Patent Publication Number: US-2002013629-A1

Title: Process control system using a process control strategy distributed among multiple control elements

Description:
[0001] This application is related to copending application by Nixon et al., entitled “Process Control System for Monitoring and Displaying Diagnostic Information of Multiple Distributed Devices”, filed on even date herewith (attorney docket no. M-3926 US), which application is hereby incorporated by reference in its entirety, including any appendices and references thereto.  
       [0002] This application is related to copending application by Nixon et al., entitled “Process Control System Including Automatic Sensing and Automatic Configuration of Devices”, filed on even date herewith (attorney docket no. M-4029 US), which application is hereby incorporated by reference in its entirety, including any appendices and references thereto.  
       [0003] This application is related to copending application by Nixon et al., entitled “A Process Control System User Interface Including Selection of Multiple Control Languages”, filed on even date herewith (attorney docket no. M-4045 US), which application is hereby incorporated by reference in its entirety, including any appendices and references thereto.  
       [0004] This application is related to copending application by Dove, entitled “System for Assisting Configuring a Process Control Environment”, filed on even date herewith (attorney docket no. M-3927 US), which application is hereby incorporated by reference in its entirety, including any appendices and references thereto.  
       [0005] This application is related to copending application by Nixon et al., entitled “Process Control System Using a Control Strategy Implemented in a Layered Hierarchy of Control Modules”, filed on even date herewith (attorney docket no. M-4028 US), which application is hereby incorporated by reference in its entirety, including any appendices and references thereto.  
       [0006] This application is related to copending application by Dove et al., entitled “System for Configuring a Process Control Environment”, filed on even date herewith (attorney docket no. M-3928 US), which application is hereby incorporated by reference in its entirety, including any appendices and references thereto.  
       [0007] This application is related to copending application by Nixon et al., entitled “Improved Process System ”, filed on even date herewith (attorney docket no. FEMR:004), which application is hereby incorporated by reference in its entirety, including any appendices and references thereto.  
       BACKGROUND OF THE INVENTION  
       [0008] 1. Field of the Invention  
       [0009] This invention relates to process control systems. MorQ specifically, the present invention relates to a process control system using a distributed process control strategy.  
       [0010] 2. Description of the Related Art  
       [0011] Present-day process control systems use instruments, control devices and communication systems to monitor and manipulate control elements, such as valves and switches, to maintain at selected target values one or more process variables, including temperature, pressure, flow and the like. The process variables are selected and controlled to achieve a desired process objective, such as attaining the safe and efficient operation of machines and equipment utilized in the process. Process control systems have widespread application in the automation of industrial processes such as the processes used in chemical, petroleum, and manufacturing industries, for example.  
       [0012] Control of the process is often implemented using microprocessor-based controllers, computers or workstations which monitor the process by sending and receiving commands and data to hardware devices to control either a particular aspect of the process or the entire process as a whole. The specific process control functions that are implemented by software programs in these microprocessors, computers or workstations may be individually designed, modified or changed through programming while requiring no modifications to the hardware. For example, an engineer might cause a program to be written to have the controller read a fluid level from a level sensor in a tank, compare the tank level with a predetermined desired level, and then open or close a feed valve based on whether the read level was lower or higher than the predetermined, desired level. The parameters are easily changed by displaying a selected view of the process and then by modifying the program using the selected view. The engineer typically would change parameters by displaying and modifying an engineer&#39;s view of the process.  
       [0013] In addition to executing control processes, software programs also monitor and display a view of the processes, providing feedback in the form of an operator&#39;s display or view regarding the status of particular processes. The monitoring software programs also signal an alarm when a problem occurs. Some programs display instructions or suggestions to an operator when a problem occurs. The operator who is responsible for the control process needs to view the process from his point of view. A display or console is typically provided as the interface between the microprocessor based controller or computer performing the process control function and the operator and also between the programmer or engineer and the microprocessor based controller or computer performing the process control function.  
       [0014] Systems that perform, monitor, control, and feed back functions in process control environments are typically implemented by software written in high-level computer programming languages such as Basic, Fortran or C and executed on a computer or controller. These high-level languages, although effective for process control programming, are not usually used or understood by process engineers, maintenance engineers, control engineers, operators and supervisors. Higher level graphical display languages have been developed for such personnel, such as continuous function block and ladder logic. Thus each of the engineers, maintenance personnel, operators, lab personnel and the like, require a graphical view of the elements of the process control system that enables them to view the system in terms relevant to their responsibilities.  
       [0015] For example, a process control program might be written in Fortran and require two inputs, calculate the average of the inputs and produce an output value equal to the average of the two inputs. This program could be termed the AVERAGE function and may be invoked and referenced through a graphical display for the control engineers. A typical graphical display may consist of a rectangular block having two inputs, one output, and a label designating the block as AVERAGE. A different program may be used to create a graphical representation of this same function for an operator to view the average value. Before the system is delivered to the customer, these software programs are placed into a library of predefined user selectable features. The programs are identified by function blocks. A user may then invoke a function and select the predefined graphical representations to create different views for the operator, engineer, etc. by selecting one of a plurality of function blocks from the library for use in defining a process control solution rather than having to develop a completely new program in Fortan, for example.  
       [0016] A group of standardized functions, each designated by an associated function block, may be stored in a control library. A designer equipped with such a library can design process control solutions by interconnecting, on a computer display screen, various functions or elements selected with the function blocks to perform particular tasks. The microprocessor or computer associates each of the functions or elements defined by the function blocks with predefined templates stored in the library and relates each of the program functions or elements to each other according to the interconnections desired by the designer. Ideally, a designer could design an entire process control program using graphical views of predefined functions without ever writing one line of code in Fortran or other high-level programming language.  
       [0017] One problem associated with the use of graphical views for process control programming is that existing systems allow only the equipment manufacturer, not a user of this equipment, to create his own control functions, along with associated graphical views, or modify the predefined functions within the provided library.  
       [0018] New process control functions are designed primarily by companies who sell design systems and not by the end users who may have a particular need for a function that is not a part of the standard set of functions supplied by the company. The standardized functions are contained within a control library furnished with the system to the end user. The end user must either utilize existing functions supplied with the design environment or rely on the company supplying the design environment to develop any desired particular customized function for them. If the designer is asked to modify the parameters of the engineer&#39;s view, then all other views using those parameters have to be rewritten and modified accordingly because the function program and view programs are often developed independently and are not part of an integrated development environment. Clearly, such procedure is very cumbersome, expensive, and time-consuming.  
       [0019] Another problem with existing process control systems is a usage of centralized control, typically employing a central controller in a network, executing a program code that is customized for specialized, user-defined control tasks. As a result, the process control systems are typically constrained to a particular size and difficult to adapt over time to arising needs. Similarly, conventional process control systems are inflexible in configuration, often requiring a complete software revision for the entire system when new devices are incorporated. Furthermore, the conventional process control systems tend to be expensive and usually perform on the functions initially identified by a user or a system designer that are only altered or reprogrammed to perform new functions by an expert who is familiar with the entire control system configuration and programming.  
       [0020] A further problem with existing process control systems is that the physical implementation of different systems is highly variable, including control devices and field devices that have a wide range of “intelligence”. For example, some field devices, such as valves, motors and regulators, may have no computational or control capability. Other field devices may have a high level of control autonomy. Still other devices may have some computational strength, but not a sufficient amount to accomplish a desired control task.  
       [0021] What is needed is a uniform or universal design environment that can easily be used, not only by a designer or manufacturer but also a user, to customize a control process to the physical constraints of the process, utilizing control capabilities various controllers and devices, supplementing these control capabilities when desired and distributing control functionality flexibly throughout the process control system to meet specific needs for developing process control functions. What is further needed is a personal computer-based process control system that is easily implemented within substantially any size process and which is updated by users, without the aid of the control system designer, to perform new and different control functions by distributing these control functions throughout the control system including all central, intermediate and peripheral levels.  
       SUMMARY OF THE INVENTION  
       [0022] In accordance with the present invention, a process controller implements an overall, user-developed control strategy in a process control network that includes distributed controller and field devices, such as Fieldbus and non-Fieldbus devices. A user defines the control strategy by building a plurality of function blocks and control modules and downloading or installing user-specified portions of the control strategy into the Fieldbus devices and the non-Fieldbus devices. Thereafter, the Fieldbus devices automatically perform the downloaded portions of the overall strategy independently of other portions of the control strategy. For example in a process control system that includes distributed field devices, controllers and workstations, portions of the control strategy downloaded or installed into the field devices operate independently of and in parallel with the control operations of the controllers and the workstations, while other control operations manage the Fieldbus devices and implement other portions of the control strategy.  
       [0023] In accordance with one embodiment of the present invention, a process control system includes a field device, a controller connected to the field device, a workstation connected to the controller, and a software system. The software system implements a control strategy for the process control system. The control strategy is selectively apportioned into a plurality of control strategy modules and selectively distributed among the field device, controller and workstation, the control strategy modules operating mutually independently and in parallel.  
       [0024] In accordance with another embodiment of the present invention, a method of operating a process control system including a distributed controller and a distributed field device includes the steps of defining a control strategy further including the substeps of building a plurality of function blocks and control modules and downloading user-specified function blocks and control modules selectively among the distributed controller and the distributed field device. The method of operating the process control system further includes the step of executing the function blocks and control modules distributed to the controller and distributed to the field device mutually independently and in parallel.  
       [0025] The described process control system and operating method has many advantages. One advantage is that the system supplies a uniform, universal design environment for users of many various expertise, experience and training levels to customize a control process to the physical constraints of the process. A further advantage is that the described system uses control capabilities of various controllers and devices, supplementing these control capabilities when desired and distributing control functionality flexibly throughout the process control system as needed. Another advantage is that the process control system is easily based on a personal computer-based design which is easily implemented within substantially any size process and which is updated by users, without the aid of the control system designer, to perform new and different control functions. This flexibility is achieved by distributing control functions throughout the control system including all central, intermediate and peripheral levels. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0026] The features of the invention believed to be novel are specifically set forth in the appended claims. However, the invention itself, both as to its structure and method of operation, may best be understood by referring to the following description and accompanying drawings.  
     [0027]FIGS. 1A, 1B and  1 C illustrate a screen display, a first schematic block diagram and a second schematic block diagram, respectively, process control systems in accordance with a generalized embodiment of the present invention which furnishes a capability to create a new control template and a capability to modify an existing control template for only one view, such as an engineering view.  
