Patent Publication Number: US-7716354-B2

Title: Control system, display device, control-use host computer, and data transmission method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a Divisional of application Ser. No. 09/674,175, filed Oct. 27, 2000, now U.S. Pat. No. 6,867,749, and PCT/JP99/02204 filed Apr. 23, 1999, and claims the benefit of Japanese Patent Application 10-117117 filed Apr. 27, 1998, Japanese Patent Application 10-120343 filed Apr. 30, 1998, Japanese Patent Application 10-148801 filed May 29, 1998, Japanese Patent Application 10-232600 filed Aug. 19, 1998, and Japanese Patent Application 10-311039 filed Oct. 30, 1998, the disclosures of which are incorporated herein by reference. 

   TECHNICAL FIELD 
   The present invention relates to (i) a control system that has a plurality of control devices each of which includes a method of control e.g., control unit such as a programmable logic controller (hereinafter referred to as PLC) and a display device displaying a control state of the method of control, and that enables transmission between the control means, (ii) a display device for the control system, (iii) a control-use host computer suitably used in a control system, and (iv) a data transmission method regarding data transmission between a first data processing device having its own communication protocol specialized for itself like the foregoing control unit and a second data processing device having a common communication protocol like a personal-computer-applied device. 
   BACKGROUND OF THE INVENTION 
   In a conventional control system, generally, a PLC is placed at the center of control, and a plurality of PLCs and host computers are connected via predetermined interface circuits directly or indirectly via dedicated adapters, so that transmission of control data between the PLCs as well as between the PLCs and the host computers is carried out with use of communication functions of the PLCs. 
   However, developed from a sequencer utilizing a relay, a PLC best performs in operations related with input/output control, such as an ON/OFF control of a switch and an operation of taking in data from a sensor, and use of a PLC as a center of data communication will overload the PLC, even though such use is possible. 
   Furthermore, data communication time becomes excessively long, for instance, upon an initial setting before control when more data transmission is required, as compared with during control. In a case where a personal computer is directly connected to a display device of a PLC to shorten the data communication time, connection change becomes extremely bothersome. 
   The following description will explain a typical conventional control device, while referring to  FIG. 24  that is a block diagram schematically illustrating an arrangement of the same. A plurality of PLCs (three PLCs in the case of  FIG. 24 ), that is, PLCs  501 α,  501 β, and  501 γ are equipped with their dedicated display devices  502 α,  502 β, and  502 γ, respectively, and are connected with a host computer  504  via a network circuit  503 . In response to a control output of the host computer  504 , the PLCs  501 α,  501 β, and  501 γ control one or a plurality of apparatuses to be controlled (not shown), receive states of the control-target apparatuses from sensors not shown, and cause the display devices  502 α,  502 β, and  502 γ to display the states. 
   The foregoing PLC  501 α,  501 β, and  501 γ may use different protocols, as indicated with the reference codes of α, β, and γ in  FIG. 24 , depending on respective manufacturers thereof, or in some cases, from one PLC to another PLC even though made of the same manufacturer. Therefore, the host computer  504  conducts communication with the PLCs  501 α,  501 β, and  501 γ via the network circuit  503  according to protocols α, β, and γ, respectively. 
   Therefore, the display devices  502 α,  502 β, and  502 γ are connected with the corresponding PLCs  501 α,  501 β, and  501 γ via connection cables  505 α,  505 β, and  505 γ, respectively. The display devices  502 α,  502 β, and  502 γ synthesize data expressing control states transmitted according to the foregoing protocols α, β, and γ, respectively, with characters and image data installed beforehand, so as to produce display screens and execute display. More specifically, for example, changeable data such as data expressing the foregoing control states are synthesized with a fixed screen display in which names of the foregoing apparatuses to be controlled are added to a background illustrating a manufacturing line, and a resultant screen display is displayed. 
   Therefore, the foregoing display screen is changed upon a change of products being produced or a producing method, and also it is changed out upon occurrence of an inconvenience. 
   As described above, the foregoing PLCs  501 α,  501 β, and  501 γ have originally been developed from control-use sequencers using relays, and they excel in ON/OFF control of switches and functions as input/output circuits for taking in data from sensors, whereas they are inferior in data communication functions. Therefore, they are capable of transmitting data like data expressing control states of the apparatuses to be controlled to the display devices  502 α,  502 β, and  502 γ, but they are not suitable for transmission of enormous data such as character and image data. 
   Therefore, conventionally, upon changes of the display screen, a personal computer  506  storing the character and image data is taken into the manufacturing site, and the display devices  502 α,  502 β, and  502 γ set in walls or manufacturing machines are taken out. After the PLCs  501 α,  501 β, and  501 γ are turned into a offline state, disconnected to the network circuit  503 , the connection cables  505 α,  505 β, and  505 γ connecting the display devices  502 α,  502 β, and  502 γ with the PLCs  501 α,  501 β, and  501 γ are disconnected from the display, devices  502 α,  502 β, and  502 γ (indicated by an arrow code φ as to the connection cable  505 α). Thereafter, a connection cable  507  extended from the personal computer  506  is connected thereto (indicated by an arrow code A as to the connection cable  507 ), so that the foregoing character and image data are installed. 
   Upon an end of the installation of data, connection of the connection cables  505 α,  505 β, and  505 γ with the display devices  502 α,  502 β, and  502 γ is restored, the display devices  502 α,  502 β, and  502 γ are returned to their own places in the walls or manufacturing machines, and the PLCs  501 α,  501 β, and  501 γ are returned into an online state, with connection of the same with the network circuit  503  restored. 
   Therefore, the operation of changing the screen display is complicated, and it is necessary to consecutively connect the personal computer  506  with the display device whose screen display is to be changed, one after another, for downloading. Therefore, there arises a problem that an offline time is prolonged. 
   Furthermore, transmission of a relatively large amount of data upon, for example, a change of processed products or drastic changes to processing operation data is also executed by connecting the personal computer  506  with the PLCs  501 α,  501 β, and  501 γ and installing preset data such as application programs and initial values, like in the foregoing case of the foregoing characters and image data. 
   Furthermore, since the PLCs differ in communication protocols for data communication, depending on respective manufacturers thereof, or in some cases from one PLC to another PLC even though produced by the same manufacturer, there are innumerable communication protocols to which a host computer has to be suited, and it is necessary to anew prepare communication protocols that are suited for the PLCs, for dedicated use of the host computer. Thus, there exists a problem of extreme inconvenience. 
   SUMMARY OF THE INVENTION 
   The inventors of the present invention examined the foregoing problems, and arrived at the following finding. Namely, having been developed as a personal-computer-applied device, a programmable display device that has been exclusively used for display is capable of data communication with a generally used personal computer without any problem with use of a generally used protocol. Furthermore, a display device is necessarily capable of data communication with a PLC, it is usual that a dedicated communication protocol specialized for a PLC to be connected with the display device has been developed. 
   Therefore, in the case where this display device is placed at center of communication and each PLC and a host computer are connected to each other with use of display devices, the display devices function as protocol converting means in data communication. 
