Universal process control device and method for developing a process control loop program

Abstract

A distributed processing unit (DPU) or drop which performs process control and data acquisition functions in a distributed processing control system having a data highway linking a plurality of such units. A DPU functional processor accesses the local process I/O interface thereby continually receiving plant information for storage in digital form and subsequent use in the functional processor or for transmission along the data highway. DPU control programs use process values in a transparent fashion, that is without regard to whether these values were obtained through local process I/O interface or via the data highway. The DPU software structure is made up of execution software and support software. The execution software is a collection of data acquisition and process control programs which are developed at an engineer's console drop using a DPU programming language which operates in text and CRT graphic display modes, the latter programming modes allowing system documentation via hard copy graphic display printout. These programs are initiated, performed consecutively, and repeated at specified intervals. The support software initiates process loop execution. Control programs which have been presented to the DPU as representations of ladder diagrams for sequential control or process flow diagrams for continuous process control are executed in the DPU functional processor to achieve the required process control operation.

Claims

What is claimed is:

1. Method of developing a process control loop program for execution in a digital processing unit (DPU) to control a process, said method comprising the steps of:

(a) storing a graphics program representation of a predetermined graphical grid pattern of an unconfigured process control loop in a memory, said grid pattern having a predetermined number of input and output process control loop entries with predefined locations therein;

(b) causing said graphics program to be executed by a controller to display said grid pattern on a visual display device;

(c) selecting at least one algorithm for said process control loop program from a library of algorithms stored in a memory;

(d) causing a predefined outlined region to appear in the displayed grid pattern at a specified location for each selected algorithm;

(e) setting parameters associated with each selected algorithm;

(f) selecting at least one input entry from said predetermined number thereof for said process control loop program and assigning a predefined input variable of said process control loop to each selected input entry;

(g) causing each said input entry selection and corresponding assignment to appear in the displayed grid pattern at its predefined location;

(h) selecting at least one output entry from said predetermined number thereof for said process control loop program and assigning a predefined output variable of said process control loop to each selected output entry;

(i) causing each said output entry selection and corresponding assignement to appear in the displayed grid pattern at its predefined location;

(j) causing graphical line interconnections to be drawn in the displayed grid pattern between the selected algorithm region locations and the selected input and output entries to configure a block diagram schematic of the desired process control loop;

(k) storing in memory information of said process control loop generated during the graphical configuration thereof in accordance with the above steps (c) through (j); and

(l) converting the stored information of the configured process control loop into instructional program code for execution in said DPU, whereby as the graphical display of the process control loop is completed on the visual display device, the process control loop program may be concomitantly completed for use by said DPU.

2. The method in accordance with claim 1 including the step of developing a process control loop program for each of a plurality of digital processing units for execution therein to control corresponding aspects of the process, said step comprising:

indexing each process control loop program to be developed and assigning it to its corresponding DPU;

performing steps (b) through (l) for the development of each indexed process control loop program; and

downloading each developed process control loop program to its assigned DPU for execution therein to control its corresponding aspect of the process.

3. The method in accordance with claim 2 including the steps of:

developing a plurality of control loop programs for execution in a common DPU; and

sequencing the order in which said plurality of control loop programs are executed in said common DPU.

4. The method in accordance with claim 3 including the step of re-ordering the executing sequence of the plurality of control loop programs in the common DPU.

5. The method in accordance with claim 1 including the step of displaying the configured block diagram schematic of the process control loop on the visual display device during the execution of the corresponding process control loop program in the DPU to trend the values of the assigned input and output variables in real time.

6. The method in accordance with claim 5 including the step of suspending the execution of the process control loop program in the DPU to modify a parameter associated with a selected algorithm of the process control loop program through interaction with the visual display device while the process control loop graphics are being displayed.

7. The method in accordance with claim 1 wherein step (j) includes the steps of:

storing in a look-up table in a memory a set of graphic design rules for allowable interconnections between the selected algorithm region locations and the selected input and output entry locations;

determining the validity of each graphical line caused to be drawn by comparing each graphical line with the set of allowable interconnections stored in said memory look-up table; and

rejecting each graphical line found to be invalid as a result of said comparison.