     [0028]FIG. 2 is a schematic block diagram showing the process control environment in a configuration implementation and a run-time implementation.  
     [0029]FIG. 3 is a block diagram illustrating a user interface for usage with both configuration and run-time models of the process control environment.  
     [0030]FIG. 4 is a schematic block diagram which depicts a hierarchical relationship among system objects of a configuration model in accordance with an embodiment of the present invention.  
     [0031]FIG. 5 is a schematic block diagram which depicts a configuration architecture that operates within the hierarchical relationship illustrated in FIG. 4.  
     [0032]FIG. 6 is a block diagram illustrating an example of an elemental function block, which is one type of system object within the configuration model definition.  
     [0033]FIG. 7 is a block diagram depicting an example of a composite function block, which is another type of system object within the configuration model definition.  
     [0034]FIG. 8 is a block diagram illustrating an example of a control module, which is another type of system object within the configuration model definition.  
     [0035]FIG. 9 is a block diagram showing a module instance, specifically a control module instance, which is created in accordance with the control module definition depicted in FIG. 8.  
     [0036]FIG. 10 is a flow chart which shows an example of execution of a control module at run-time.  
     [0037]FIG. 11 is a flow chart which shows an example of a module at a highest layer of a control structure.  
     [0038]FIG. 12 is a block diagram which illustrates an object-oriented method for installing a process I/O attribute block into a PIO device.  
     [0039]FIG. 13 is a block diagram depicting an object-oriented method for linking a control module input attribute to a PIO attribute.  
     [0040]FIG. 14 is a block diagram showing an object-oriented method for linking a control module output attribute to a PIO attribute.  
     [0041]FIG. 15 is a block diagram showing an object-oriented method for reading values of contained PIO attributes.  
     [0042]FIG. 16 is a block diagram which illustrates an organization of information for an instrument signal tag.  
     [0043]FIG. 17 is a flow chart illustrating a method for bootstrap loading a control system throughout a network in the process control environment.  
     [0044]FIG. 18 is an object communication diagram illustrating a method for creating a device connection for an active, originating side of the connection.  
     [0045]FIG. 19 is an object communication diagram illustrating a method for creating a device connection for a passive, listening side of the connection.  
     [0046]FIG. 20 is an object communication diagram illustrating a method for sending request/response messages between devices.  
     [0047]FIG. 21 is an object communication diagram illustrating a method downloading a network configuration. 
    
    
     DETAILED DESCRIPTION OF THEE PREFERRED EMBODIMENTS  
     [0048] Referring to FIG. 1A, a control system is shown. In general, the system  1  includes a main processing device, such as personal computer  2 , that is connected to a local area network (“LAN”)  3  via a local area network card. Although any local area network protocol may be used, a non-proprietary ethernet protocol is beneficial in many applications because it allows for communications with the local area network  3 . The local area network  3  is dedicated to carrying control parameters, control data and other relevant information concerned in the process control system. As such, the LAN  3  may be referred to as an area controlled network or ACN  3 . The ACN  3  may be connected to other LANs for sharing information and data via a hub or gateway without affecting the dedicated nature of ACN  3 .  
     [0049] In accordance with standard ethernet protocol, a plurality of physical devices may be connected to the ACN  3  at various “nodes.” Each physical device connected to the ACN  3  is connected at a node and each node is separately addressable according the LAN protocol used to implement ACN  3 .  
     [0050] To establish a redundant system, it may be desirable to construct ACN  3  from two or more ethernet systems such that the failure of a single ethernet or LAN system will not result in the failure of the entire system. When such “redundant ethemets” are used the failure of one ethernet LAN can be detected and an alternate ethernet LAN can be mapped in to provide for the desired functionality of ACN  3 .  
     [0051] The main personal computer (“PC”) A forms a node on the ACN  3 . The PC  2  may, for example, be a standard personal computer running a standard operating system such as Microsoft&#39;s Window NT system. Main PC  2  is configured to generate, in response to user input commands, various control routines that are provided via the ACN  3  to one or more local controllers identified as element  4  and  5  which implement the control strategy defined by the control routines selected and established in main PC  2 . Main PC  2  may also be configured to implement direct control routines on field devices such as pumps, valves, motors and the like via transmission across the ACN  3 , rather than through a local controller  4  or  5 .  
     [0052] Local controllers  4  and  5  receive control routines and other configuration data through the ACN  3  from PC  2 . The local controllers then generate signals of various types to various field devices (such as pumps, motors, regulator valves, etc.)  6  through  15  which actually implement and perform physical steps in the field to implement the control system established by the routines provided by PC  2 .  
     [0053] Two types of field devices may be connected to local controller  4  and  5  including field devices  6  through  10  which are responsive to specific control protocol such as FieldBus, Profibus and the like. As those in the art will appreciate, there are standard control protocols (e.g. FieldBus) according to which specific protocol instructions are provided to a protocol-friendly field devices (e.g., a Fieldbus field devices) will cause a controller located within the field device to implement a specific function corresponding to the protocol function. Accordingly, field devices  6  through  11  receive protocol specific (e.g., FieldBus) control commands from either the local controllers  4  and  5  or the personal computer  2  to implement a field device-specific function.  
     [0054] Also connected to local controllers  4  and  5  are non-protocol field devices  12  through  15 , which are referred to as non-protocol because they do not include any local processing power and can respond to direct control signals. Accordingly, field devices  12  through  15  are not capable of implementing functions that would be defined by specific control protocol such as the FieldBus control protocol.  
     [0055] Functionality is supplied to allow the non-protocol field devices  12  through  15  to operate as protocol-friendly (e.g., FieldBus specific) devices  6  through  11 . Additionally, this same functionality allows for the implementation of the protocol-specific control routines to be distributed between the local field devices  6  through  11 , the local controllers  4  and  5  and the personal computer  2 .  
     [0056] The distribution of protocol-specific control routines is illustrated in more detail in FIG. 1B. FIG. 1B refers to one portion of the system shown in FIG. 1A, specifically the personal computer  2 , the ethernet  3 , local controller  4 , a smart field device  6  and a dumb device  12 , in greater detail.  
     [0057] Personal computer  2  includes program software routines for implementing standard functional routines of a standard control protocol such as the FieldBus protocol. Accordingly, personal computer  2  is programmed to receive FieldBus commands and to implement all of the functional routines for which a local field device having Fieldbus capabilities could implement. The ability and steps required to program personal computer  2  to implement FieldBus block functionality will be clearly apparent to one of ordinary skill in the art.  
     [0058] Connected to personal computer  2  by the ethernet  3  is local controller  4 . Local controller  4  included as central processing unit connected to a random access memory which provides control signals to configure the central processing unit to implement appropriate operational fictions. A read only memory is connected to the random access memory. The read only memory is programmed to include control routines which can configure the central processing unit to implement all of the functional routines of a standard control protocol such as FieldBus. Personal computer  2  sends signals through ethernet  3  to the local controller  4  which causes one, more or all of the programmer routines in the read only memory to be transferred to the random access memory to configure the CPU to implement one, more or all of the standard control protocol routines such as the FieldBus routines.  
     [0059] The smart field device  5  includes a central processing unit which implements certain control functions. If the devices is, for example, a FieldBus device then the central processing unit associated with the field device  5  is capable of implementing all of the FieldBus functionality requirements.  
     [0060] Because the local controller  4  has the ability to implement FieldBus specific controls, controller  4  operates so that non-protocol device  12  acts and is operated as a FieldBus device. For example, if a control routine is running either in personal computer  2  or on the CPU of local controller  4 , that control routine can implement and provide FieldBus commands to FieldBus devices  5  and  12 . Since field device  5  is a FieldBus device, device  5  receives these commands and thereby implements the control functionality dictated by those commands. Non-protocol device  12 , however, works in conjunction with the central processing unit of local controller  4  to implement the FieldBus requirements such that the local controller in combination with the field device implements and operates FieldBus commands.  
     [0061] In addition to allowing non-FieldBus device  12  to act and operate as a FieldBus device, the described aspect allows for distribution of FieldBus control routines throughout the system  1 . For example, to the extent that a control routine initially requests field device  5  to implement more than one FieldBus control routines, the system I allows for control to the divided between the local controller  4  and the field device  5  such that a portion of the FieldBus control routines are being implemented by local controller  5  and other FieldBus routines are implemented by the use of the FieldBus routines stored on local controller  4 . The division of FieldBus routine implementation may allow for more sophisticated and faster control and more efficient utilization of the overall processing power of the system. Still further, the fact that personal computer  2  has the ability to implement FieldBus control routines, the FieldBus routines are further distributed between the local controller  4  and the personal computer  2 . In this manner, the system allows personal computer  2  to implement one or all of the FieldBus routines for a particular control algorithm.  
     [0062] Still further, the system allows for the implementation of FieldBus controls to a non-FieldBus device connected directly to the ethernet  3  through use of the FieldBus control routines stored on personal computer  2  in the same manner that FieldBus routines are implemented on non-FieldBus device  12  through use on the FieldBus routines stored on local controller  4 .  
     [0063] A process control environment  100  is shown in FIG. 1C and illustrates a control environment for implementing a digital control system, process controller or the like. The process control environment  100  includes an operator workstation  102 , a laboratory workstation  104 , and an engineering workstation  106  electrically interconnected by a local area network (“LAN”)  108  for transferring and receiving data and control signals among the various workstations and a plurality of controller/multiplexers  110 . The workstations  102 ,  104 ,  106  are shown connected by the LAN  108  to a plurality of the controller/multiplexers  110  that electrically interface between the workstations and a plurality of processes  112 . In multiple various embodiments, the LAN  108  includes a single workstation connected directly to a controller/multiplexer  110  or alternatively includes a plurality of workstations, for example tree workstations  102 ,  104 ,  106 , and many controller/multiplexers  110  depending upon the purposes and requirements of the process control environment  100 . In some embodiments, a single process controller/multiplexer  110  controls several different processes  112  or alternatively controls a portion of a single process.  
     [0064] In the process control environment  100 , a process control strategy is developed by creating a software control solution on the engineering workstation  106 , for example, and transferring the solution via the LAN  108  to the operator workstation  102 , lab workstation  104 , and to controller/multiplexer  110  for execution. The operator workstation  102  and lab workstation  104  supply interface displays to the control/monitor strategy implemented in the controller/multiplexer  110  and communicates to one or more of the controller/multiplexers  110  to view the processes  112  and change control attribute values according to the requirements of the designed solution. The processes  112  are formed from one or more field devices, which may be smart field devices or conventional (non-smart) field devices. The process  112  is illustratively depicted as two Fieldbus devices  132 , a HART (highway addressable remote transducer) device  134  and a conventional field device  136 .  