   Aspects of the present invention have been made based on this finding, and an object of the present invention is to provide a control system in which a display device that conventionally was used exclusively for display of a control state is placed at a center of data communication. This ensures that a control unit such as a PLC can be exclusively used for I/O control whereby a load on the control unit is decreased. At the same time, it ensures that innumerable communication protocols of control units need not be considered on the side of a data processing device of the host computer or the like or on the display device side, whereby software operations are reduced. 
   Furthermore, based on the foregoing finding, an object of the present invention is to provide a data transmission method for a system having (i) a first data processing device having its own communication protocol such as the foregoing control unit, and (ii) a second data processing device having a common communication protocol such as a personal-computer-applied device, the data transmission method ensuring normal communication, without causing the second data processing device to consider all innumerable specialized communication protocols upon data transmission between the first and second data processing devices, so that software operations for development, management, and maintenance of the system are reduced. 
   More specifically, a control system in accordance with an aspect of the present invention is a control system including a plurality of control devices, each control device including a control unit and a display device connected with the control unit via a dedicated communication line and being capable of display corresponding to a control state of the control unit, the control system being capable of transmitting data between the control devices. The control system includes data transmitted between the control unit and the display device according to a dedicated protocol specialized for the control unit, and (ii) all of the display devices in the control devices are connected with each other via a common communication line, and data is transmitted between the display devices according to a common protocol. 
   Incidentally, protocols in a wide sense include protocols (transmission protocols) that have to be standardized in a network covering a transmitter and a receiver, for, otherwise data per se cannot be transmitted. Such protocols include: codes indicative of a start character, an end character, etc.; transmitting/receiving timings of each character; and methods for identifying the transmitter or the receiver. The protocols also include command systems that have to be standardized between the transmitter and the receiver, for, otherwise, an operation requested by one part and an operation executed by the other part do not coincide, thereby making it impossible to normally execute operations such as control operations. The aforementioned dedicated and common protocols are realized as combination of transmission protocols and command systems. 
   This allows the control unit to be used in exclusively for I/O control, thereby enabling a reduced load on the control unit. Moreover, since the display device should be at least capable of communication according to a dedicated protocol of a control unit connected thereto and according to the foregoing common protocol, it is possible to decrease the number of steps required for production (development) of software. 
   In addition to the foregoing arrangement, a data processing device may be connected to the common communication line, and data may be transmitted between the data processing device and each display device according to the foregoing common protocol. 
   In the foregoing arrangement, the data processing device is communicable with any control device as long as the data processing device can communicate according to the common protocol. Therefore, in spite of the presence of the data processing device, time and labor for development, management, and maintenance of the whole control system can be saved. 
   Furthermore, a display device according to an aspect of the present invention includes (i) a first data communication port connected with a control unit having a predetermined dedicated communication protocol specialized for the control unit itself, (ii) a second data communication port connected with a certain data processing device, (iii) a data processing section that processes data inputted and outputted through the first and, second data communication ports, in accordance with a procedure set beforehand, and (iv) a display section that provides a display corresponding to information processed by the data processing section, and (a) the display device executes data communication with the control unit according to the dedicated communication protocol of the control unit, and (b) the display device executes data communication with the data processing device according to the common communication protocol. 
   By using a display device arranged as above, like in the case of the above-described control system, a load on the control unit is reduced, while the number of steps required for production (development) of software is decreased. 
   Furthermore, in addition to the foregoing arrangement, the display device may be arranged so that the data processing section is also connected with an operating section, so that a timing of data processing and contents of the processing by the data processing section is manually instructed by means of the operating section. This arrangement allows the display device to be used as operating means of the control system. 
   Furthermore, a display device arranged as described above may be further arranged so as to further include a data input section for downloading, from outside the device, data for communication according to the dedicated communication protocol. 
   The foregoing arrangement ensures communication with the control unit by downloading programs for communication, data indicative of a communication format, and other necessary data for a dedicated protocol, without preparing in the display device beforehand all data that allows communication according to any one of all dedicated protocols likely connected with the display device. 
   Furthermore, a display device arranged as described above is further arranged so as to further include a conversion table storing section that stores a conversion table showing correspondence between special information of the control unit that is extracted from the dedicated protocol on one hand and common information transmitted according to the common protocol on the other hand, so that the data processing section converts the common information received from the common communication line, into the special information, referring to the conversion table. 
   According to the foregoing arrangement, a program for communication is not prepared for each control unit, but rather a conversion table is prepared for each control unit. Therefore, by only changing the conversion table, communication according to each dedicated protocol is enabled. 
   Furthermore, in addition to the foregoing arrangement, the display device is further arranged to include a format information storing section for storing a data transfer format of transfer information transmitted according to the dedicated protocol, so that the data processing section produces the transfer information by substituting the converted special information for an undefined portion of the data transfer format. 
   According to the foregoing arrangement, the display device applies necessary data to undefined portions of the data transfer format and produces a command to be transferred with a specific control unit at real time during an operation of the control system. Therefore, irrespective of where data inputted come from, transmission of data is enabled. Furthermore, even when a control unit to which the data should be transmitted is changed, necessary actions can be taken without stopping the system. 
   Furthermore, a display device arranged as described above is further arranged so as to include a protocol determining section that (a) sends out preset data according to a communication protocol selected from among a group of protocols applicable for communication of itself, prior to data transmission with the control unit, and that (b) in the case where a response from the control unit coincides with a predetermined response, determines the communication protocol as a communication protocol used for the data transmission with the control unit. 
   According to the foregoing arrangement, the dedicated protocol that the display device uses for data transmission is determined prior to establishment of data transmission with the control unit. This enables reduction of time and labor spent by the user, and also prevents errors in setting, as compared with the case where the user sets the dedicated protocol. 
   On the other hand, a data transmission method in accordance with an aspect of the present invention is a data transmission method applied when a data processing device transmits data with a first data processing device and a second data processing device, the data processing device being provided between the first data processing device and the second data processing device, the first data processing device transmitting data according to a first communication protocol, and the second data processing device transmitting data according to a second communication protocol. The method includes (i) a dedicated protocol communication operation of transmitting data with the first data processing device according to a communication protocol specialized for the first data processing device, and (ii) a common protocol communication operation of transmitting data with the second data processing device according to a communication protocol common to data processing devices that are likely connected with the data processing device. 
   According to the foregoing arrangement, each data processing device is capable of communication only according to a dedicated protocol of the first data processing device connected thereto and according to the common protocol. Therefore, as is the case with the foregoing control system, it is possible to decrease the number of operations required for production (development) of software. 