8. The method in accordance with claim 1 including the step of printing a graphical representation of the configured process control loop in accordance with the stored information of step (k) for hard copy self-documentation thereof.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to two groups of previously filed patent applications and one concurrently filed patent application, all assigned to the same assignee as the present application, the first group all filed by Houser et al. on June 29, 1983 (Ser. Nos. 508,951; 508,795; 508,771; 509,122, 508,769; 509,071; 509,251; and 508,770) and Sept. 13, 1983 (Ser. No. 531,821), the second group all filed by Bukowski et al. on Dec. 16, 1983 (Ser. Nos. 562,378; 562,507; and 562,508) and on July 30, 1984 (Ser. No. 635,387), and the concurrently filed application entitled "Universal Process Control Device and Method", the disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to distributed process control systems, more particularly to distributed process control systems which are integrated to perform sequential and continuous control and data acquisition.

Process control technology involves the application of control techniques to industrial processes to permit achievement of a desired level of performance and desired results. The historical development of process control technology reflects the evolution of a wide variety of control techniques to solve particular process control problems. However, the core issue in the solution of process control problems centers on the availability of tools with which to provide these solutions. The evolution of control techniques has in large part been based on the evolutionary development of control tools.

The industrial control business of the 1950's was divided into two different groups, the first being sequential control which used relays to provide motor control, and the second being continuous or modulating process control which used pneumatic devices to sense and control valves and pumps. Because the apparatus, i.e. motors, valves and pumps, for which the control tools were developed was manufactured by independent manufacturing industries, the control techniques evolved independently and there was no need for any integration.

With the solid state revolution came the ability to provide control tools based upon electronic circuits. The 1960's saw logic directors replace motor control relays, and also saw electronic analog devices replacing pneumatic modulating instruments. Even with control tools based on similar technology, sequential and continuous control systems remained independent along the lines of the supplier apparatus manufacturing industries. This period also saw the introduction of plant computers for data acquisition and monitoring, although not performing control functions.

At this stage, the goal of total process management appeared on the horizon. As plant computers grew in processing capability, it became evident that they could also absorb control responsibilities. Thus, the evolution of the direct digital control (DDC) computer system permitting integration of control and data acquisition functions, with the added capability of total process optimization and management. For a while, this seemed to be the ultimate solution for process control. However, application experience proved otherwise. A combination of factors was at work, notably the tendency to funnel all process control functions into a given piece of equipment, the plant computer. System response times became intolerable. Added to this was the difficulty of coordinating the motor control and instrumentation engineering design groups responsible for different portions of a given plant. Thirdly, this approach gave birth to the industry expression "the system is down" which meant that an entire plant control system was inoperable, often with disastrous results.

The middle 1970's brought about radical changes in the approach to sequential and continuous process control. The introduction of programmable controllers (PC's) allowed sequential control techniques to be programmed in the form of familiar relay ladder diagrams which were representative of the sequentially controlled process. These PC's also allowed continuous process control functions, traditionally analog in nature, to be performed in digital fashion. At this stage, process control was divided into essentially independent segments, each segment executing in a low cost microprocessor. Distribution of different control functions among many independent processing units eliminated the sluggishness and reliability problems of the DDC system. A serial process control data highway allowed an operator to communicate with the process via a color CRT screen. The data acquisition functions were kept in a separate minicomputer, and a data port allowed transfer of separately gathered process data onto the data highway.

This stage of evolution presented three essentially independent systems, a distributed system for sequential control, a distributed system for continuous process control, and a central computer system for data acquisition and optimization. Each had been designed independently, and none had been designed with the idea of integration in mind. When used for primarily one type of control, each of these systems was successful. It is important to note that at this stage of evolution, three different programming languages and programming techniques were required to implement these systems. This cumbersome programming problem was compounded by the lack of simple and meaningful user documentation for all three independent systems. This is the stage of development from which most process control systems in use today are derived.