     [0065] In addition, the operator workstation  102  and lab workstation  104  communicate visual and audio feedback to the operator regarding the status and conditions of the controlled processes  112 . The engineering workstation  106  includes a central processing unit (CPU)  116  and a display and input/output or user-interface device  118  such as a keyboard, light pen and the like. The CPU  116  typically includes a dedicated memory  117 . The dedicated memory  117  includes a digital control system program  115  that executes on the CPU  116  to implement control operations and functions of the process control environment  100 . The operator workstation  102 , the lab workstation  104  and other workstations (not shown) within the process control environment  100  include at least one central processing unit (not shown) which is electrically connected to a display (not shown) and a user-interface device (not shown) to allow interaction between a user and the CPU. In one embodiment, the process control environment  100  includes workstations implemented using a Motorola 68040 processor and a Motorola 68360 communications processor running in companion mode with the 68040 with primary and secondary ethernet ports driven by the 68360 processor (SCC 1  and SCC 3  respectively).  
     [0066] The process control environment  100  also includes a template generator  124  and a control template library  123  which, in combination, form a control template system  120 . A control template is defined as the grouping of attribute functions that are used to control a process and the methodology used for a particular process control function, the control attributes, variables, inputs, and outputs for the particular function and the graphical views of the function as needed such as an engineer view and an operator view.  
     [0067] The control template system  120  includes the control template library  123  that communicates with the template generator  124 . The control template library  123  contains data representing sets of predefined or existing control template functions for use in process control programs. The control template functions are the templates that generally come with the system from the system designer to the user. The template generator  124  is an interface that advantageously allows a user to create new control template functions or modify existing control template functions. The created and modified template functions are selectively stored in the control template library  123 .  
     [0068] The template generator  124  includes an attributes and methods language generator  126  and a graphics generator  128 . The attributes and methods language generator  126  supplies display screens that allow the user to define a plurality of attribute functions associated with the creation of a new control template function or modification of a particular existing control template function, such as inputs, outputs, and other attributes, as well as providing display screens for enabling the user to select methods or programs that perform the new or modified function for the particular control template. The graphics generator  128  furnishes a user capability to design graphical views to be associated with particular control templates. A user utilizes the data stored by the attributes and methods language generator  126  and the graphics generator  128  to completely define the attributes, methods, and graphical views for a control template. The data representing the created control template function is generally stored in the control template library  123  and is subsequently available for selection and usage by an engineer for the design of process control solutions.  
     [0069] The process control environment  100  is implemented using an object-oriented framework. An object-oriented framework uses object-oriented concepts such as class hierarchies, object states and object behavior. These concepts, which are briefly discussed below, are well known in the art. Additionally, an object-oriented framework may be written using object-oriented programming languages, such as the C++ programming language, which are well-known in the art, or may be written, as is the case with the preferred embodiment, using a non-object programming language such as C and implementing an object-oriented framework in that language.  
     [0070] The building block of an object-oriented framework is an object An object is defined by a state and a behavior. The state of an object is set forth by fields of the object. The behavior of an object is set forth by methods of the object Each object is an instance of a class, which provides a template for the object. A class defines zero or more fields and zero or more methods.  
     [0071] Fields are data structures which contain information defining a portion of the state of an object. Objects which are instances of the same class have the same fields. However, the particular information contained within the fields of the objects can vary from object to object Each field can contain information that is direct, such as an integer value, or indirect, such as a reference to another object.  
     [0072] A method is a collection of computer instructions which can be executed in CPU  116  by computer system software. The instructions of a method are executed, i.e., the method is performed, when software requests that the object for which the method is defined perform the method. A method can be performed by any object that is a member of the class that includes the method. The particular object performing the method is the responder or the responding object When performing the method, the responder consumes one or more arguments, i.e., input data, and produces zero or one result, i.e., an object returned as output data The methods for a particular object define the behavior of that object.  
     [0073] Classes of an object-oriented framework are organized in a class hierarchy. In a class hierarchy, a class inherits the fields and methods which are defined by the superclasses of that class. Additionally, the fields and methods defined by a class are inherited by any subclasses of the class. I.e., an instance of a subclass includes the fields defined by the superclass and can perform the methods defined by the superclass. Accordingly, when a method of an object is called, the method that is accessed may be defined in the class of which the object is a member or in any one of the superclasses of the class of which the object is a member. When a method of an object is called, process control environment  100  selects the method to run by examining the class of the object and, if necessary, any superclasses of the object.  
     [0074] A subclass may override or supersede a method definition which is inherited from a superclass to enhance or change the behavior of the subclass. However, a subclass may not supersede the signature of the method. The signature of a method includes the method&#39;s identifier, the number and type of arguments, whether a result is returned, and, if so, the type of the result. The subclass supersedes an inherited method definition by redefining the computer instructions which are carried out in performance of the method.  
     [0075] Classes which are capable of having instances are concrete classes. Classes which cannot have instances are abstract classes. Abstract classes may define fields and methods which are inherited by subclasses of the abstract classes. The subclasses of an abstract class may be other abstract classes; however, ultimately, within the class hierarchy, the subclasses are concrete classes.  
     [0076] All classes defined in the disclosed preferred embodiment, except for mix-in classes which are described below, are subclasses of a class, Object. Thus, each class that is described herein and which is not a mix-in class inherits the methods and fields of class Object.  
     [0077] The process control environment  100  exists in a configuration model or configuration implementation  210  and a run-time model or run-time implementation  220  shown in FIG. 2. In the configuration implementation  210 , the component devices, objects, interconnections and interrelationships within the process control environment  100  are defined. In the run-time implementation  220 , operations of the various component devices, objects, interconnections and interrelationships are performed. The configuration implementation  210  and the run-time implementation  220  are interconnected by downloading. The download language creates system objects according to definitions supplied by a user and creates instances from the supplied definitions. Specifically, a completely configured Device Table relating to each device is downloaded to all Workstations on startup and when the Device Table is changed. For controller/multiplexers  110 , a downloaded Device Table only includes data for devices for which the controller/multiplexer  110  is to initiate communications based on remote module data configured and used in the specific controller/multiplexer  110 . The Device Table is downloaded to the controller/multiplexer  110  when other configuration data is downloaded. In addition to downloading definitions, the download language also uploads instances and instance values. The configuration implementation  210  is activated to execute in the run-time implementation  220  using an installation procedure. Also, network communications parameters are downloaded to each device when configuration data are downloaded and when a value is changed.  
     [0078] The process control environment  100  includes multiple subsystems with several of the subsystems having both a configuration and a run-time implementation. For example, a process graphic subsystem  230  supplies user-defined views and operator interfacing to the architecture of the process control environment  100 . The process graphic subsystem  230  has a process graphic editor  232 , a part of the configuration implementation  210 , and a process graphic viewer  234 , a portion of the run-time implementation  220 . The process graphic editor  232  is connected to the process graphic viewer  234  by an intersubsystem interface  236  in the download language. The process control environment  100  also includes a control subsystem  240  which configures and installs control modules and equipment modules in a definition and module editor  242  and which executes the control modules and the equipment modules in a run-time controller  244 . The definition and module editor  242  operates within the configuration implementation  210  and the run-time controller  244  operates within the run-time implementation  220  to supply continuous and sequencing control functions. The definition and module editor  242  is connected to the run-time controller  244  by an intersubsystem interface  246  in the download language. The multiple subsystems are interconnected by a subsystem interface  250 .  
     [0079] The configuration implementation  210  and the run-time implementation  220  interface to a master database  260  to support access to common data structures. Various local (non-master) databases  262  interface to the master database  260 , for example, to transfer configuration data from the master database  260  to the local databases  262  as directed by a user. Part of the master database  260  is a persistent database  270 . The persistent database  270  is an object which transcends time so that the database continues to exist after the creator of the database no longer exists and transcends space so that the database is removable to an address space that is different from the address space at which the database was created. The entire configuration implementation  210  is stored in the persistent database  270 .  
     [0080] The master database  260  and local databases  262  are accessible so that documentation of configurations, statistics and diagnostics are available for documentation purposes.  
     [0081] The run-time implementation  220  interfaces to the persistent database  270  and to local databases  262  to access data structures formed by the configuration implementation  210 . In particular, the run-time implementation  220  fetches selected equipment modules, displays and the like from the local databases  262  and the persistent database  270 . The run-time implementation  220  interfaces to other subsystems to install definitions, thereby installing objects that are used to create instances, when the definitions do not yet exist, instantiating run-time instances, and transferring information from various source to destination objects.  
     [0082] Device Tables are elements of the configuration database that are local to devices and, in combination, define part of the configuration implementation  210 . A Device Table contains information regarding a device in the process control environment  100 . Information items in a Device Table include a device ID, a device name, a device type, a PCN network number, an ACN segment number, a simplex/redundant communication flag, a controller MAC address, a comment field, a primary internet protocol (IP) address, a primary subnet mask, a secondary IP address and a secondary subnet mask.  
     [0083] Referring to FIG. 3, a block diagram illustrates a user interface  300  for usage with both the configuration and run-time models of the process control environment  100 . Part of the user interface  300  is the Explorer™  310 , an interfacing program defined under the Windows NT™ operating system which features a device-based configuration approach. Another part of the user interface  300  is a module definition editor  320  for interfacing using a control-based configuration approach.  
     [0084] The Explorer™  310  is operated by a user to select, construct and operate a configuration. In addition, the Explorer™  310  supplies an initial state for navigating across various tools and processors in a network. A user controls the Explorer™  310  to access libraries, areas, process control equipment and security operations. FIG. 3 illustrates the relationship between various tools that may be accessed by a task operating within the process control environment  100  and the relationship between components of the process control environment  100  such as libraries, areas, process control equipment and security. For example, FIG. 3 shows that, when a user selects a “show tags” function from within an area, a “tag list builder” is displayed, showing a list of control and I/O flags. From the tag list builder, the user can use an “add tag” function to add a module to a list, thereby invoking a “module editor”.  
     [0085] Referring to FIG. 4, a schematic block diagram illustrates a hierarchical relationship among system objects of a configuration model  400 . The configuration model  400  includes many configuration aspects including control, I/O, process graphics, process equipment, alarms, history and events. The configuration model  400  also includes a device description and network topology layout.  