   Furthermore, a data transmission method in accordance with an aspect of the present invention is a data transmission method applied when a data processing device transmits data with a first data processing device and a second data processing device, the data processing device being provided between the first data processing device and the second data processing device, the first data processing device transmitting data according to a first communication protocol, and the second data processing device transmitting data according to a second communication protocol. The method includes (i) a dedicated protocol communication step of transmitting data with the first data processing device according to a communication protocol specialized for the first data processing device, and (ii) a data sending operation of, prior to the dedicated protocol communication operation, selecting a communication protocol from among a group of protocols possessed by itself and sending out preset data according to the selected communication protocol, and (iii) a protocol determining operation of waiting for a response from the first data processing device, and determining a communication protocol to which a predetermined response is obtained as a communication protocol used for data transmission with the first data processing device. 
   According to the foregoing arrangement, the data sending operation and the communication protocol determining operation are performed before the dedicated protocol communication operation. This enables reduction of time and labor spent by the user, and also prevents errors in setting, as compared with the case where the user sets the dedicated protocol. 
   On the other hand, a control-use host computer in accordance with an aspect of the present invention is a control-use host computer used in a control system that includes (a) a control unit for controlling a control target, (b) a display device that communicates with the control unit via a dedicated network so as to display or control a control state of the control unit and that, in the case where a common network apart from the dedicated network has a communication protocol different from that of the dedicated network, converts one of the protocols into the other protocol, and (c) the control-use host computer connected with the display device via the common network. The control-use host computer includes an interface section that sends data streams containing instruction contents for the control unit via the common network to the display device connected with the control unit. 
   According to the foregoing arrangement, the interface section of the control-use host computer gives the display control device information of instruction contents to the display device via the common network, and the display control device of the control system converts the communication protocol so as to transfer the information to the control device. Therefore, the interface section of the control system communicates with the common network according to one communication protocol irrespective of the communication protocol of the control device. Consequently, this allows control devices with different communication protocols to exist together in the control system, and therefore, saving time and labor in incorporating a control device into the control system. 
   Furthermore, since the communication protocol of the interface section is uniform irrespective of the communication protocol of the control device, there is no need to change the interface section and the transmission control means of the control-use host computer, even in a case where a new control device is developed. Therefore, as compared with the conventional cases where when a new control device is incorporated a program for a communication protocol of the new control device is created on both the sides of the display control device and the control-use host computer, time and labor required upon incorporation of the control device into the control system is saved for the time and labor required relating to the control-use host computer, and time and labor required for management and maintenance of the whole control system is saved. 
   Furthermore, in addition to the foregoing arrangement, the control-use host computer according to an aspect of the present invention is arranged so as to further include a plurality of transmission control sections that are provided between the interface section and a host-side display section for controlling or displaying a state of the control target and that controls the interface section in response to a request from the host-side display section, so as to transmit or receive data for controlling or displaying the state of the control target, wherein one of the transmission control sections is a simple transmission control section that is capable of specifying the request through a simpler procedure than the other transmission control section does. 
   According to the foregoing arrangement, since at least one of the transmission control sections is a simple transmission control section, the developer on the host-side control section is allowed to send a request regarding a degree of difficulty according to his/her own expertise, by selecting the simple transmission control section or the other transmission control section. Furthermore, the developer of the transmission control section is allowed to recommend which transmission control section should be used, considering expertise of the developer of the host-side display control section. Consequently, it is possible to prevent errors of the control system caused by a mistake of the developer of the host-side display control section. 
   Furthermore, a control system in accordance with an aspect of the present invention is a control system in which a control device controls a control target in response to a control output transmitted from a host device, and transmits information about the control to a display device so that the information is synthesized and displayed with character and image data installed beforehand in the display device. The control system includes a display device between the host device and the control unit, so that the character and image data are downloaded from the host device so as to be installed in the display device. 
   According to the foregoing arrangement, noting that a display device dealing with data of a relatively larger amount such as image data has a higher computing competence and higher applicability to data communication as compared with a control unit dealing with data of a relatively smaller amount such as ON/OFF data, this display device is provided between the host device such as a host computer and the control unit such as a programmable logic controller, and a control output from the host device or control state data from the control unit is transmitted without any trouble via the display device to the control unit or to the host device, respectively. On the other hand, upon installing character and image data that are produced by writing names of control-target devices on a background screen illustrating, for example, manufacturing lines, the data is transmitted directly to the display device via a normal communication circuit. Therefore, there is no need to carry out complex operations such as taking the host device into a manufacturing site, changing the wire connection, and the like. Moreover, in the case where a plurality of display devices, that is, control units are connected via a network circuit, character and image data can be transmitted to the display devices successively, whereby the installing operation can be completed within a short time. 
   Furthermore, a control system in accordance with an aspect of the present invention is a control system in which a plurality of control units respectively control targets in response to control outputs transmitted from a host device, and information about the controls is displayed in corresponding display devices, respectively. The control system includes a display device between the host device and the control unit, so that preset data are downloaded from the host device so as to be installed in the display device. 
   According to the foregoing arrangement, like in the case where character and image data are installed, the display devices transfer data to the control units and the host computer, respectively, thereby providing transmission of data without problems. Furthermore, upon installing preset data such as application programs and initial values not only to the display devices but also to the control units, it is possible to directly transmit the data via normal communication lines. This ensures a reduction of work upon installation, while also ensuring that the installing operation is completed within a short time even in a case where a plurality of display devices, that is, control units are connected. 
   Furthermore, the control system that enables installation of character and image data or preset data from the host device is according to an aspect of the present invention arranged so that communication between the control unit and the display device corresponding thereto is carried out according to a certain protocol, while communication via a network circuit between the display device and the host device should be carried out according to the predetermined common protocol. 
   According to the foregoing arrangement, taking advantage that each control unit is connected with the host device via the display device that has a high computing competence and high applicability to data communication, the display device provides protocol conversion. In so doing, the control unit is allowed to use an arbitrary protocol that differs depending on the manufacturer or the product, while the network is made to use the predetermined common protocol, and consequently, extension or change of the control system easily executed. 
   For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating a structure of a principal part of a control system illustrating an embodiment of the present invention. 
       FIG. 2  is a block diagram illustrating a structure of a principal part of a control system of another embodiment of the present invention. 
       FIG. 3  is a block diagram illustrating a structure of a principal part of a control system of still another embodiment of the present invention. 
       FIG. 4  is a block diagram illustrating a network structure of a control system of still another embodiment of the present invention. 
       FIG. 5  is a block diagram illustrating an example of an arrangement of hardware of a PLC and a programmable display device in the foregoing control system. 
       FIG. 6  is a perspective view of an example of an outward, partially cut-away appearance of the programmable display device. 
       FIGS. 7A and 7B  are explanatory views illustrating an example of control operations of the foregoing control system. 
       FIGS. 8A through 8C  are explanatory views illustrating an arrangement of a processing instruction word used in the foregoing control system. 
       FIG. 9  is a block diagram illustrating a principal part of the foregoing control system. 
       FIG. 10  is a block diagram illustrating a principal part of a control system in accordance with still another embodiment of the present invention. 
       FIG. 11  is an explanatory view illustrating an example of a data transfer format used in a common protocol in the foregoing control system. 
       FIG. 12  is an explanatory view illustrating an example of data transfer format used in a dedicated protocol in the foregoing control system. 