However, the evolution in hardware as described above was accompanied by an evolution in the approach to solution of process control problems. Entering the ranks of the control design engineers were those trained to approach process control system problems so as to maximize process efficiency. Therefore, their ability to apply ladder diagrams for sequential control and process flow diagrams for continuous process control had to be matched by available hardware in which to implement these two techniques. Because of the independent hardware development in each of these areas, the control design engineer was thwarted in his attempt at integration. In addition, the desirability of including data acquisition capability further compounded the problem because of the tendency for a designated plant computer to burden the system response time in accessing information along the data highway. In order to advance to the next stage of distributed process control, there had to be a means to enable data highway communication that would eliminate the system sluggishness and allow integration of sequential and continuous process control functions along with data acquisition in a single control system. Just such a significant development was the subject matter of the applications outlined in the cross-reference at the beginning of the present application. There, a data highway communication system for a distributed process control system was disclosed in great detail, indicating the advantage of having a global data base available for distributed processors along a data highway. The global data base contains all of the information necessary to permit sequential and continuous process control while also handling data acquisition functions. However, that series of applications, while alluding to the capability of performing sequential and continuous control and data acquisition functions in a single system, provided only a general description of the distributed processing unit or drop connected to the data highway and the method of programming it to perform these functions.

The preceding discussion of the evolution of distributed processing control systems points to the desirability of a distributed processing control unit or drop in such a system which is connected to a data highway and which is capable of performing sequential and continuous process control and data acquisition functions all in the same unit. Such a universal process control device would enable a control design engineer to distribute the various process control tasks among several such drops while integrating in each of them the capability for performing a combination of control techniques. It would also be desirable if such a distributed processing control unit were easily programmed through a single programming language and technique. It would also be desirable if such a programming technique provided the user with simplified documentation of the control system configuration.

SUMMARY OF THE INVENTION

The present invention provides a distributed processing unit (DPU) or drop which performs process control and data acquisition functions in a distributed processing control system having a data highway linking a plurality of such units. Each single DPU drop is capable of performing all of these operations, and is actually an independent subsystem used to interface plant process and control signals to other drops on the data highway. This subsystem includes support for system-wide communications as well as functional processing capabilities and local process I/O hardware interface. A DPU functional processor accesses the local process I/O interface thereby continually receiving plant information for storage in digital form and subsequent use in the functional processor or for transmission along the data highway. The plant information accessed by the functional processor is converted into engineering units and is limit-checked to detect abnormalities in the system. Control programs are executed sequentially in the functional processor without interrupts to ensure security of control and data acquisition functions. The control programs use process values in a transparent fashion, that is without regard to whether these values were obtained through local process I/O interface or via the data highway. A data highway controller allows for this by obtaining all process values not local to the DPU by listening on the data highway for these values needed by the functional processor and storing them in shared memory. The present invention provides, in one drop, both control and data acquisition functions. This eliminates the duplication of plant sensor instrumentation, and allows for an integrated approach to sequential and continuous process control as well as data acquisition. The DPU software structure is made up of execution software and support software. The execution software is a collection of data acquisition and process control programs which are developed at an engineer's console drop using a DPU programming language which operates in text and CRT graphic display modes, the latter programming modes allowing system documentation via hard copy graphic display printout. These programs are initiated, performed consecutively, and repeated at specified intervals. The support software initiates process loop execution. It also ensures error-free interaction between the hardware and the execution software by monitoring timing, making calls to subroutines and algorithmic procedures, and running diagnostic testing. Control programs which have been presented to the DPU as representations of ladder diagrams for sequential control or process flow diagrams for continuous process control are executed in the DPU functional processor to achieve the required process control operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by reference to the accompanying drawings, in which:

FIG. 1 shows an idealized perspective, cut-away view of a plant employing the distributed processing system according to the invention;

FIG. 2 shows a similar view of the processing system according to the invention in more detail;

FIG. 3 shows building blocks used in manufacture of the system of the invention;

FIG. 4 shows a schematic view of the components making up a typical drop;

FIG. 5 shows a more detailed view of a typical drop showing use of redundant additional highway components;

FIG. 6 shows the physical layout of a typical drop;

FIG. 7 shows a typical ladder diagram;

FIG. 8 shows a block diagram of the single board linesolver;

FIG. 9 shows a basic hardware configuration of an engineer's console;

FIGS. 10, 11 and 12 show a portion of a flowchart for programming a drop;

FIG. 13 shows a digitizer pad template;

FIG. 14 shows a CRT screen pattern for a graphic control loop diagram;

FIG. 15 shows a typical process flow diagram for a portion of a process control system;

FIG. 16 shows a portion of a flowchart for a method programming a drop;

FIG. 17 shows a typical ladder logic diagram for a portion of a process control system;

FIGS. 18 and 19 show a portion of a flowchart for a method of programming a drop; and

FIG. 20 shows a flowchart of the sequence of software execution in a drop.