     [0086] The configuration model hierarchy  400  is defined for usage by a particular set of users for visualizing system object relationships and locations and for communicating or navigating maintenance information among various system objects. For example, one configuration model hierarchy  400 , specifically a physical plant hierarchy, is defined for usage by maintenance engineers and technicians for visualizing physical plant relationships and locations and for communicating or navigating maintenance information among various instruments and equipment in a physical plant An embodiment of a configuration model hierarchy  400  that forms a physical plant hierarchy supports a subset of the SP88 physical equipment standard hierarchy and includes a configuration model site  410 , one or more physical plant areas  420 , equipment modules  430  and control modules  440 .  
     [0087] The configuration model hierarchy  400  is defined for a single process site  410  which is divided into one or more named physical plant areas  420  that are defined within the configuration model hierarchy  400 . The physical plant areas  420  optionally contain tagged modules, each of which is uniquely instantiated within the configuration model hierarchy  400 . A physical plant area  420  optionally contains one or more equipment modules  430 . An equipment module  430  optionally contains other equipment modules  430 , control modules  440  and function blocks. An equipment module  430  includes and is controlled by a control template that is created according to one of a number of different graphical process control programming languages including continuous function block, ladder logic, or sequential function charting (“SFC”). The configuration model hierarchy  400  optionally contains one or more control modules  440 . A control module  440  is contained in an object such as a physical plant area  420 , an equipment module  430  or another control module  440 . A control module  440  optionally contains objects such as other control modules  440  or function blocks. The control module  440  is thus a container class, having instances which are collections of other objects. The control module  444  is encapsulated so that all of the contents and the implementation of the methods of the control module are hidden.  
     [0088] Referring to FIG. 5, a schematic block diagram shows a configuration architecture  500  that operates within the configuration model hierarchy  400  illustrated in FIG. 4. The configuration architecture  500  includes a several objects and classes at multiple levels of abstraction. At an organizational level of abstraction  510 , the configuration architecture  500  includes a site class  512  which contains “named” objects and classes within the configuration architecture  500 . Named objects and classes are definitions, display components such as screens and graphics and other items. The named objects and classes include function blocks, user accounts, modules, plant areas, events, libraries and other site-wide information. Examples of named items are block definitions, equipment module definitions, control module definitions, plant area names and the like.  
     [0089] At a primitive level of abstraction  520 , the configuration architecture  500  includes primitives that define the interfaces to functions within the configuration architecture  500 , including hard-coded functions such as “+”. The primitive level of abstraction  520  includes the classes of functions  522  and parameters  524 . Functions  522  are operational functions at the lowest level of abstraction in the configuration architecture  500 . Functions  522  are typically coded in the C or C++ languages. In one embodiment of the configuration architecture  500 , the full set of implemented function blocks  522  are primitives. Objects and classes at the primitive level of abstraction  520  are defined throughout the site class  512 . Parameters  524  are classes and objects at the lowest level of abstraction in the configuration architecture.  
     [0090] Parameters  524  include integer numbers, real numbers, vectors, arrays and the like.  
     [0091] Attribute values are mapped into parameters  524  for usage within a function block  522 . In one embodiment, function blocks  522  at the primitive level of abstraction  520  include the function block primitives listed in TABLE I, as follows:  
               TABLE I                          Function Blocks                     Function Block   Description/Comments               Action   Handles simple assignment statements using           Hawk expression capability.       ADD   Simple Add function with extensible inputs.       Al   FF Standard Analog Input       AI Lite   A scaled back version of the FF analog input.       AI HART   The EF Standard Analog Input with some extra           ability to handle HART devices.       AND   Simple And function with extensible inputs.       AO   FF Standard Analog Output (From FF standard           specification)       Arithmetic   FF Standard Arithmetic Blo&amp; (From FF standard           specification)       BDE_TRIGGER   Simple bi-directional edge trigger.       BIASGAIN   FF Standard Bias/Gain. (From FF standard           specification)       CALC/LOGIC   Advanced calculation and logic block that has its           own language as well as the ability to           handle simple ST (1131). It has           extensible inputs, extensible outputs, and the           ability to create temporary variables.       Condition   Handles simple condition statements using           Hawk expression capability.       Counter   Simple up/down counter that handles several different           Accumulation methods.       CTLSEL   FF Standard Control Selector. (From FF standard           specification)       DI   FF Standard Discrete Input (From FF standard           specification)       DI Lite   A scaled back version of the FF discrete input.       DIVIDE   Simple Divide.       DO   FF Standard Discrete Output. (From FF standard           specification)       DT   FF Standard Deadtime with advanced control research           implemented. (From FF standard specification)       DtoI   A boolean fan in that converts up to 16 discrete           inputs to a 16-bit integer value.           Also has some special abilities for capturing           input patterns.       FILT   Simple filter.       H/L MON LIMIT   Simple high/low signal monitor and limiter.       INTEGRATOR   FF Standard Integrator block. (From FF standard           specification)       ItoD   Boolean fan-out. Takes a 16-bit integer and           translates it into 16 discrete outputs.       L/L   FF Standard LeadLag with 2 additional           types of equations to           select. (From FF standard specification)       LOOP   An I/O and control block with the abilities           of AI PID, and AO           rolled into one block.       LOOPD   An I/O and control block with the           abilities of DI, Device           Control, and DO rolled into one block.       MAN   FF Standard Manual Loader. (From FF standard           specification)       MULTIPLEX   Simple multiplexor with extensible inputs.       MULTIPLY   Simple multiply with extensible inputs.       NDE_TRIGGER   Simple negative edge trigger.       NOT   Simple not.       OFF_DELAY   Simple off-delay timer.       ON_DELAY   Simple on-delay timer.       OR   Simple logical or with extensible inputs.       P/PD   FF Standard P/PD. (From FF standard specification)       PDE_TRIGGER   Simple positive directional edge trigger.       PERIOD   Simple monitor that triggers when an input is true for a           specified period       PI   FF Standard Pulse Input (From FF standard           specification)       PID   FF Standard PID with many additions including           the ability to           choose algorithm type, form, and structure.           (From FF standard specification)       RAMP   Simple ramp generator.       RATELIM   Simple rate limiter generator.       RATIO   FF Standard Ratio block.           (From FF standard specification)       RETENTIVE   Simple retentive timer.       RS   Simple reset dominant flip-flop.       RUNAVE   Simple running average calculator.       SCALER   Simple sealer.       SIGGEN   Generates square waves, sin waves,           random waves, or any           combination of the three.       SIGNALCHAR   FF Standard Signal Characterizer. (From FF standard           specification)       SIGSEL   Simple signal selector.       SPLITTER   FF Standard Splitter. (From FF standard specification)       SR   Simple set dominant flip-flop.       SUBTRACT   Simple subtract block.       TP   Simple timed pulse block.       TRANSFER   Simple transfer block.       XOR   Simple exclusive or block.                  
 
     [0092] At a definition and usage level of abstraction  530 , the configuration architecture  500  includes definitions  532  and usages. Definitions  532  and usages, in combination, define the algorithm and the interface for objects including function blocks, control modules, equipment modules, links and attributes. The definitions  532  define algorithms and interfaces for function blocks, modules, links and attributes. Usages are objects and classes at the definition and usage level of abstraction  530  that represent the usage of one definition within another.  
     [0093] At an instance level of abstraction  540 , the configuration architecture  500  includes instances, which are “tagged” items within the configuration. Plant areas  542 , modules  544 , attributes  546 , and PIO blocks  548  are tagged instances. Instances are defined according to definitions  532 . A plant area  542  represents a geographical or logical segmentation of a process site class  512 . All objects and classes at the instance level of abstraction  540  are defined throughout the plant area level so that all module instances have a 0 or 1 association with a plant area  542 . To be installed in a run-time system, the module instances must have a 1 association, meaning that the module is viewed as being “contained by” or “scoped” in this context of a plant area A module instance  544  is an installable object that is associated to a specific object of plant equipment An attribute instance  546  is a visible parameter in a module instance  544 , a plant area instance  542  or other device. An attribute instance  546  may be used for an input signal, an output signal, data storage or the like.  
     [0094] At a device level of abstraction  550 , the configuration architecture  500  includes devices  552  such as controllers, smart devices and consoles, and input/output devices (IO)  560  such as a PIO block, and the like, which represent physical process control equipment in the physical plant A process input/output (PIO) block is an abstraction that represents various high density and low density conventional input/output devices including Hart, FieldBus and other input and output devices that are interfaced into the configuration architecture  500 . High or low density relates to the number of channels on an I/O card. For example,  8  channels are typical on a low density card while a high density card may have  32  channels. Devices  552  are process control equipment in the configuration architecture  500  and include objects such as controllers, input/output devices, consoles and the like. Input/output devices (IO)  560  are the physical process input and output devices in the configuration architecture  500 .  
     [0095] A smart device is a field device that is implemented to transmit and receive digital data pertaining to a device, including data relating to device calibration, configuration, diagnostics and maintenance. Typically, the smart device is also adapted to transmit a standard analog signal that is indicative of various information including, for example, a process value measured by a field device. Examples of smart field devices include field devices which follow a HART (highway addressable remote transducer) protocol, a Fieldbus protocol, a Modbus protocol and a device net protocol.  
     [0096] Various hierarchical relationships among system objects are implemented to facilitate navigation through the process control environment  100  by different users and to accomplish different tasks. Four different hierarchical relationships are defined including control, control system, operations and physical plant hierarchies. A specific system object may be implemented in multiple hierarchical systems.  
     [0097] The control hierarchy is a subset of a standard SP88 hierarchy and has system objects including site, physical area, equipment module, control module and control element objects. The control hierarchy is used to organize control operations and to define the scope of named objects. A user interacts with the control hierarchy on the basis of a site instance, equipment module definitions, control module definitions, a plant area instance, equipment module instances, control module instances, display module instances, and process I/O module(block instances, having signal tags.  
     [0098] The control system hierarchy includes operator/configuration stations, host computers, controllers, I/O devices, smart devices, gateways and the like, which are associated using various network standards including area control network (ACN), process control network (PCN) and other I/O network standards. In one embodiment, the ACN hardware includes standard 10-base-T ethernet communication ports and a workstation contains standard Ethernet 10-base-T interface cards and software drivers. A user interacts with the control system hierarchy on the basis of a defined site instance, a network definition, a defined network instance, devices, and subsystems such as files, cards, channels, controllers, operation stations, and Fieldbus segments.  