       FIG. 13  is an explanatory view illustrating an example of a conversion table used in protocol conversion between a dedicated protocol and a common protocol in the foregoing control system. 
       FIG. 14  is a block diagram explaining the foregoing control system in more detail. 
       FIG. 15  is a block diagram illustrating a principal part of a control system in accordance with still another embodiment of the present invention. 
       FIGS. 16A through 16B  are explanatory views illustrating an example of tables of correspondence between dedicated protocols and response codes in the case where a protocol-identification command is sent out by the foregoing control system. 
       FIG. 17  is a flowchart illustrating an automatic judging procedure of the dedicated protocol in the foregoing control system. 
       FIG. 18  is a block diagram illustrating a principal part of a control system in accordance with still another embodiment of the present invention. 
       FIG. 19  is a block diagram illustrating a principal part of a control system in accordance with still another embodiment of the present invention. 
       FIG. 20  is an explanatory view illustrating an example of a data transfer format used in the foregoing control system. 
       FIG. 21  is a timing chart for explaining a download operation of image data from a host side to a display device side in the foregoing control system. 
       FIG. 22  is a block diagram illustrating a principal part of a control system in accordance with still another embodiment of the present invention. 
       FIGS. 23A through 23G  are explanatory views illustrating an example of a format of data used in the foregoing control system. 
       FIG. 24  is a block diagram illustrating a principal part of a typical control system of prior art. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following description will explain the aspects of the present invention, referring to examples of the present invention and comparative examples. Note that however the aspects of the present invention are not limited by the examples at all. 
   [First Aspect] 
   The following description will explain an aspect of the present invention while referring to  FIG. 1 . As shown in  FIG. 1  that schematically illustrates an overall structure of a control system  1  in accordance with the aspect of the present invention, the control system  1  is provided with a plurality of control devices  5  each of which includes a control unit  2 α,  2 β, a display device  3 α,  3 β capable of display suitable to a controlled state of the control unit  2 α,  2 β and a dedicated communication line  4  for connection between the control unit  2 α,  2 β and the display device  3 α,  3 β so that data can be transferred between the control devices  5 . In each control device  5 , the control unit  2 α,  2 β and the display device  3 α,  3 β execute data communication by means of a dedicated protocol inherently specialized for each control unit  2 α,  2 β. On the other hand, the display devices of the control devices  5  are connected with each other via a common communication line  6 , and execute data communication by means of a common protocol. 
   In the foregoing arrangement, the display devices  3 α,  3 β execute protocol conversion thereby enabling each control device  5  to communicate each other by means of a common protocol predetermined. Therefore, even in the case where the dedicated protocols of the control units  2 α,  2 β are different from each other, transfer of data between the control devices  5  is enabled without any difficulties. As a result, control units  2 α,  2 β using different communication protocols are allowed to exist in one control system, without problems. 
   Furthermore, the display devices  3 α,  3 β that have been used exclusively for display of control states are now used as center of data communication, and the display devices  3 α,  3 β execute protocol conversion between a dedicated protocol and a common protocol. Therefore, it becomes possible to use the control units  2 α,  2 β exclusively for I/O control which is its own function, thereby enabling to reduce a load on the control units  2 α,  2 β. 
   Furthermore, if the display device  3 α is capable of protocol conversion between a dedicated protocol for the control unit  2 α connected with the display device  3 α and the foregoing common protocol, data transfer is enabled between the foregoing control unit  2 α and a control unit  2 β via a display device  3 β that corresponds to the control unit  2 α, irrespective of the dedicated protocol of the control unit  2 β. Therefore, communication protocols specific to the control units  2 α,  2 β that are innumerable need not be considered, while only communication by means of a dedicated protocol of a control unit that can possibly be connected thereto, communication by means of the common protocol, and the protocol conversion between the two may be taken into consideration. Therefore, operations required in production (development) of software can be decreased. 
   Additionally, even in the case where a control device including a control unit  2 β that communicates with a new dedicated protocol is added in the control system  1 , there is no need to alter another display device. Therefore, time and labor upon incorporating the control unit in the control system is saved, whereby time and labor for management and maintenance of the whole control system is saved. 
   [Second Aspect] 
   A control system  20  in accordance with an aspect of the present, as shown in  FIG. 2 , includes a data processing device  7  connected with a common communication line  6 , to ensure transfer of data between the data processing device  7  and the display devices  3 α,  3 β using the common protocol. This enables various operations such as centralized control by the host computer, and download of a protocol conversion procedure that will be described later. 
   In this arrangement as well, like in the foregoing control system  1 , the data processing device  7  is capable of transfer of data to and from each control device  5  by communication using the common protocol alone. As a result, the data processing device  7  need not produce different sets of software for dedicated protocols respectively, and therefore, time and labor necessary for production of software can be reduced. Furthermore, even in the case where a new control device  5  is incorporated in the control system  20 , there is no need to alter the data processing device  7 . Consequently, in spite of the provision of the data processing device  7  communicable with a certain control device  5 , time and labor for development, management, and maintenance of the whole control system  20  can be saved. 
   [Third Aspect] 
   The following description of an aspect of the present invention will explain in more detail an arrangement of a display device while referring to  FIG. 3 . 
   Display devices  31 α,  31 β of a control system  30  in accordance with an aspect of the present invention include a dedicated protocol interface section (first data communication port)  11 , a common protocol interface section (second data communication port)  12 , a data processing section  13 , and a display section  14 . The dedicated protocol interface section  11  is connected, via a dedicated communication line  4 , with a control units  2 α,  2 β in which a dedicated protocol specific to the control units  2 α,  2 β is specified beforehand. The common protocol interface section  12  is connected with a certain data processing device such as a data processing device  7  or another display device  31 α,  31 β, via a common communication line  6  for communication using a predetermined common protocol. The data processing section  13  processes data inputted/outputted via the protocol interface sections  11  and  12  according to a procedure set beforehand. The display section  14  executes display corresponding to information processed by the data processing section  13 . 
   Furthermore, for example, the protocol interface sections  11  and  12  execute communication using their own communication protocols, respectively, while a data format is converted by the data processing section  13  as required; consequently, data communication between the dedicated protocol interface  11  and the control unit  2 α,  2 β is executed using the dedicated protocol, and data communication between the common protocol interface section  12  and the data processing device  7  (another display device  31 α,  31 β) is executed using the common protocol. 
   Furthermore, the data processing section  13  is connected with an operating section  15  accepting an input from a user, such as a touch panel or a keyboard, so that instruction of data processing times and processing contents of the data processing section  13  can be inputted manually via the operating section  15 . 
   Additionally, the display device  31 α,  31 β in accordance with an aspect of the present invention is equipped with a data input section  16  for input of data from outside the display device  31 α,  31 β, such as a memory card reader, so that data and programs for processing the foregoing dedicated protocols (data for dedicated protocols) can be downloaded from outside the display device  31 α,  31 β. Incidentally, using the foregoing common protocol interface section  12  as the data input section  16 , the foregoing data or programs may be downloaded from the data processing device  7  or another display device  3 β, for example. 