DESCRIPTION OF THE PREFERRED EMBODIMENT

1. System Outline

A primary object of the invention is to provide a distributed process control system in which a single data highway connects all the various input/output terminals, data acquisition stations, control devices, record keeping devices, error and alarm indicators, and means for communication with other data processing systems. FIG. 1 shows an overview of the system of this kind according to the invention. A single data highway 10 connects a wide variety of input/output devices such as process control devices 12, input/out terminals 14, sensors 16, control display devices 18 and the like. According to the invention and as will be further detailed below, the various inputs and outputs to the data highway 10 are termed "drops"; all interact with the data highway in substantially an identical way, although, of course, not all of them utilize the same properties of the data highway. This allows the system to be considered modular because any number of any type of drops may be added to the system. In a preferred embodiment up to 254 different drops can be used. They all have access to the entire data base of the system, which is copied or stored in the respective memories of each drop, and all can communicate with one another where required. This is done to ensure maximum flexibility and also to avoid use of a single central computer. As discussed above, this is desirable for a variety of reasons, one of which is that the system can keep operating even if one or more of the drops is disabled, whereas if a central computer were disabled, the entire system would effectively be shut down. The fact that the drops all communicate with each other allows different portions of the system to provide inputs and outputs to various control loops and the like. The mixed-mode operation of the highway discussed below, allows such traditional central computer functions as program download, status report generation and the like to be performed by drops configured as operator terminals, and without interruption of the system functions. Complete "data transparency" is achieved, that is, each processor can "see into" the memories of the other drops, eliminating the requirement of central memory, and providing very fast cathode ray tube displays which may be customized according to the operator's desires, allowing flexibility in the choice of programming languages.

FIG. 2 shows a more detailed view of a portion of a distributed processing system according to the invention. A data highway 10 connects a plurality of drops which are arranged to perform varying functions. Each drop comprises a processor for performing such functions; as used herein the term processor refers to the hardware and software at the drop location, which is connected by suitable means to the highway. Processors may include, for example, a distributed processing unit 20 which performs a data acquisition and control functions and interfaces to the various industrial processes in the plant under control, indicated generally at 22. A batch processing unit 24 may also be used. The operator's alarm console 26 provides a cathode ray tube based control display and alarm console for a human operator. An engineer's console 28 can be used to provide functions provided by the operator's alarm console and also for programming of the system as desired. A data logger 30 may also be provided to record information generated during a production run. Historical storage and retrieval at 32 can be used to archive this data. One of the drops may also include a calculator function at 34 to provide special calculations required to optimize the plant operations, for example. One drop may be a "gateway" 36, providing interface to some other computer, and another interface unit may be provided at 38 for interface via a telephone line to a remote location as indicated at 40. A programmable controller master 42 is also shown as one of the drops. This provides the interface to a programmable controller highway 44 which is another bus to which a number of varying types of programmable controllers 46 may be connected. These, in turn, may connect by way of a process input/output bus 48 to other programmable controller devices as at 50. In this way, a large number of differing controllers of varying types can all be interfaced to one drop to save access spots on the data highway 10, and to allow complete flexibility in drop design; each drop may be configured to suit its purpose, not merely to suit the overall system design.

FIG. 3 shows in further detail the three sets of hardware which comprise the "building blocks" of the system according to the invention. To the data highway 10 are connected data highway controller (DHC) subsystems at 52. These are common to all drops and consist of a shared memory, a modulator/demodulator (Modem) for serial-to-parallel and parallel-to-serial data conversions and a data base highway processor which, in the preferred embodiment is a custom bit-slice microprocessor. The shared memory is dual ported and forms the interface between the data highway processor and the functional processor 54 (discussed below). The functional processor performs the specified tasks associated with the particular drop. The functional processor comprises a commercially available microprocessor, having one or more chips. As used herein, the term microprocessor may refer to a single chip or interconnected chips, as well as associated memory, and thus embraces microprocessor systems as known in the art.