     [0099] The area control network (ACN) includes communication functionality at two levels, a remote object communications (ROC) level and a low level communications level. ROC level controls the interface between the Hawk applications and the ACN communications system. The low level communications support the interface with the TCP/IP sockets and the actual transmission of messages.  
     [0100] Remote Object Communications (ROC) are system operations supporting communication of objects in the process control system  100  and particularly supporting communication between objects whether the objects reside in the same device or in remote devices. The ROC communication level supports communications message services including request/response, unsolicited reporting, event/alarm reporting and broadcast message service.  
     [0101] Request/Response is a service by which applications send messages to a remote device and receive a response from the device. Unsolicited Reporting is a service for periodically sending updated data to a remote device. Unchanged data is not reported. Event/Alarm Reporting is a guaranteed delivery message service which is used for the transmission of events, alarms and other vital information for delivery to a remote device. The broadcast message service is used to send messages to all Hawk devices on the communications network. The ROC level also sets communications policies for the communications subsystem. This means that it is responsible for managing what message get sent and when as well as how incoming messages are processed. Communications flow control will also be the responsibility of the ROC portion.  
     [0102] Low level communications support is included for device connection management, ACN redundancy and communications systems diagnostics. Device connection management establishes a communications connection between two devices and manages the transmission of messages between the two devices. ACN Redundancy handles the detection of communications link failures, controls the switch from one link to another and tracks the status of communication links between a host device and connected remote devices. Communications subsystem diagnostics tracks communication integrity and statistical information, responds to requests for communications diagnostic data.  
     [0103] Device connection management in an ACN communications system supports both UDP and TCP type device connections. UDP connections are used for normal real time data transfers between devices. TCP connections are used for special applications using a streaming protocol such as file transfers, device flash downloads, and the like. Communications between devices is managed by a Device Connection Object. The Device Connection Object is transmits data and maintains the status of the communications links between two communicating devices.  
     [0104] All normal device connection message traffic is directed through a single UDP communications port. A Device Connection Object starts the communications system by creating the communication socket associated with this UDP port as well as creating the queues needed for management of the device connection message traffic. The Device Connection Object receives all incoming messages on a Device Connection communications socket and routes messages to the proper device connection instance for processing. The Device Connection Object handles timing functions of device connections, including notifying device connection instances when messages time out waiting to be acknowledged, when communications link checks are due and when device connection resyncs have timed out.  
     [0105] UDP type communications are used for the transfer of real-time data among devices. The remote object communications (ROC) subsystem passes messages to a UDP Device Connection for transmission to a destination device. A pool of message buffers is created on startup of each device. The message pool is used for all data transferred between devices, preventing the communications subsystem from exhausting memory and ensuring that no other task exhausts memory, thereby preventing the communication subsystem from running. Communication flow control is implemented in the Device Connection Object. If the number of message buffers in the communications buffer pool reaches a predefined low level, all remote devices are inhibited from sending messages until the low buffer problem is resolved in the affected device preventing loss of messages.  
     [0106] TCP-type communications are used for applications using a streaming-type protocol such as file transfers and device flash downloads. TCP-type connections are temporary connections established for the duration of the applications needs and terminated once the application has completed a communications task. For reasons of interoperability, compatibility, and availability, a TCP/IP protocol stack is employed. The TCP/IP stack supplies a connection-oriented, byte stream protocol (TCP) and a connectionless, message oriented protocol (UDP). The device connection supports request/response messages, unsolicited data, and event and alarm data between devices. The communication system maintains the device connection through one of two available communications links in the event of a single communications failure, typically a cable fault. Detection of a fault and switch to an alternate communications path transpires in a short, deterministic time span which is less than one second.  
     [0107] The operations hierarchy is defined for a particular set of users, specifically operators and maintenance engineers, generally for the purpose of accessing displays, reports, and other informational items. A user interacts with the operations hierarchy on the basis of a site instance, User Group definitions, a plant area instance, equipment module instances, control module instances, display instances, and report instances.  
     [0108] The physical plant hierarchy is defined for a particular set of users, specifically maintenance engineers and technicians, typically for the purpose of determining physical relationships among objects and navigating maintenance information about plant instruments and equipment. A user interacts with the physical plant hierarchy on the basis of a site instance, a maintenance area instance, a plant area instance, room instances, cabinet instances, node/device instances and display instances.  
     [0109] The system objects that are implemented in the multiple hierarchical systems are arranged into a plurality of subsystems including control, process I/O, control system hardware, redundancy management, event/alarm management, history services, process graphics, diagnostics presentation, user environment, management organization and field management system (FMS) subsystems. The control subsystem includes routines for configuring, installing and executing control modules and equipment modules. The process I/O subsystem is a uniform interface to devices including HART, Fieldbus, conventional I/O and other input/output systems. The control system hardware subsystem defines a control system topology, devices within the topology and capabilities and functions of the devices. The control system hardware subsystem also includes objects and data structures for accessing device level information indicative of status and diagnostics.  
     [0110] The redundancy management subsystem establishes a redundant context between primary and secondary control applications and manages switching in context between the primary and secondary control applications. The redundancy management subsystem also maintains and monitors redundant context diagnostic information including state information and data latency information. Network redundancy is accomplished using two separate Ethernet communications links or networks. The primary communication link is the preferred communications path. The secondary link is only used if the primary has failed. Communications switchovers are performed on a per device basis. For example, if device A is communicating with devices B and C and the primary link to device C falls, device A continues to communicate with device B on the primary link but switches to the secondary link to communicate with device C. Each Ethernet link is a separate, dedicated network having a dedicated set of IP addresses and a subnet mask.  
     [0111] The device connection object manages redundant communications including controlling when to switch to the secondary link and when to switch back to the primary link. Each device in the process control system tracks the communication status of all current links to remote devices by periodically sending link test messages when no other communications is occurring, to check the status of the communications links to each device. Redundancy switchovers are performed on a device connection basis.  
     [0112] The event/alarm management subsystem configures, monitors, and supplies notification of significant system states, acknowledgments and priority calculations. The history services subsystem stores and retrieves process and event information. The process graphics subsystem supplies user-defined views for display and operator interfacing onto the defined system architecture. The diagnostics presentation subsystem furnishes displays of diagnostic information, typically at the request of a user. The user environment subsystem supplies a user interface, allowing a user to enter commands to control operation of the process control environment  100 . The management organization subsystem sets an organizational structure of the process control environment  100  including specification of site, area, primitives, access to user libraries, and location of defined objects and instances. The FMS supplies user interfaces, views, and organization structure for the configuration, installation and monitoring of HART and Fieldbus devices.  
     [0113] A Fieldbus device implements localized control of a process within the process, in contrast to a longer-used and more conventional approach of controlling device functions from a main or centralized digital control system. A Fieldbus device achieves localized control by including small, localized controller/multiplexers  110  which are closely associated with field devices within the Fieldbus device. The small, localized controllers of a Fieldbus implement standardized control functions or control blocks which operate on the field devices within the Fieldbus device and which also operate on other smart field devices that are connected to the Fieldbus device.  
     [0114] Thus, the process control environment  100  implements smart field device standards, such as the Fieldbus Hi standard, Profibus standard, CAN standard and other bus-based architecture standards so that communications and control among devices, particularly Fieldbus devices, are performed so that Fieldbus-type control operations are transparent to a user.  
     [0115] The process control environment  100  allows attachment to a substantially unlimited number and type of field devices including smart devices, such as Fieldbus and HART devices, and conventional non-smart devices. Control and communication operations of the various numbers and types of devices are advantageously performed simultaneously and in parallel.  
     [0116] The process control environment  100  implements and executes a standard set of function blocks or control functions defined by a standard Fieldbus protocol, such as the Fieldbus HI standard, so that Fieldbus-type control is achieved with respect to non-Fieldbus-type devices, such as a HART device  134  and a conventional device  136 . In addition, the process control environment  100  enables Fieldbus devices to implement the standard set of function blocks and control functions. Advantageously, the process control environment  100  implements an overall strategy as if all connected devices are Fieldbus devices. This implementation is achieved, in part, by the usage of a function block as a fundamental building block for control structures. These function blocks are defined to create control structures for all types of devices. Usage of function blocks as fundamental building blocks is described in FIGS. 6, 7,  8  and  9 .  
     [0117] The process control environment  100  implements an overall, user-developed control strategy through the definition of function blocks and control modules by downloading or installing specific portions of the control strategy into Fieldbus devices and non-Fieldbus devices. Thereafter, the Fieldbus devices automatically perform the downloaded portions of the overall strategy independently of other control system operations. For example, the portions of the control strategy downloaded or installed into the devices operate independently of and in parallel with the control operations of the controller/multiplexers  110  and the workstations, while other control operations manage the Fieldbus devices and implement other portions of the control strategy. In effect, the process control environment  100  implements a control strategy using the controller/multiplexers  110  within the Fieldbus devices.  
     [0118] An example of the definition of a function block  522  is shown in FIG. 6. Specifically, FIG. 6 depicts an “elemental” function block definition  600  which is defined to contain only primitive objects. The elemental function block definition  600  defines a sum function and includes a “+” primitive  610 , a first input attribute  612  which is a first parameter  614  applied to the primitive  610 , and a second input attribute  622  which is a second parameter  624  applied to the primitive  610 . The primitive  610  produces a result that is supplied as an output attribute  630 . In this example, the elemental function block definition  600  is a block definition that is created and named SUM.  
     [0119] A second example of the definition of a function block  522  is shown in FIG. 7. Specifically, FIG. 7 depicts a “composite” function block definition  700  which is defined to contain one or more elemental function blocks  600  and, optionally, one or more primitive objects. The composite function block definition  700  defines a composite sum function and includes a first SUM elemental function block  710  and a second SUM elemental function block  712 , each of which is the same as the SUM elemental function block  600  illustrated in FIG. 6. The composite function block  700  has a first input attribute  720  and a second input attribute  722  which are respective first and second parameters  724  and  726  applied to the first SUM elemental function block  710 . The first elemental function block  710  produces a result that is applied to the second SUM elemental function block  712  as a first parameter  730 . The composite function block  700  has a third input attribute  728  that is a second parameter  732  applied to the second SUM elemental function block  712 . The second SUM elemental function block  712  produces a result that is supplied as an output attribute  734 . In this example, the composite function block definition  700  is a block definition that is created and named SUM 3 .  