   The foregoing arrangement enables the display device  31 α,  31 β to, even after being completely fabricated, download data for its dedicated protocol when necessary. Therefore, data for dedicated protocols need not be prepared beforehand as to all the control units  2 α,  2 β that are connectable, and consequently, a memory capacity required for the display device  31 α,  31 β is reduced. Furthermore, even in the case where a control unit  2  of a new dedicated protocol is developed, the display device has to only download data for the dedicated protocol so as to communicate with the foregoing control unit. Furthermore, the data for the dedicated protocol may be managed by, for example, the data processing device  7  and downloaded as required. Therefore, management of the data is also facilitated. 
   [Fourth Aspect] 
   The following description of an aspect of the present invention will explain in more detail an arrangement of the control device, while referring to  FIGS. 4 through 9 . Specifically, as shown in  FIG. 4 , a control system  40  in accordance with an aspect of the present invention is a system provided with a plurality of PLCs  22 α,  22 β as control units associated with each other, so that the system is capable of sequence control of a target system  8  such as a belt-conveyer-type automatic assembly system. In the control system  40 , a programmable display device  32 α,  32 β is used as a control panel of each PLC  22 α,  2 β. Furthermore, the control system  40  includes a host computer  71  for centralized control of the control devices  5   c  and for managing data of the control devices  51 α,  51 β all together. The programmable display devices  32 α,  32 β connected with the forgoing PLCs  22 α,  22 β constitute a set of a control devices  51 α,  51 β. The control system  40  includes two control devices  51 α,  51 β including a first control device  51 α and a second control device  51 α, respectively, or alternatively it may include more than two of such control devices. In the following explanation, for convenience, Greek characters for distinguishing the sets of the control devices  51 α,  51 β are attached to ends of referential codes like PLC  22 α,  22 β in the case where the respective sets are individually referred to. 
   In the present aspect of the invention, two types of the PLCs are used, PLC  22 α and PLC  22 β. Each of the PLCs  22 α and  22 β has its own dedicated communication protocol specialized for the manufacturer and the type of the apparatus. PLC  22 α and the PLC  22 β are capable of executing controls associated with each other as will be described later, in a state of, for example, being individually connected with control-target devices  9  that are individually provided a belt conveyer. 
   In the following explanation, for convenience, a case where two PLCs and two programmable display device are provided is taken as an example, but according to an aspect of the present invention this number can be increased to three or more, respectively. Furthermore, apart from the PLC, any one of various personal-computer-applied apparatuses may be used as the control unit. 
   In the PLC  22 α,  22 β, like a CPU unit or a memory unit, a circuit is formed into a unit for each individual circuit required, and by appropriately adding a unit as required via a bus line, functions achieved by the PLC  2   c  as a whole can be increased/decreased. 
   The PLC  50  shown in  FIG. 5  includes a CPU unit  101  for controlling the PLC  50 , an input unit  102  enabling direct input of a detected signal sent from the target system  8  an output unit  103  enabling output of a processing operation control signal to the target system  8  a memory unit  104  for storing various kinds of data and a computer-linked unit  105  enabling predetermined data communication with the programmable display device  3   c . The foregoing CPU unit  101  observes changing states of the input, output, and computer-linked units  102 ,  103 , and  105 . Upon recognition of changes in data such as input of new data, after applying necessary calculating operations thereto the CPU unit  101  makes access to a corresponding address on the memory unit  104  to rewrite contents and send data to the output unit  103 . 
   Here, in the memory unit  104 , a place for storing state data MD that are inputted/outputted directly to/from the target system  8  or used for control of the target system  8  is provided as to each control-target device  9  to be controlled or displayed, such as a passive member like a level meter or a limit switch provided on the target system  8 , an active member like a relay or a motor, or a data setting member such as a counter or an inker. Further, a word device is appropriated to data to be inputted/outputted, in the case where the data are word data such as numerical values, while a bit device is appropriated for bit data such as on/off information. Each device has its own device name such as “X000,” and by designating a device name, a place where the data is to be stored is specified on the memory unit  104 . Therefore, only by specifying and accessing a certain word or bit device in the memory unit  104  from inside or outside the PLC  50 , a corresponding part of the target system  8  can be controlled, or information relating to an operation state of the corresponding part can be individually taken out. 
   Incidentally, a PLC has according to an aspect of the present invention includes a passive data transmission function, like receiving a command sent from another data processing device such as the programmable display device, and executes writing/reading of data corresponding to the command to/from the memory unit  104 , or returning a command or data sent thereto without applying changes thereto, but needless to say, a substantially identical embodiment can be achieved in the case where the PLC has an active data transmission function. 
   On the other hand, the programmable display device  51  is, along with the foregoing PLC  50 , incorporated in an operation table of the target system  8 , or alternatively it is independently provided, so as to be used as a control panel for the target system  8 . 
   A programmable display device  51  in accordance with an aspect of the present invention is, as shown in  FIG. 6  for example, arranged so that a display screen  112  of a display  141  as the foregoing display section  14  is provided on a front side of a main body case  111  formed in a substantially rectangular shape, and a touch panel  151  as the foregoing operating section  15  is provided so as to closely adhere to the display screen. On the other hand inside the main body case  111 , a display control circuit  113  shown in  FIG. 5  is housed. According to an aspect of the present invention, a liquid crystal display panel is used as the foregoing display  141 . Furthermore, needless to say, any one of various manual operating devices such as a pointing device like a mouse or a key board may be used instead of, or in addition to, the touch panel  151 . 
   A basic hardware arrangement of the foregoing display control circuit  113  is connected as shown in  FIG. 5  via a bus line  114  with a CPU  115  and memories, e.g., a ROM  116 , and a RAM  117 , a touch panel controller  118  for controlling the foregoing touch panel  151 , and a graphic controller  119  for controlling the foregoing display  141 . The CPU  115  executes a predetermined calculating operation in accordance with a system program stored in the ROM  116 . Results of calculations appropriately stored in the RAM  117  are developed into bit images on a video RAM  121  by the graphic controller  119  using various display data stored beforehand in a graphic data storing memory  120  including font data for display of characters and still picture data. Then, contents corresponding to the contents written in the video RAM  121  are displayed on the display screen  112  of the display  141 . 
   Furthermore, the foregoing bus line  114  is connected to a communication controller  511  as the dedicated protocol interface section  11  that is actuated with a dedicated protocol specialized for the circuit arrangement of the PLC, so as to establish communication with the computer-linked unit  105  of the foregoing PLC via a dedicated communication line  4  in parallel or in serial. 
   On the other hand, the foregoing bus line  114  is connected to a generally used communication adapter  512  such as Ethernet (trademark: Xerox Corp.) as the common protocol interface section  12 , so that, for example, connection with the host computer or another programmable display device is provided via the common communication line  6  composed of communication cables, whereby a local area network (LAN) is formed. In the foregoing LAN, a generally used communication protocol (common protocol) such as TCP/IP is used, so as to enable transfer of various kinds of data between the host computer and each control device, or between the control devices. 