The functional processor 54 has transparent communication through the DHC to the remainder of the distributed processing system according to the invention. All data transferred to and from shared memory appears to the functional processor as part of its internal data base, independent of origin. In the preferred embodiment, where data is supplied over the data highway to other drops, the data highway controller looks at each data message and compares it to a stored table to determine whether its associated functional processor will have use for this data. If so, the message is stored, or copied, into the drop's shared memory. The functional processor, therefore, is freed from such communications tasks and can concentrate on its own control tasks, using as memory the memory it shares with the data highway processor. The use of two processors with a shared memory greatly simplifies the data highway interface while providing additional local processing capability for the functional processor.

The functional processors 54 perform the specific functions associated with the drop, which may include such things as man/machine interface by means of an operator input/output terminal 56, as well as all forms of data acquisition and control processing, by means of process input/output devices 58. The functional processor obtains amd stores data from the shared memory, and its communication with other hardware as required, including mass memory process input/output and peripherals, is carried out by way of the data highway processor, thus freeing the functional processor from communications tasks.

The input/output interface 58 provides communication to the various processes in the plant controlled. According to this arrangement a wide variety of input/output devices are available, such as the operator console displays at 56, as well as all forms of process controller devices.

2. Communications Format

The communication format used according to the invention will now be described briefly in order to enable understanding of subsequent subject matter. Communication over the data highway bus is controlled by means of the data highway controllers at each drop. Communication is performed both by periodic broadcasts of process data and in response to requests made by any one of the drops. In the preferred embodiment, the system operates according to a mixed mode communications arrangement comprising repetitive and non-repetitive transmission modes. In a first repetitive portion of each hundred millisecond communications cycle, the system is operated according to a time division multiplexing (TDM) scheme during which each drop has at least one "slot" in time into which it injects a message onto the data highway. All the other drops can select data from the messages as desired. In the latter or non-repetitive half of each 100 ms interval, the "democratic" mode, the highway is available for other messages, such as requests for specific data from other drops and the like. Specifically, every 100 milliseconds, each drop (of which up to 254 may be provided in the preferred embodiment) has access to the highway allowing it to broadcast process values stored in its shared memory along with appropriate message identifiers and status information. Each non-broadcasting drop listens to broadcasts made by other drops, selects points of interest to it and pulls them off the highway to store them in its shared memory.

At the conclusion of each periodic broadcast cycle, the remaining time in each 100 millisecond time slice is available on a demand basis for other communications such as downloading of programs, transfer of English description of points, etc. The drops also send and respond to specific data requests from other drops where required.

In practice, all the process variables are broadcast at least once per second, but since each drop has access to the highway every 100 milliseconds, a drop can broadcast and update data concerning key process points as frequently as every 100 milliseconds if conditions warrant. The data highway has a transfer rate of two megabaud which provides a system broadcast rate of at least 10,000 process points per second. The broadcast technique used in the practice of this invention eliminates both the need for a master or traffic director and the high overhead associated with conventional send/acknowledge schemes in which an acknowledgement is passed back to the originating point in the system. According to the present system, the drops are not provided with acknowledgement that their messages have been received. Instead, the information is simply put on the highway to be taken by other drops as they require. Each drop is master of the highway for a moment and in its transmission includes a token, used to indicate the next drop to have highway access.

When the data base for a drop is created, data base information such as English descriptions of data points, alarm limits, etc., are defined and stored in the memory of the same drop where the process value is obtained or calculated. The system data base is thus distributed in many drops, as is the processing system; however the mixed mode broadcast means of communication allows any drop on the highway access to any process data residing anywhere in the system, as if it were part of its local data base. The data highway in essence thus acts as a distributed global data base available to any drop on the system, and the speed and architecture of the communications system ensures that this global data base is always current, never more than one second old.

This transparent access of each drop to the distributed global data base means that control loops can run in one drop using process values generated or calculated by other drops. In addition, transparent access to the global data base permits functions which would normally be constrained to run in one processor to be distributed anywhere on the highway, a feature which is greatly convenient in systems of large physical extent or complexity and also permits additional drops to be added to an existing system if increase in system performance is required without modification of the existing system or degradation of its performance. For example, more calculators, more historical storage, and more data acquisition drops can be added. Drops can be added without affecting the operation of the pre-existing drops, if desired, since each functional processor can be programmed to accept or ignore the broadcasts from the additional drops. In a like manner, drops can be removed without perturbing operation of the remaining drops.