     [0120] An example of the definition of a control module  440  is shown in FIG. 8. Specifically, FIG. 8 depicts a control module definition  800  which is defined and contains various input attributes  810 , elemental function blocks  820 , a first SUM 3  composite function block  830  and a second SUM 3  composite function block  832 . The exemplary control module  440  includes five input attributes  810  which are connected to five respective elemental function blocks  820 , three of which are parameters applied to the first SUM 3  composite function block  830 . The first SUM 3  composite function block  830  produces a result that is supplied as a parameter to the second SUM 3  composite function block  832  in combination with parameters supplied by the remaining two elemental function blocks  820 . The second SUM 3  composite function block  832  produces a result that is supplied as an output attribute  840 . In this example, the control module  800  is a control module definition that is created and named CM 1 .  
     [0121] Examples of a module instance  544 , specifically a control module instance, are shown in FIG. 9. Control module instances  910  and  920  are created in accordance with the CM 1  control module definition so that each control module instance  910  and  920  includes five input attributes  912  and  922 , respectively, that correspond to the five input attributes  810  shown in FIG. 8. Each control module instance  910  and  920  also includes one output attribute  914  and  924 , respectively, that correspond to the output attribute  840  shown in FIG. 8. In this example, the control module instances  910  and  920  are control module instances that are created and tagged CALC 1  and CALC 2 , respectively.  
     [0122] Using a definition editor, a system user creates and names definitions, such as the SUM elemental function block definition, the SUM composite function block definition and the CM 1  control module definition. Then, using a module editor, the system user creates and tags instances, such as the CALC 1  and CALC 2  control module instances.  
     [0123] Referring to FIG. 10, a flow chart shows an example of control module execution at run-time. A run-time program includes a scheduler routine. Scheduler routines are well-known in the computing arts. The scheduler routine requests execution  1010  of a composite control module, for example the composite control module  440  with tag CM 1  shown in FIG. 8. The CM 1  composite control module  440  initiates transfer  1012  of the input attributes  820 , causing any associated links, or attribute associations, to transfer  1014 . A database definition typically refers to “associations” while a runtime definition relates to “links”. In steps  1016  through  1056  the CM 1  composite control module  440  requests each elemental function block  820 , first SUM 3  composite function block  830  and second SUM 3  composite block  832  to execute in turn.  
     [0124] Specifically, in step  1016  the CM 1  composite control module  440  requests each elemental function block  820  to execute. The elemental function blocks  820  initiate transfer  1018  of input attributes, for example first input attribute  612  shown in FIG. 6. The input attributes of the elemental function blocks  820  request  1020  loading of values from the links transferred in step  1014 . The links copy  1022  values from source attributes to destination attributes. The elemental function blocks  820  execute block algorithms  1024 . Upon completion of block algorithm execution, the elemental function blocks  820  initiate transfer of output attributes  1026 , for example output attribute  630  shown in FIG. 6.  
     [0125] In step  1028  the CM 1  composite control module  440  requests first SUM composite function block  830  to execute. First SUM 3  composite function block  830  initiates transfer  1030  of input attributes, for example input attributes  722 ,  724  and  726  shown in FIG. 7, from the elemental function blocks  820 . In step  1032 , first SUM 3  composite function block  830  requests internal function blocks, for example, the first SUM elemental function block  710  and the second SUM elemental function block  712  shown in FIG. 7, to execute in turn. First SUM elemental function block  710  reads input attributes, executes a block algorithm and sets an output attribute in step  1034 . Second SUM elemental function block  712  reads input attributes, executes a block algorithm and sets an output attribute in step  1036 . First SUM 3  composite function block  830  initiates transfer of output attributes in step  1038 . The output attribute of first SUM 3  composite function block  830  requests an associated link to copy the value from the output attribute in step  1040 .  
     [0126] In step  1042  the CM 1  composite control module  440  requests second SUM 3  composite function block  832  to execute. Second SUM 3  composite function block  832  initiates transfer  1044  of input attributes from the links connected to the first SUM 3  composite function block  830  output attributes. In step  1046 , second SUM 3  composite function block  832  requests internal function blocks, for example, the first SUM elemental function block  710  and the second SUM elemental function block  712  shown in FIG. 7, to execute in turn. First SUM elemental function block  710  reads input attributes, executes a block algorithm and sets an output attribute in step  1048 . Second SUM elemental function block  712  reads input attributes, executes a block algorithm and sets an output attribute in step  1050 . Second SUM 3  composite function block  832  initiates transfer of output attributes in step  1052 . The output attribute of second SUM 3  composite function block  832  requests an associated link to copy the value from the output attribute in step  1054 .  
     [0127] In step  1056  the CM 1  composite control module  440  initiates transfer of output attributes and output attribute  840  requests a link from second SUM 3  composite function block  832  to copy the value of the second SUM 3  composite function block  832  output attributes. In some embodiments, output function blocks push output values to a user-configured PIO block attribute (not shown). Thus, PIO attributes are “pulled” by function blocks while output function blocks push output values to PIO Block attributes. Similarly, input function blocks pull input attribute values from PIO Block attributes.  
     [0128] A user defines a module control strategy by specifying function blocks that make up control modules and determine the control strategy. The user modifies or debugs a module control strategy by adding, modifying and deleting function blocks, configuring parameters associated with the function blocks and creating a view to new attributes.  
     [0129] By defining function blocks and control modules, a user-defined control strategy, application program or diagnostic program is represented as a set of layers of interconnected control objects identified as modules. A layer of the control strategy includes a set of modules which are interconnected in a user-specified manner. A module typically includes an algorithm for performing a specific function and display components which are used to display information to a user. A module is optionally represented to include a set of input and output connections for connecting to other modules. A module may be considered to be a “black box” which performs a specified function and is connected to other modules via specified input and output connections.  
     [0130] Referring to FIG. 11, a display screen serves as a flow chart which shows an example of a module or application program LOOPSIM  1060  at a highest layer of a control structure. The illustrated layer of the LOOPSIM  1060  application program includes an input attribute (AIN) module  1062  called AI 1 , a deadtime module  1064 , a proportional, integral, differential (PID) control module  1066 , an output attribute (AOUT) module  1068  and a simulate module  1070 . Each of the illustrative modules includes named input connections and output connections which are connected to the other modules via lines. The set of modules, the input connections and the output connections of the set of modules, and the interconnections between modules define the operation of the LOOPSIM  1060  application.  
     [0131] At a lowest level, a module is a set of interconnected function blocks. At higher levels, a module is a set of interconnected submodules which, in turn, may include a further set of submodules. For example, the PID control module  1066  is typically a set of interconnected submodules which perform the different functions included in a PID functionality. The input and output connections of the PID module  1066  are an input connection and an output connection of one or more of the submodules within a next lower layer of the PID module  1066 . The submodules in the PID module  1066  optionally include other input and output connections sufficient to define the interconnections between the submodules.  
     [0132] An application, a module or a submodule, at any module level, is optionally modified by a user to perform a slightly different function or to perform the same function in a different manner. Thus, a user optionally modifies the module, thereby modifying the control structure, in a desired manner. Specifically, a user optionally adds input and output connections to modules and extends the input and output connections of a module to a higher level module so customize modules for various applications. For example, a user optionally adds a new input connection or output connection to the PID module  1066  to the “edge” of the PID module  1066  which makes the input connection and output connection appear as input and output connections to the PID module  1066 .  
     [0133] The process control environment facilitates the definition and modification of the control structure by furnishing editing operations in a plurality of control languages including IEC- 1131  standard languages such as Field Blocks, Sequential Function Charts (SFC), Ladder Logic and Structured Text. Accordingly, different types of users, from different control backgrounds use the different languages to write different modules for implementing the same or different applications.  
     [0134] Control modules are specified to have several advantageous characteristics. Some control modules allow direct access to attributes. For example, some attributes, called “heavy” attributes, support direct (maximum performance) communications. Direct communications are advantageously used for connecting function blocks and Control Modules, supporting event/alarm detection, and high performance trending, for example. Some attributes are created automatically upon definition of a control module with a user having the option to promote or force a parameter to be exposed as an attribute of a Control Module. Other parameters are made accessible through a module, such as a Control Module, an Equipment Module, a PIO Block, or a Device, which contains the parameter but direct communications performance of the attributes does not warrant the overhead incurred in supplying this performance. These parameters are advantageously accessed to supply information relating to control system tuning, debugging and maintenance. In some embodiments, these parameters are accessed by a general purpose parameter browser applications, which use services provided by tagged containers to reveal attributes, invokeable services, and subcomponents within the containers.  
     [0135] Referring to FIG. 12, a block diagram illustrates an object-oriented method for installing a process I/O attribute block into a PIO device through the operation of the control subsystem. A block of defined objects  1110  includes a site object  1112 , a controller device  1114 , a controller I/O subsystem  1116 , a PIO interface device  1118  and a PIO device  1120 . Prior to installation of the PIO device, the controller I/O subsystem  1116  is previously created. The PIO device  1120  is also previously created, either by installation or downloading. The block of defined objects  1110  directs a detail pointer  1122  to a list of block definitions  1124  to specify a particular type of object to be created by a create pointer  1126  directing the operation of a create block  1128 . The block definitions  1124  includes a PIO input attributes (AIN) block definition either as hardwired or by previous installation. Attributes of the specified object are set by a user through the operation of an editor  1130 . Prior to installation of the PIO device, an input attribute (AIN) block  1132  does not exist.  
     [0136] Prior to installing the AIN block  1132 , a user creates the PIO device  1120  then sets up initial values for AIN block attributes using the editor  1130 . The user also sets a period for view parameter acquisition. The AIN block  1132  is saved and then installed.  
     [0137] Referring to FIG. 13, a block diagram illustrates an object-oriented method for linking a Control Module input attribute to a process I/O attribute. Prior to linking of the control module input attribute to the PIO attribute, the PIO block AIN  1220  is previously installed and the control module  1210  is also installed. The user specifies that a PIOIN attribute  1212  of the control module  1210  is connected to an attribute input process variable PV  1214  and requests that a link be made. A link  1216  is made as the control module finds the PIOIN attribute and returns a corresponding attribute index, locates PIO AIN in a plant area, find the process variable PV attribute and returns a corresponding attribute index, instructs the run-time linker  1216  to create a link with a source at the process variable (PV)  1214  and a destination at the PIOIN attribute  1212 , creates the link and connects the link  1216 . At end of a download, links are resolved by the linked objects.  