   Furthermore, an example of data transfer between the PLC and the programmable display device in the control system in accordance with an aspect of the present invention is as follows: state data storing memories  17  and  32  that will be described later are provided in the programmable display device and the PLC, respectively, and they transfer data with each other so as to possess the same contents. By so doing, the PLC and the programmable display device are allowed to hold state data of substantially identical contents including the bit device or word device provided on the PLC side, with a minimum time lag. Incidentally, for instance, the state data storing memory  17  is set as one region on the RAM  117  on the programmable display device, while the state data storing memory  32  is set as one region on the memory unit  104 . 
   Here, the programmable display device, as shown in  FIG. 7(   a ), manages a screen in which one or a plurality of still pictures B or member pictures J 1  is placed on a base screen, as a unit screen. The foregoing still picture B is a picture such as name plate whose display contents are not to be changed, while the foregoing member picture J 1  is a picture with changes in shape or color or flickering, such as a picture of a switch. Furthermore, the programmable display device includes a plurality of unit screens for use of data processing, as well as it is arranged so that file numbers F e.g., FL 1 , FL 2  . . . are respectively allocated to unit screens, and that a screen displaying required operation contents can be obtained only by switching unit screens by designating a file number F. 
   More specifically, in the foregoing programmable display device, as shown in  FIG. 5 , an event data storing memory  122  that stores processing instruction words corresponding to the unit screens, respectively, is connected with the bus line  114 . When one unit screen is selected, a processing instruction word W, e.g., WL 1 , WL 2 , . . . related to the selected unit screen is read intermittently at extremely short intervals. Such a processing instruction word W includes a set composed of a file number F, an event name N, and a reference information r, as shown in  FIG. 8A : the file number F is indicative of a base screen to which a display control operation is to be applied the event name N specifies an operation content to be executed on the base screen and the reference information r is composed of one or a plurality of pieces of data that are referred to as to each event to be executed. The programmable display device executes operations of the contents identified by the event names N of the processing instruction word W successively, while referring to the data in the foregoing state data storing memory  17  having identical contents of the state data storing memory  32  of the PLC  70 . By so doing, the member picture J 1 , display data, and the like on the base screen are displayed immediately in accordance with changes to the contents of the bit device or the word device of the PLC  70   c.    
   For instance, the processing instruction word WT that is to enable to invert the bit device set at a predetermined address position in the state data storing memory  32  in association of pressing instruction operations with respect to the touch panel  151  is as follows: as shown in;  FIG. 8B , the processing instruction word WT includes a file number F 1  of the base screen, and an event name N 1  specifying an operation of the touch panel  151  that starts with, for example, “T”, and further include as the reference information r an input coordinates range X·Y making an input operation through the touch panel  151  valid and an address A at which data should be rewritten in association with pressing operations at the touch panel  151 . 
   Furthermore, the processing instruction word WL that is to enable to display a predetermined picture at a corresponding position on the base screen in association with operations at the touch panel  151  is as follows: as shown in  FIG. 8C , the processing instruction word WL includes a file number F 1  of the base screen, and an event name N 2  specifying display of a member picture J 1  that starts with, for example, “L”, and further includes as the reference information r a member picture display coordinates range X·Y, a file number FL specifying a member picture to be recalled, and an address A to be referred to upon display of a member picture. 
   Furthermore, as shown in  FIG. 7A , upon setting processing instruction words WL 1  and WTI in the event data storing memory  122 , the words WL 1  and WTI are set so that respective coordinates ranges X·Y of the words WL 1  and WTI coincide with each other, and that the respective reference addresses A thereof coincide with each other. Furthermore, a state of “0” of a bit device at the address A in the state data storing memory  17  is made to correspond to an OFF state of a switch beforehand, so that the PLC  70  executes an OFF operation with respect to the actual switch setting at the time to be controlled, when the bit device exhibits a state of “0”. Furthermore, in the graphic data storing memory  119 , data indicative of a member picture J 1  corresponding to an OFF state of the switch are stored in association with a reference file number FL 1 , and data indicative of a member picture J 2  corresponding to an ON state thereof are stored in association with a reference file number FL 2 . 
   Here, as shown in  FIG. 7A , in the case where a reference address A of the state data storing memory  17  is “0”, a programmable display device reads data indicative of the member picture corresponding to the OFF state of the switch, from the reference file numbered as FL 1  of the graphic data storing memory  120  based on the processing instruction word WL 1 , and develops and displays the read data in the coordinates range X·Y on the base screen. 
   Upon pressing the switch-like member picture J 1  on the touch panel, the programmable display device searches for a touch-panel-use processing instruction word WT in the event data storing memory  122 , and judges, based on a coordinates instructed by the touch panel, whether or not exists the processing instruction word WT including as coordinates information a range including the foregoing coordinates. 
   For instance, when a point in the coordinates range X·Y of the processing instruction word WT 1  is pressed, the programmable display device finds the processing instruction word WTI and inverts the data value at the corresponding address A from “0” shown in  FIG. 7A  to “1” shown in  FIG. 7B . Consequently, the PLC  2   c  executes a controlling operation that turns on a switch point corresponding to the address A. Further, since the value of the address A is turned to “1”, the member picture J 2  illustrating the ON state is read from the reference file numbered “FL 2 ” in the graphic data storing memory  120  based on the processing instruction word WL 1 , and is displayed in the coordinates range X.Y. Thus, the switch-like member picture displayed on the base screen is also turned from the OFF state (JI) to the ON state (J 2 ). 
   This arrangement enables display on the display screen  112  by the PLC  70  that corresponds to a control state of the target system  8 , data input by fingers through the touch panel, as well as indirect control of the target system  8  by the PLC  70 . 
   Here, not only state data MD of the PLC  70  directly connected with the programmable display device but also state data of the PLC  2   c  indirectly connected thereto via the common communication line  6  can be designated as the state data MD that are referred to by the foregoing processing instruction word W. For instance, information indicative of the PLC  70  or information indicative of the programmable display device connected with the PLC  70  is added in the address A of the reference information r, based on which each control-target device  9  in the control system is may be distinguished. This allows all the PLCs to be controlled in association with each other, via the programmable display device. 
   A control system  90  is shown in  FIG. 9 . Here, as function blocks in the PLC, a dedicated protocol interface section  31 , a state data storing memory  32 , and a data processing section  33  are provided. The dedicated protocol interface section  31  is realized by the foregoing computer-linked unit  105  and the like, for communication using the dedicated protocol specific to the PLC. The state data storing memory  32  is realized as a region of the foregoing RAM  117 , for storing state data MD including contents of a bit device or a word device set on the PLC side. The data processing section  33  is realized by the CPU  115  executing a program stored in the ROM  116  or the like, for processing data inputted/outputted to/from the target system  8 , the dedicated protocol interface, etc. Likewise, the host computer  71  includes, a screen display section  41  for executing screen display, a data processing section  42  for controlling the entirety of the host computer  7   c , and a common protocol interface section  43  for communication with the common protocol. 