Certain key functions of any processing system such as overall plant optimization, historical storage and retrieval and plant-wide logging require access to the entire data base. Traditionally, such functions have been performed through the use of a central host computer which periodically obtains plant data from the highway and then creates a data base within itself for use by these systemwide programs. The major deficiency of this approach has been the data bottleneck created at this one point and the potential saturation of the host computer since it has to provide many functions simultaneously. For example, traditional operator terminals have required access to the entire system data base, and have accordingly been configured as peripherals attached to host computers, which have access to a main memory in which is stored the entire data base. According to the present invention, the ability of any drop to access the total global data base transparently allows functions which would previously have required a host computer to be distributed among many drops and permits the host function to be carried out, in effect, by a distributed computer system. One drop can be arranged to provide historical storage and retrieval, another can be a calculator to provide plant optimization, and a third a logger drop to provide other functions which would previously have required a host computer. Higher system availability and elimination of performance degradation associated with host computers are obvious benefits. On the other hand, if for hierarchical or other interface considerations, a host computer should be required, one can easily be accommodated via a "gate-way" drop. Finally, the transparency of communications provided according to the invention allows additional drops to be connected to the system with ease.

In the preferred embodiment up to 254 drops can be connected to the coaxial cable forming the data highway, which can be up to 6 km in length without repeaters. In another envisioned embodiment, a fiber optic highway is used which has the capability of supporting up to 64 drops. As is well understood in the art, fiber optic cables have substantially better noise immunity than ordinary coaxial cables and this may be of significance in certain plant applications. While present system capacities are set by practical noise and time delay factors, the system and method of this invention are limited only by state-of-the-art engineering constraints. Further, by integrating data acquisition and local control functions in a single drop, duplication of sensors so often needed by systems which perform only one or the other function is avoided. The dual drop functions also permit easy integration of systems, for example, which start with data acquisition only and add control later. Moreover, this arrangement permits an integrated approach to process modulation and sequential control as well as to data acquisition.

3. Drop Overview

Referring now to FIG. 4, the system according to the invention uses a first functional processor 60 and ladder logic linesolver combination at the drop level for such functions as process monitoring, sequential and continuous process control and operator interface, while a second data highway processor 64 is used to obtain data from the highway as required by the functional processor and make its communications to the highway. In this way, the functional processor is free to concentrate on its data acquisition and control tasks and is relieved from the complex requirements of communications interface. The functional processor 60 is connected by means of a shared memory 62 to the data highway processor 64. Use of this shared memory 62 is very desirable in that it automatically provides the interface between the two processors without requirement of simultaneous transfer of data directly from one processor to the other. Instead, one or the other processors simply accesses the shared memory 62 as required. The functional processor 60 then is connected to various conventional input/output units 66 by a process input/output unit 68. As will be discussed in further detail below, the functional processor is connected to a conventional industry standard bus so that any form of presently available input/output equipment which is adapted to be connected to that bus can be used. In this way, the user of a system according to the invention is not constrained to use of any one manufacturer's process input/output units but can connect substantially any device desired.

The data highway processor 64 makes connection to the data highway 70 (corresponding to the highway 10 of FIGS. 1-3) which is shown duplicated for redundancy. The dual highways thus constitute physically separate transmission lines, or paths. As used herein, the term highway means a coaxial cable, optical fiber cable, or the equivalent. Further redundancy is provided by means of duplicated communication modules 72 and transceivers 76. These will be discussed in detail below. In particular the transceivers 76 form the subject matter of previously filed application Ser. No. 508,770 filed June 29, 1983, incorporated herein by reference. The data highway processor 64, the communications module 72, and the shared memory 62 correspond to the data highway controller 52 of FIG. 3.