     [0138] Referring to FIG. 14, a block diagram shows an object-oriented method for linking a control module output attribute (AOUI)  1312  attribute to a PIO output attribute (PIOAOUT)  1320 . A control module  1310  is previously installed and the control module output attribute (AOUT)  1312  is installed within the control module  1310 . The user specifies that the control module output attribute (AOUT)  1312  is connected to the a PIO output attribute (PIOAOUI)  1320 . The link is made as the run-time implementation of the control module  1310  is sent a message to form the connection, the control module  1310  finds the AOUT attribute, requests location of the PIOAOUT attribute  1320 , creates a link  1322  and connects the AOUT attribute  1312  and the PIOAOUT attribute  1320  to the link  1322 .  
     [0139] Referring to FIG. 15, a block diagram shows an object-oriented method for reading values of contained PIO attributes. A PIO block  1410  is previously installed and an output attribute (AOUT)  1412  is previously installed within the PIO block  1410 . A user, for example an engineer, requests a detailed view of the block in which all attribute values are displayed. The detailed display includes one or more sets of display groups, also called view definitions, associated with the PIO block  1410 . A proxy is previously established for the PIO Block  1410 . A user requests detail for the output attribute (AOUT)  1412 . Attribute names and values for the AOUT block are presented by an application program requesting a proxy client routine to access a view, an AOUT proxy client setting a return view definition and creating an attribute proxy object, and the application program requesting the AOUT proxy client to read out values for attributes named with granted privileges. The application program formats and displays the data Display group parameters are part of an I/O block definition and are, therefore, not configurable. Display groups are defined for attributes. Information is advantageously updated while a PIO block is not linked since display groups and view groups control updating of non-linked PIO attributes associated with a block.  
     [0140] The process control environment  100  implements an overall strategy as if all connected devices are Fieldbus devices not only by the usage of a function block as a fundamental building block for control structures but also by implementing an input/output architecture that treats Fieldbus and nonFieldbus devices in the same manner. The fundamental character of the input/output architecture is based on instrument signal tags (ISTs) that furnish user-configurable names for all I/O signals including Fieldbus and nonFieldbus I/O signals.  
     [0141] From the perspective of a user, an IST binds a user-defined name to a signal type, to a specific signal in the I/O subsystem, to a signal path including an attribute and to a set of signal property settings.  
     [0142] ISTs are not installed in the manner of other system objects. Instead, signal properties inherent to the IST tag are combined with I/O Port and I/O Device properties that are made available when an I/O Card is installed. The combination of IST, I/O Port and I/O Device properties furnish information for creating a PIO function block in the run-time system. The signal path from ISTs is included in the script that defines I/O Function Blocks during installation of a module.  
     [0143] To communicate throughout the process control environment  100 , an I/O type Function Block uses an I/O reference definition. An IST satisfies the specification for an I/O reference. Conventional I/O devices, such as MTL devices, have an IST for each channel. Hart and Fieldbus I/O devices may include an IST for each distinct “I/O signal” on a Port or in a field Device. IST names have system-wide scope and share the name space of Modules, Devices, and Areas. In large systems, ISTs typically correspond to instrument signal names on instrumentation drawings. In small systems, formal instrument drawings may not exist so that no obvious IST names are inferred. Typically, ISTs are automatically generated as cards are configured based on a device hierarchy path representing a controller node, I/O subsystem, card and port so that arbitrary IST names are avoided. Typically most ISTs are created automatically when a new I/O card is defined. For multiple-signal I/O devices, an IST is automatically created for only a single “primary signal”. In addition to automatic IST creation, a user may also create ISTs using an “Assign . . . ” menu available from the Explorer Node/IOsubsys/Port/Device tree with a Port or Device selected or using a “New . . . ” menu available from the Explorer IST tree.  
     [0144] ISTs have a “signal type” property to ensure compatibility between the I/O signal and the I/O Function Block(s) that accesses the I/O signal. Signal type is one of: AIN, AOUT, DIN, DOUT, PCIN, PCOUT. ISTs have a set of “signal-related” attributes specific to the signal type (e.g. EUO and EU100 for a AIN, MOMENTARY or LATCHED for a DOUT, etc.). All signal sources with the same signal type have the same set of “signal attributes”. All other properties of the I/O subsystem objects are held in card, port, or device attributes.  
     [0145] Fully configured ISTs have a fully qualified path to a corresponding signal in the I/O system, e.g. “CON1/I01/S01/C01/FIELD_VAL”. An IST may be created without a defined path defined so that module configuration may be completed before I/O structure details are fully defined. Modules with I/O Function Blocks using ISTs with no defined path may be configured and installed but the run-time system must deal appropriately with missing I/O paths of missing ISTs on I/O Function blocks. A signal source has no more than one IST. Attempts to configure more than one IST with the same path are rejected.  
     [0146] A user may delete an IST, thereby deleting associated signal properties and possibly leaving some unresolvable IST references in I/O Function Blocks. A deleted IST does not affect card/port/device properties with a normal display of the IST on the Port/Device in the Explorer tree indicating no associated IST.  
     [0147] I/O-interface Function Blocks have at least one IST-Reference property. An IST-Reference property is either left blank to indicate that the function block does not connect to a IST, or is designated with a valid IST name. An IST-Reference property in an I/O Function Block is compatible with exactly one IST signal type. For example, the IST-Reference in the AI Function Block has an IST with a signal type “AIN” only. Several I/O Function Blocks are compatible with the same IST signal type.  
     [0148] For compatibility with Fieldbus I/O Function Block definitions, Fieldbus I/O Function Blocks have attributes such as XD_SCALE, OUT_SCALE which overlap with some of the signal properties in ISTs. When a valid IST-Reference is made, the configured values of these overlapped Function Block attributes are ignored in the Run-time system and the corresponding properties from the IST are used instead. An engineer configuring Fieldbus I/O Function Blocks uses an indication of ignored attributes when a IST reference is in place. Such an indication is typically presented on a display as grayed out and non-editable text with values copied from the IST. The I/O Function Block holds a private setting for the ignored attributes which are typically downloaded and promptly overridden. If the IST-Reference is removed, the setting for these attributes retains utility.  
     [0149] I/O Cards, Ports and Devices are incorporated into a configuration by a user operating a user interface, either the Explorer™ or the Module Definition Editor. The channels on conventional I/O cards are called “ports” and treated as an I/O Port so that special case terminology for conventional I/O is avoided. The user interface also allows a user to delete I/O Cards, Ports or Devices. Multiple I/O Card types are supported including, for example, 8-chan MTL AI, 8-chan MTL AO, 8-chan MTL DI, 8-chan MTL DO, 4-chan MTL Thermocouple/RTD, 8-chan HART input, 8-chan HART output, and 4-chanSolenoid. Some I/O Card types have a combination of I/O Port types on the same I/O Card. Deletion of an I/O Card deletes all subordinate Ports. Deletion of an I/O Port deletes all subordinate Devices. Deletion of I/O Ports or I/O Devices does not delete related instrument signal tags (ISTs), but the path of the IST path to the associated I/O signal no longer is operable. If another I/O Port or I/O Device is created which has the same path, the IST automatically rebinds to the I/O Port or I/O Device, so long as the signal type is compatible.  
     [0150] A user can initiate the Install of an I/O subsystem, which installs or reinstalls all I/O Cards defined in the Subsystem. The user can initiate the Install of a single I/O Card, which installs the card properties and all properties for subordinate I/O Ports and I/O Devices.  
     [0151] The Explorer™ and the Module Definition Editor configure the I/O subsystem by accessing current signal values, status, and selected properties that are directly addressable as Attributes in the I/O subsystem. The user displays a graphic indicative of the current status of cards, ports, devices, and signal values and status by accessing the respective cards, ports, devices and signal values and status using device hierarchy attribute path addressing (for example, “CON1/I01/C01/P01/FIELD_VAL”). I/O subsystem attributes are communicated using the physical device path (for example, CON1/I01/IC01/P01/D01/FIELD_VAL) for addressing in communications between devices. Communication of I/O subsystem attributes is advantageously used to transmit attributes from a controller/multiplexer  110  to a workstation  102 ,  104 ,  106  for display and from a first to a second controller/multiplexer  110  for virtual I/O handling.  
     [0152] Referring to FIG. 16, a block diagram illustrates an organization of information for an instrument signal tag. An system IST table  1510  contains information relating to an IST including path information and pointers to a system object. A first pointer  1512  designates a signal type which points to an attribute signal table  1520 . A second pointer  1514  designates an entry in the attribute signal table  1520 .  
     [0153] Accessing of information using device hierarchy attribute addressing advantageously allows system diagnostic displays to be designed and built for system integration checkout before Control Module work is complete. Device hierarchy attribute addressing also supports direct addressing of I/O signals from Modules, bypassing the use of I/O function blocks and avoiding I/O function block behavior. I/O Card, I/O Port and I/O Device identifiers are generally defined automatically according to slot position information and the like.  
     [0154] Referring to FIG. 17, a flow chart illustrates a method for bootstrap loading a control system throughout a network in the process control environment  100 , including the operations of assigning the controller/multiplexers  110  a set of IP Addresses, a node name and other startup information that is not stored in flash ROMs of a controller/multiplexer  110 . A process  1600  for assigning internet protocol (IP) Addresses to a Controller upon its initial bootup includes the step of associating a MAC address in a Boot server, a Windows NT™ workstation, with a controller/multiplexer name  1610 . The MAC address alone designates the controller/multiplexer identity. In step  1612 , the name of the controller/multiplexer is assigned an arbitrary device ID, and an ACN link number and a PCN network number that are determined by the cable attached to the controller/multiplexer. In step  1614 , an IP address of a device is calculated from the device ID, the ACN link number and the PCN network number. In step  1616 , a UDP datagram, which designates default primary and secondary IP addresses that are reserved for booting nodes and includes the controller/multiplexer MAC address in the UDP user data, is broadcast to a special UDP reserved boot port using the default primary IP address for the source address on the primary interface. In step  1618 , the boot server matches the MAC address with the assigned name and IP addresses, and broadcasts the assigned name and IP addresses with an echo of the MAC address to the UDP boot port. By broadcasting, the problem of doing any routing or ARP static entry manipulation is avoided. In step  1620 , the controller/multiplexer receives the datagram, checks the MAC address, and if the MAC address matches, sets the IP addresses and saves the node name and device ID. If the datagram is not received, the procedure is repeated using the secondary interface through the operation of branch step  1622 . In step  1624 , the controller/multiplexer, using the new address, sends a message to the boot server saying indicating that the controller/multiplexer is operational.  