   The foregoing data processing section  42  of the host computer  7   c  carries out a predetermined data processing operation, and displays a result of the operation on the display screen of the screen display section  41 . The data processing section  42  sends data to be transmitted to the common communication line  6  via the common protocol interface section  43 . By so doing the data are transmitted on the common communication line  6  according to the common protocol. 
   On the other hand, the data processing section  33  of each PLC inputs/outputs signals with respect to the control-target device  9  of the target system  8  in accordance with the program set beforehand, as well as stores data of contents corresponding to a control state at that time (state data MD) in the state data storing memory  32  set in the memory unit  104 . Incidentally, an address in the state data storing memory  32  at which the data are stored is determined, for example, according to the control-target device  9  and the like. The state data MD changed are sent according to the dedicated protocol to the programmable display device via the dedicated protocol interface section  31  of the PLC and the dedicated communication line  4 , so as to be stored in the state data storing memory  17 . Conversely, when data to change the state data MD in the PLC are fed via the dedicated communication line  4  from the programmable display device, a control operation corresponding to the changes in the data is carried out with respect to the control-target device  9 . 
   According to an aspect of the present invention, it is arranged beforehand that various kinds of data generated in the programmable display device or taken into the programmable display device from the PLC are, automatically at uniform intervals, or appropriately in response to a command, fed to the host computer  71 , via the common communication line G. This enables a lessening of memory capacity provided in each programmable display device, and managing of all data together by building up a database of data sent from all the programmable display devices. 
   Furthermore, in each programmable display device, necessary data can be retrieved from the host computer at an appropriate timing and displayed on the display screen D of each programmable display device, or utilized in the control operation of the PLC. Furthermore, it is also information regarding all the control devices  5  can be obtained by the host computer  7   c  via any one of the display devices. 
   Furthermore, according to an aspect of the present invention, to manage all data for communication according to the dedicated protocols (protocol information) together, a protocol database is built up in the host computer with the protocol information suitable to the PLC that could be connected with the programmable display devices. 
   In the foregoing control systems, at an initial setting of the programmable display device, the programmable display device selects protocol information corresponding to the PLC connected therewith from the protocol database stored in the host computer, and downloads the same via the common communication line according to the common protocol. This enables mutual data transmission between the programmable display device and the PLC according to the dedicated protocol of the PLC. 
   Furthermore, the programmable display device is set so that contents of data that the programmable display device processes are specified by using the foregoing processing instruction word W and the processing instruction word W is stored in the event data storing memory  122 , while the state data MD of the PLC referred to according to each processing instruction word W are maintained in the state data storing memory  17 . 
   Upon an end of the initial setting, a normal control operation is carried out. For instance, in the case where control of a second PLC  22 β is conducted using state data of a first PLC  22 α, a processing instruction word is set beforehand as the processing instruction word set for the second display device  32 β and the state data of the first PLC  22 α is referred to and the second PLC  22 β executes a control operation based on the foregoing state data. 
   In this state, when state data MD is sent to the first programmable display device from the first PLC  22 α using the dedicated protocol thereof, the corresponding state data MD is updated in the state data storing memory  17  of the first programmable display device  3   cα . The state data MD thus updated are sent to the state data storing memory in the second programmable display device  32 β using the common protocol in response to a reading operation on the second programmable display device  32 β side. 
   Here, in the case where the processing instruction word W is set beforehand so that the updated state data MD are utilized in a control operation of the second PLC  22 β, the state data MD is further transferred to the second PLC  22 β, using the dedicated protocol. Thus, the control operations of the second PLC  22 β is carried out based on the data of the first PLC  22 α. 
   Conversely, in the case where state data MD taken in by the second programmable display device  32 β is data to be processed by the foregoing programmable display device  32 β, the data is processed by the second programmable display device without being transferred to the second PLC  22 β. Then, display corresponding to the state data MD is executed on the display screen of the display. 
   Furthermore, in the case where data are sent from the host computer to the first PLC  22 α so as to control the control-target device connected with the foregoing PLC  22 α, data is produced by adding an address specifying the first PLC  22 α to the data to be transmitted, and is fed to the first programmable display device  32 α via the common communication line according to the common protocol. 
   On the other hand, programmable display device  32 α, where data is fed from another programmable display device, judges whether the data fed thereto is data to be used by that programmable display device  32 α or the data is to be used by the first PLC  22 α. In the case where they are judged to be the data for the PLC  22 α, they are transmitted to the PLC  22 α according to the dedicated protocol. 
   Thus, even in the case where the data communication protocols are dedicated for the PLCs, respectively, and hence different from each other, state data MD corresponding to a control state in the PLCs can be transferred via the programmable display device between a plurality of the PLCs or between the PLC and the host computer. 
   According to an aspect of the present invention, the programmable display device has the state data (MD) in itself at all times, that is, the case where the state data storing memories  17  and  32  are provided in the programmable display device and the PLC, respectively, and the programmable display device accesses the PLC side regularly at predetermined short intervals so that the data contents of the state data storing memories  17  and  32  should coincide with each other. As explained in the descriptions of the embodiments below, however, the state data may be taken from the PLC side as required, upon occurrence of a specific event or the like, for example. In the case where the state data storing memories  17  and  32  are provided on both sides, respectively, however, what has to be done is only designation of the programmable display device connected with the PLC and an address in the state data storing memory  17  in the programmable display device  3   c , and there is no need to specify a PLC in a format of a counterpart code, etc. 
   [Fifth Aspect] 
   The following description will explain in more detail a protocol converting operation display device, while referring to  FIGS. 10 through 13 . Members having a similar structure (function) as those in the above-mentioned aspects of the invention are designated by the same reference numerals and their description will be omitted. 
   A display device  103  includes a protocol converting section  133  and a protocol information storing section  18 , in addition to the protocol interface sections  11  and  12  and the display section  14 . The protocol converting section  133  executes protocol conversion between a dedicated protocol and a common protocol. The protocol information storing section  18  stores protocol information that is referred to upon protocol conversion. 
   Protocol information is divided into (i) a conversion table that shows correspondence between information specific to the PLCs such as command data and information of the common protocol corresponding to the specific information, and (ii) a format of transfer information transferred to/from the PLC (format information), and are stored in a conversion table storing section  181  and a format information storing section  182 , respectively. Incidentally, the storing sections  18  e.g.,  181 ,  182  can be regions on the RAM  117  shown in  FIG. 5 , for instance. 
   More specifically, in a dedicated or common protocol used in a control system  101  in accordance with an aspect of the present invention, a format in which information to be transferred is provided between a start code and an end code as shown in  FIG. 11 , like a general asynchronous system data transfer format, is specified as a asynchronous system data transfer format C. Data transfer formats specified by PLC manufacturers, however, differ in not only start codes and end codes, but also contents of data contained in the transferred information and the order of data provided therein. 