FIG. 5 shows schematically further details how a data highway drop is arranged, and shows redundancy of certain components which in some environments may be desirable. The data highways 70 as shown are redundant. They are connected by means of transceivers (MBT) 76 to data highway communications controllers (MBC) 72 which are connected to a data highway controller (DHC) bus 82. The data highway communications cards (MBC) 72 are thus provided in redundant fashion. To the data highway controller bus 82 is connected the data highway processor (MBD) 64 which as discussed above performs the functions of communications between the functional processor 60 and the data highway 70. The data highway controller bus 82 makes its connection to the functional processor via the shared memory (MBS) 62 which is connected to the functional processor 60 by way of a second bus 84. In the preferred embodiment this second bus 84 conforms to the industry standard "Multibus" (trademark Intel Corporation). This industry-standard bus (defined in IEEE Standard No. 796) was adopted so that a user's choice of functional processor would not be limited to goods available from any one manufacturer, and so that users could select from the very wide variety of peripheral devices now available which interface to the industry standard Multibus data communications interface. This provides a great deal of flexibility in designing of drops. Effectively, the drop can be designed in accordance with the user's needs and system devices. At the present time there are literally thousands of peripherals available which are amenable to the Multibus interface, and the possible permutations of the system according to the invention are thus practically endless. A particularly versatile 16 bit microcomputer available from Intel Corp. under Model No. SBC 86/05 can be modified by the addition of hardware as described herein to provide sequential control capability based on programs which use a ladder diagram representation of the portion of the process suited for this control technique. This combined hardware forms a functional processor 60 which can readily be programmed as described herein to perform a wide variety of other useful functions, including man/machine interface, which includes video display generation, as well as process interface and sequential and continuous process control. See Intel Manual Order No. 143153-001.

An interface unit (MBU) 94 may be used to interface the Multibus 84 to a third bus 86, referred to as a distributed input/output bus. Connection can be made from bus 86 to input/output devices which may have different interface specifications. The functional processor 60 is also connected by way of Multibus 84 to input/output devices 88, as required for the various functions indicated, including process monitoring, process control and operator interface. Other drop functions are possible such as archival storage and the like.

FIG. 6 shows the physical location in a drop of the components discussed above with respect to FIG. 5. The data highway 70 is connected to the transceiver 76, and a cable runs from it to the data highway communications (MBC) card 72 which is inserted in a Multibus card cage indicated generally at 90. The transceiver 76 may alternatively be carried in cage 90 where it is convenient to juxtapose this closely to the highway 70. Also carried in this cage and plugged into the Multibus are the components which are Multibus compatible, such as the shared memory system (MBS) 62, the functional processor 60, and the ladder logic single-board linesolver (SBL) 95. The Multibus connectors are shown as dotted lines 92 running across the back of the card cage. Thus, simply by inserting the cards into the cage, connection is automatically made to the Multibus. The data highway controller (DHC) bus 82 is also shown as a dotted line connecting the data highway processor 64, the shared memory (MBS) 62 and the data highway communications card MBC 72. The DHC bus is defined in the Appendix which forms a part of the specification in the first group of previously filed applications referred to in the Cross-Reference.

The Multibus 92 is connected to the functional processors indicated at 60 and also to the MBU unit 94 which provides connection by way of a distributed input/output bus 86 to a second card cage referred to as the Q-line card cage 96. Cage 96 may contain, for example, other input/output devices, for example, those available from the Westinghouse Electric Corporation, assignee of the present invention, and sold under the trade name "Q-line point cards". These then make actual connection to plant sensors, position actuators and the like, as indicated on the drawing of FIG. 6. Accordingly, if the drop is to function in accordance with a peripheral which is directly Multibus compatible such as an operator terminal or the like, it is simply connected to the Multibus 92. On the other hand, if specialized process control is desired, an MBU unit 94 can be used to interface the Multibus to the distributed input/output bus 86 and the plant sensors can then be attached to the Q-line card cage 96 (or any other desired bus system) as indicated in FIG. 6. In FIG. 6 it will be observed that the MBC 72, which is the data highway communications (Modem) card, the MBD 64 which is the data highway processor card, and the MBS 62 which is the shared memory card, are all referred to as making up the DHC or data highway controller 98. The MBT or transceiver can also be mounted here. These four cards then comprise the means making the interface between the functional processor 60 and the data highway bus 70.

Details of the components of the data highway controller (DHC) 98 and the communications scheme are discussed in the first group of previously filed applications referred to in the Cross-Reference.