     [0155] In step  1626 , a user enters a Device Name, Device MAC Address, ACN Link Number and PCN Network Number. The device ID can be automatically assigned by configuration software. The communications subsystem calculates the devices three IP addresses from the configured ACN Link number, PCN Network Number and the assigned device ID. In step  1628 , controller/multiplexer or I/O card software is flash downloaded over the ACN network by passing messages and S-Record files between devices on the ACN.  
     [0156] Referring to FIG. 18, an object communication diagram shows a method for creating a device connection for the active, originating side of a connection. An application program in either a workstation or a controller/multiplexer requests access to an attribute which is contained in another device. A UDP communications connection to the other device is established by the communication services so that the attribute can be accessed. Creation of a device connection spans two separate application programs. The application program which initiates the connection by requesting data located in another device and the Remote Object Communications (ROC) Services application program that actually sends the messages to the other device. If no connection exists when the ROC Services process is ready to send a message to a device, the ROC services create a connection to that device.  
     [0157] Prior to creating the device connection, a device to be connected has a valid Device Table containing the source device, is operating and includes an object RtDeviceConnection which monitors messages on the device connection port. After the device connection is created, a connection is established between the two devices and an RtDeviceConnection instance is created in the active device to handle the connection.  
     [0158] In step  1710 , an application program sends a message getContainer to object RtSite which returns the object ID of the module found or created. In step  1712 , object RtSite sends a Locate message to object RtPlantArea which locates the module and return its object ID. In step  1714 , object RtSite sends a GetDevice message to object RtDevice which returns the object ID of the device containing the module. In step  1716 , assuming that a proxy for the remote device does not exist, object RtDevice sends a Create message to object RtDeviceProxy. In step  1718 , object RtDeviceProxy creates an instance of object RtDeviceProxy using template RtNew. In step  1720 , object RtDeviceProxy asks object RtDeviceConnection to GetDeviceConnectionlndex which returns the index of the device name in the device connection table managed by object RtDeviceConnection. In step  1722 , object RtDeviceProxy registers the pointer to the RtDeviceProxy instance for the connected device by sending a RegisterPointer message to the object RtRegistry and returns the device proxy Object ID to object RtDevice. In step  1724 , object RtPlantArea sends a Create message to object RtModuleProxyClient to create a proxy client for the remote module. In step  1726 , object RtModuleProxyClient sends a Create message to object RtModuleProxyServer to create a proxy server for the module in the remote device. In step  1728 , object RtModuleProxyServer builds a create proxy server message and asks object RtRocReqRespService to SendRequest to the remote device. In step  1730 , object RtRocReqRespService Appends the message to the Outbound Message Queue for the ROC Communications Services process to send to the remote device. In step  1732 , object RtRocReqRespService in the ROC Comm Services process issues a RemoveFirst command to the Outbound Message Queue and gets the create proxy server message. In step  1734 , the RtRocReqRespService sends the message by issuing a sendMsg command to the aRtDeviceProxy instance for the destination device. In step  1736 , the aRtDeviceProxy instance issues a GetDeviceConnection command to RtDeviceConnection to get the Object ID for the RtDeviceConnection instance for the destination device. Assuming that a device connection does not already exist, in step  1738 , object RtDeviceConnection performs a createDeviceConnection. In step  1740 , object RtDeviceConnection creates an instance of RtDeviceConnection using template RtNew. In step  1742 , object RtDeviceConnection registers the pointer to the RtDeviceConnecfion instance by sending a RegisterPointer message to the object RtRegistry and returns the device connection Object ID to object RtDeviceConnection. In step  1744 , object RtDeviceConnection sends a startActiveConnection message to the aRtDeviceConnection instance. The aRtDeviceConnection instance performs the necessary steps to establish the connection to the other device. In step  1746 , the RtDeviceProxy instance issues a sendMsg to the aRtDeviceConnection instance to send the create server message to the remote device. In step  1748 , the aRtDeviceConnection instance sends the message to the remote device over the newly created connection.  
     [0159] Referring to FIG. 19, an object communication diagram shows a method for creating a device connection for the passive, listening side of a connection. A request to establish a device connection is received from another workstation or controller/multiplexer. The communications services establishes a UDP communications connection with the requesting device.  
     [0160] Previous to creation of the connection, a device to be connected to is operating and contains an object aRtDeviceConnection which is ready to establish a connection. Object RtDevice Connection exists in the device and is listening for input messages in the form of a sync request. After the connection is created, a connection is established between the two devices and an RtDeviceConnection instance is created in the passive device to handle the connection.  
     [0161] In step  1810 , object RtDeviceConnecfion receives a sync request message from a remote device. In step  1812 , object RtDeviceConnection sends a Create message to object RtDeviceConnection to create a connection to the requesting device. Assuming that a device connection does not already exist, object RtDeviceConnection performs a createDeviceConnection in step  1814 . In step  1816 , object RtDeviceConnection creates an instance of RtDeviceConnection using template RtNew. In step  1818 , object RtDeviceConnection registers the pointer to the RtDeviceConnection instance by sending a RegisterPointer message to the RtRegistry and returns the device connection object ID to object RtDeviceConnection. In step  1820 , object RtDeviceConnection sends a Create message to object RtDeviceProxy to create a device proxy for the requesting device. In step  1822 , object RtDeviceProxy creates an instance of RtDeviceProxy using template RtNew. In step  1824 , object RtDeviceProxy sends a GetDeviceConnectionIndex message to the object RtDeviceConnection to have the index of the device in the device connection table managed by RtDeviceConnection for later use. In step  1826 , object RtDeviceProxy registers the pointer to the RtDeviceProxy instance by sending a RegisterPointer message to the RtRegistry and returns the device proxy object ID to RtDeviceConnection. In step  1828 , object RtDeviceConnection passes the sync request message to the aRtDeviceConnection instance for processing via the handleInboundMessage method. In step  1830 , object aRtDeviceConnection sends a sync response message back to the remote device to indicate successful completion of the Device Connection creation.  
     [0162] Referring to FIG. 20, an object communication diagram illustrates a method for sending request/response messages between devices. The remote object communications (ROC) service in one device sends a request message to the ROC service in another device. The request message is processed and a response message is sent back to the originating device.  
     [0163] Prior to sending messages, a UDP device connection is established between devices. Following the sending of request/response messages between devices, a response message from a remote device has been received and is ready for processing by ROC services.  
     [0164] In step  1910 , a read attribute request is issued by an application program to an aRtDeviceProxy instance associated with a remote device. In step  1912 , the aRtDeviceProxy instance builds a request message to be sent to the remote device to read the attribute value and asks the RtRocReqRespService to send the message using the SendRequest method. In step  1914 , object RtRocReqRespService sends the message to the instance of RtDeviceConnection associated with the connection to the remote device using the send msg method. In step  1916 , the instance of RtDeviceConnection then transmits the message to the remote device over the device connection. In step  1918 , the instance of RtDeviceConnection in the remote device receives the message and requests the RtRocRouter class to route the message to the correct inbound message service. In step  1920 , object RtRocRouter determines that the message is a request/response message and requests object RtRocReqRespService to ProcessInboundReqResp. After the message is processed by the ROC services and the message consumer a response message is built, in step  1922  object RtRocRqstRespService sends the response message to the originating device using the SendResponse method. In step  1924 , the outbound message queue processing of RtRocReqRespService sends the response message to the instance of RtDeviceConnection associated with the connection to the source device using the send_msg method. In step  1926 , the instance of RtDeviceConnection then transmits the response message back to the original device. In step  1928 , the instance of RtDeviceConnection in the original device receives the message and requests the RtRocRouter class to route the message to the correct inbound message service. In step  1930 , object RtRocRouter determines that the message is a request/response message and requests RtRocReqRespService to ProcesslnboundReqResp.  
     [0165] Referring to FIG. 21 an object communication diagram illustrates a method downloading a network configuration. A user, following completion of the device configuration for a system, initiates a download to a controller/multiplexer. A device table configuration script is built by the configuration application. Using communications services , the configuration application establishes a device connection with the controller/multiplexer to receive the download and sends a download script to the controller device. The controller/multiplexer receives the download script messages and processes the device table. In step  2010 , a configuration download application program builds remote object communications (ROC) script download messages containing the device table download script. In step  2012 , the Download application issues a GetDevice message to RtDevice to get the Object ID for the RtDeviceProxy for the remote device. In step  2014 , the RtDeviceProxy does not yet exist so a Create message is sent to RtDeviceProxyC to create the necessary device proxy object. In step  2016 , RtDeviceProxyC sends a GetDeviceConnlndex message to RtDeviceConnection to get the index of the device connection for the remote device in the device connection table. In step  2018 , the device connection does not yet exist so aRtDeviceConnection object is created to manage the connection to the remote device. A lookup is performed in the database to find the remote device entry. The device communications data (for example, ID and IP Addresses) is retrieved from the database and a new entry is added to the configuration devices connection table. This connection is marked permanent in the connection table since the device initiated the connection. In step  2020 , a startActiveConnection message is sent to the aRtDeviceConnection object to establish a connection to the remote device. In step  2022 , the aRtDeviceConnection sends an RtSyncMessage to the remote device. In step  2024 , the remote device receives the RtSyncMessage and attempts to find an entry in the device connection table for the sending device. In step  2026 , no entry is found so a new entry is added to the device connection table for the sending device and aRtDeviceConnection object is created to handle the connection in the receiving device. In step  2028 , a RtSyncReplyMessage is created and sent back to the sending device containing the device connection index from the device table. The device connection is now established and ready to send and receive messages. In step  2030 , the RtDeviceProxyC sends a create RtDeviceProxyS message to the remote device. In step  2032 , the RtDeviceProxyS is created in the remote device. In step  2034 , the Download Application sends the download scripts to the remote device via RtRocReqRespServices using the SendMsg call In step  2036 , RtCommScriptDownload receives the Device Table script and processes each device table item and stores the data in a database Registry used to hold configuration data. For controller/mulitplexers this processing is used to create RtDeviceConnection objects and add the objects to the device connection table, allowing the memory to be acquired on download rather than subsequently.  
     [0166] While the invention has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the invention is not limited to them. Many variations, modifications, additions and improvements of the embodiments described are possible.