carries the data input by the user can carry the action codes and helper codes. There is no need to provide separate command wiring between the various the user locations and the central location of the switch.  
         [0062]    Additional users can connect to the system in the same manner. The program running on switch control computer switch server  200  maintains separate threads associated with each user location. The helper PCs are treated as shared resources by the program running on the switch control computer or switch server  200 . If a user seeks access to a helper PC while all helper PCs currently occupied, the switch control computer will block such access and will display a message to the user indicating that the helper PC is currently unavailable. A separate device for generating an apology message and providing the appropriate video output with the apology message may be connected to a server port  80  to provide this message. This output can be connected to all user locations which have requested access to the helper but which have not yet been connected to the helper. The switch control computer may act as the apology server. Alternatively, the apology server may be a computer programmed to display educational messages to the users while they are waiting to connect with a helper computer. The program on the switch control computer will allow numerous users to have video or output only access to any one server  20 , but will only allow one user at a time to have keyboard or input access to a particular server.  
         [0063]    The administration PC  204  can be used to perform tasks such as updating the database when the servers  20  or helper computers  202  are installed and updating user profiles. The administration computer may also be connected into a server port  80  of switch  70  so that a user having appropriate privileges can gain access to the administration PC through switch  70 .  
         [0064]    The RS 232 data communications ports provided on the control cards can be used for service and maintenance procedures. For example, a terminal or computer connected to these ports can be used to send commands to an individual control card to make or break a connection to a particular server port.  
         [0065]    The various elements of switch  70  can be provided with status and fault reporting features. For example, each of the matrix cards is arranged to report its status and to report successful or unsuccessful operations, such as successful or unsuccessful connection and disconnection, to the microprocessor in the control card of the chassis. The various microprocessors are arranged to send error reporting signals to the switch control computer. The switch control computer may handle mew reports according to a variety of rules depending upon factors such as the nature of the new error report and previous error reports. These rules may be explicitly coded rules set by a program, or may incorporate learned behaviors using techniques commonly referred to as “artificial intelligence”. For example, if a particular matrix card reports a failure to disconnect a particular user&#39;s data channel from the data bus of the backplane, the switch control computer may repeat the disconnect command. If the repeated disconnect command fails after a selected number of retries, or if a certain percentage of disconnect or connect commands fail, the switch control computer may treat the matrix card as defective and may issue a command to the control card to disable that matrix card, as by disconnecting the entire card from the power supply.  
         [0066]    The rules for deciding which action to take in response to which error reports will vary with the application and the goals of the system administrator. For example, in an environment where security is paramount, and where entry of garbled data due to connection of two keyboards to one server could pose a substantial risk to the overall mission, the rules may call for disabling a matrix card, or even the entire system, in response to only a few error reports. In an educational environment where the system is being used to train operators in the use of publicly available software, the inconvenience caused by tolerating a few erroneous connections may be less significant than the inconvenience caused by shutting down part or all of the system. Thus, there is no particular set of error-handling rules which is best for all applications. Desirably, the switch control program utilizes an event-reporting and event-logging capability, such as those included in the Windows NT® operating system. The event handler in the operating system may be arranged to take appropriate actions, such as establishing a connection to a service facility and sending the appropriate report.  
         [0067]    The system discussed above can be configured for an unlimited number of servers. Thus, any desired number of server chassis  72  can be added to the system. However, each server interface chassis can only accommodate  512  user channels, i.e., 32 user channels per matrix card, and up to 16 matrix cards per server interface chassis. As discussed above with reference to FIG. 5, the server interface cards  78  are provided with expansion connectors  92 . These expansion connectors can be used to connect additional server interface chassis as illustrated in FIG. 10. A second stack of server interface chassis  72 ′ is connected alongside of the first stack of chassis  72 . The additional server interface chassis are provided with server interface cards  78 ′ similar to the server interface cards  78  discussed above. The server video and data channels coupled to each server interface card  78  in a chassis  72  is coupled through the expansion port  92  of that card to the inputs  93  of a server interface card  78 ′ in the corresponding chassis  72 ′ in the second stack. The video and data channels are coupled to the backplanes of the chassis  72 ′ in substantially the same manner as discussed above. The second stack of chassis  72 ′ can accommodate an additional  512  user channels, using matrix cards as discussed above. The server interface cards  78 ′ of the second stack may have additional expansion ports  92 ′, so that the server video and data channels can be coupled to still further chassis. Thus, the system can accommodate an essentially unlimited number of users.  
         [0068]    Numerous variations and combinations of the features discussed above can be utilized without departing from the invention as defined by the claims. For example, the particular keystroke sequences selected to designate command codes in the embodiment discussed above are arbitrary; other keystroke sequences can be employed. Desirably, the keystroke sequences used to designate command codes are those which do not occur during normal interaction between the user and a server. Also, the particular formats for video and data transmission used in the preferred embodiments discussed above are not essential to the invention. Also, the computers may have video output formats other than the VGA format discussed above, and may have different keyboard and mouse output and input formats. Any of these different formats can be utilized. Further, input and output devices at some or all of the user locations named include devices other than video monitors, keyboards and mice. For example, some or all of the user locations may incorporate printers, audio speakers, tactile feedback devices or other computer-controllable devices such as computer controlled numerical machine tools, “solid modeling” devices and the like. In each case, the communication links routed through the switch would be configured to carry the information normally sent by the computer to such devices. Also, the input devices may include more complex input devices such as joy sticks or controls which simulate the control inputs to a vehicle as, for example, simulated pilots yokes and rudder pedals for flight simulation. These devices may include features such as force feedback, vibration and the like controlled by output sent from the computer. Here again, the communications channels routed through the switch would be configured to carry the necessary information. In the embodiments discussed above, the switch acts on electrical signals. However, where the output and input are routed along optical communications such as fiber optic communication channels, the switching device may include appropriate components for switching optical signals. Further, the communication links between the computers and the switch and between the user location devices and the switch need not be hard-wired connections. For example, these links can be replaced by appropriate RF or optical communications links. Also, the particular schemes for connection between the switch control computer and the elements of the switch can be varied. For example, the RS 422 serial ports of the control cards on the server interface chassis can be connected to individual communications ports on the switch control computer, instead of being connected together in a common channel as described above. Data communication schemes other than serial communication channels can be employed.  
         [0069]    As these and other variations and combinations of the features discussed above can be utilized without departing from the invention as defined by the claims, the foregoing description of the preferred embodiment should be taken by way of illustration rather than by way of limitation of the invention as defined by the claims.