4. Ladder Diagram Linesolver

The single board linesolver (SBL) modification to the general purpose functional processor 60 allows the capability of solving ladder logic diagrams with the speed of programmable controllers. The aforementioned Intel Corp. Model No. SBC 86/05 single board microcomputer can be modified to interface the SBL module 95 via the Intel iSBX.TM. expansion port interface as defined by Intel Manual Order Number 142686-002. The SBL module 95 permits the drop to perform sequential control of the process based upon the solution of free-form ladder diagrams. The ladder diagrams provide a means of representing Boolean logical expressions in a form that may be visually interpreted. The logical AND function is represented by a series connection of two "contacts", while a logical OR function is represented by a set of "contacts" in parallel connection. FIG. 7 shows a typical ladder diagram 99 which is the equivalent of the Boolean expression D=A.times.(B+C). The solution to the ladder diagram 99 is in the form of a coil 100 whose digital value is determined by the power flow from the positive power leg 102 on the left through the closed contacts in the ladder diagram to the coil 100 and the negative power leg 104 on the right. Thus, coil 100 (D) is TRUE if contact 106 (A) AND either contact 108 (E) OR 110 (C) are also TRUE. The topography of the ladder diagram 99 may be converted to executable code which represents its topography. It is this code, generated by the functional processor 60 in response to application program instructions, which is solved by the SBL module 95.

For a ladder diagram which has been defined to include seven rows of contacts, one or more contact columns, and from one to seven coils, the executable code includes the following information items:

1. The memory address in the functional processor storing the digital information representing the existing status of each contact in the ladder (I/O 16-bit image word);

2. For each contact in the ladder, the corresponding bit number or position in the I/O image word containing the existing status of that contact;

3. The contact number of a contact in a column which for a seven row column allows contact numbers 0 to 6;

4. An identifier flag bit which indicates whether the contact is a normally open or normally closed contact;

5. For each of the six possible vertical connections between the seven contact rows in a given column which form logical OR terms, an identifier bit indicating whether such a connection exists;

6. An identifier flag indicating the last contact in a column has been reached;

7. Seven bits indicating the seven power flows each corresponding to a row of a seven-row column of contacts.

These information items are represented in tabular form as follows:

    __________________________________________________________________________
    DATA
    F  E  D  C  B  A  9  8  7  6  5  4  3  2  1 0   Function
    __________________________________________________________________________
    x  x  x  x  x  x  x  x  x  P6 P5 P4 P3 P2 P1
                                                P0  Power Flow
    EN x  05 04 03 02 01 00 NC C2 C1 C0 B3 B2 B1
                                                B0  Contact Type
    WF WE WD WC WB WA W9 W8 W7 W6 W5 W4 W3 W2 W1
                                                W0  I/O Image
    __________________________________________________________________________
                                                    Word
    Item 1
        WF-WO =
               I/O Image Word bits (0 through OFFFFHex)
    2   B3-BO =
               Bit Number of a contact in its I/O image Word
               (0 through OFHex)
    3   C2-C0 =
               Contact Number (0 through 6) of a contact in a column
    4   NC =   Normal Closed (0 or 1) = Normally open/
    5   05-00 =
               OR Terms (0 through 3FHex) for the contacts in a
               column
    6   EN =   Enable the calculation of the new Power Flow after
               the I/O lmage Word of the last contact is loaded
    7   P6-PO =
               Power Flow (0 through 7FHex) of a particular column

    ______________________________________
    Type Description     Equation
    ______________________________________
    LN   Linear          Y = C.sub.1 X + C.sub.2
    P5   Fifth Order Polynomial
                         Y = C.sub.1 + C.sub.2 X + C.sub.3 X.sup.2 +
                         C.sub.4 X.sup.3 + C.sub.5 X.sup.4 + C.sub.6 X.sup.5
    SQ   Square Root (SQRT)
                         Y = C.sub.1 [SQRT (X + C.sub.2)] + C.sub.3
    EX   Exponential (EXP)
                         Y = C.sub.1 [EXP (C.sub.2 X)] + C.sub.3
    SP   SQRT of Fifth Order
                         Y = SQRT
         Polynomial      (Fifth Order Polynomial Y)
                         (See Entry P5 above)
    F6   Six-Segment Function
                         Coefficient x, y pairs are:
         Generator       (C.sub.1, C.sub.8)
                         (C.sub.2, C.sub.9)
                         .dwnarw.
                         (C.sub.7, C.sub.14)
                         If x .ltoreq. C.sub.1, y = C8
                         If x .gtoreq. C.sub.7, y = C14
                         If C.sub.1 .ltoreq. x .ltoreq. C.sub.8, y =
                         value of the
                         y-coordinate for a corresponding
                         x-coordinate.
    ______________________________________