   Here, a format Cr for data reading and a format Cw for data writing are specified as data transfer formats to be transferred via the common communication line  6 . More specifically, in addition to a counterpart code for specifying a device to which data are transmitted (for instance, “A·B . . . ”) and a common code (for instance, “01”) for uniformly specifying a command to be executed (reading command), the data-reading-use format Cr further includes an address (for instance, “X0001”) at which the reading should be started, and a size of data to be read, as relevant information attached to the common code. Furthermore, in addition to the counterpart code and the common code that instructs the data writing, the format Cw specified for data writing further includes an address at which the writing should be started, and data to be written as relevant information. Each of the formats Cr and Cw is expressed as data obtained by providing the included data in series with use of ASCII codes of hexadecimal notation. 
   Between commands having the same contents among commands comprehensible to each PLC, the foregoing common codes are made uniform, irrespective of command codes at the dedicated communication lines  4  (command at each PLC). Besides, for instance, an order of data of relevant information such as a reading start address and a reading data size in which the data are transmitted, and expression of data when the data are transmitted, are also made uniform irrespective of the dedicated protocols. This allows the common protocol to uniformly specify a command to be executed, irrespective of PLCs. 
   Here, since correspondence between a certain common code and a command code (specific code) at the dedicated communication line  4  in one PLC differs from that of another PLC, a conversion table in which each common code and a specific code of the PLC currently connected corresponding to the foregoing common code are paired is formed, as shown in  FIG. 13 . Therefore, the display device  103  can convert a common code to a specific code or vice versa, referring to the conversion table corresponding to the dedicated protocol. Furthermore, for instance, the dedicated protocol for protocol conversion can be changed by changing a conversion table used for protocol conversion from a conversion table TBLα for a dedicated protocol a to a conversion table TBLβ for a dedicated protocol β. 
   On the other hand, as shown in  FIG. 12 , the foregoing format information FMT is, among data streams Da transmitted via the dedicated communication line  4 , a data stream on a skeleton in which portions that may change depending on contents of data to be actually transmitted, such as the contents of data to be read/written per se, a size of the data, or addresses for reading/writing are undefined, and only purposes of use are defined regarding the undefined portions. Incidentally, if the regions for command are undefined, in the case where format information FMT extracted from data streams actually transmitted is common among a plurality of commands, the regions for the foregoing command may be undefined also, so that format information FMT common among these commands can be used. 
   The foregoing format information and conversion tables are stored in a protocol data base of the host computer as the aforementioned protocol information corresponding to each dedicated protocol, while protocol information corresponding to the PLC connected to the display device is read upon initial setting of the display device and is stored via the common communication line  6  in the foregoing conversion table storing section  181  and the format information storing section  182 . 
   In the foregoing arrangement, pieces of protocol information corresponding to respective dedicated protocols are individually provided as format information FMT and conversion tables. Therefore, by switching the protocol information depending on a type of an apparatus, data communication according to a particular dedicated protocol is enabled. 
   Here, the control system  101  in accordance with the has a data transfer format specified as common communication protocol as described above used as often as possible at the reading/writing of data from/to the display device to the PLC, as well as at other occasions (for instance, upon display at the display device, and upon setting of preset data). Consequently, operations except control, such as a displaying operation and an operation of setting preset data, need not be changed for each dedicated protocol. Therefore, steps in the processes of development, management, and maintenance of software can be decreased. 
   The following description will explain protocol conversion using the foregoing format information FMT and conversion table while referring to  FIG. 14 . Incidentally, the control system  141  shown in  FIG. 14 , unlike the control system  90  shown in  FIG. 9 , adopts a method in which the state data storing memory is provided only in the PLC and the display device reads the state data MD from the state data storing memory  32  in the PLC upon necessity, as another method for state data transfer between the PLC and the display device. 
   For instance, in the case where the second PLC  22 β executes a control operation by utilizing a change in the state data MD in the first PLC  22 α, the second display device  34 α is set to refer to the state data MD of the first PLC  22 α in response to a processing instruction word set in the second display device  34 α, and the second PLC  22 β is set so as to execute a control operation based on the state data MD. 
   In this state, via the common communication line  6 , a data reading command is launched in the data-reading-use data transfer format Cr shown in  FIG. 11  to the first PLC  22 α, the protocol converting section  133  of the first display device  34 α refers to the conversion table storing section  181 , to convert the common code in the transferred information to a specific code. Further, the protocol converting section  133  refers to the format information storing section  182 , to apply necessary data such as a specific code, an address, a size, etc. to undefined portions of the format information FMT, thereby converting the same into a dedicated format. Thereafter, the data are transferred to the first PLC  22 α via the dedicated communication line  4 . Incidentally, as to an address and a size of data transmitted according to the dedicated protocol, expression formats (for instance, a bit width), if different from that of the common protocol, are converted by a predetermined procedure. 
   The first PLC  22 α receiving the data analyzes contents of the data, and retrieves necessary data from the state data storing memory  32 . Thereafter, necessary data are sent from the first PLC  22 α to the first display device  34 α according to the dedicated communication protocol. 
   On the other hand, the first display device  34 α analyzes contents of the received data, referring to the conversion table storing section  181  and the format information storing section  182 . In the case where the data are judged to be the data last time requested for reading and further in the case where a processing instruction word W is beforehand set in the data so as to be used in a control operation of the second PLC  22 β, the first display device  34 α designates the second PLC  22 β as the counterpart code, and sends the same to the second display device  34 β by using the common protocol. Thus, an operation of controlling the second PLC  22 β based on the first PLC  22 α is executed. 
   Incidentally, the data sent to the second display device  34 β is processed only by the display device  34 β, the data is processed in the second display device  34 β without being transferred to the second PLC  22 β, and display corresponding to the display screen D of the display section  14  is executed. 
   Furthermore, in the case where data is transferred from the host computer  71  to the first PLC  22 α so that the control-target device  9  connected to the first PLC  2 α should be controlled, the counterpart code and address for specifying the first PLC  22 α are attached to the data to be transmitted. The resultant data is transmitted to the first display device  34 α via the common communication line  6  according to the common communication protocol. 
   The first display device  34 α determines whether the data transmitted thereto is to be used in the display device  34 α or to be used by the first PLC  22 α, referring to the counterpart code. In the case where the data is to be used by the first PLC  22 α, the data is transmitted to the first PLC  22 α side in a manner identical to that described above, by using the dedicated communication protocol. 
   Incidentally, in the above description of the present embodiment, a case where a plurality of devices to be subjected to data access are present and a counterpart code is specified is taken as an example. However, in the case where the counterpart of data communication is specified beforehand, the counterpart code is unnecessary. 
   [Sixth Aspect] 
   Prior to protocol conversion of the foregoing display device, the dedicated protocol subjected to conversion has to be selected according to a control unit (PLC). This selection of the dedicated protocol may be carried out beforehand by the user of the display device. However, to save labor and time for setting and to prevent errors in setting, a dedicated protocol is preferably selected automatically by the display device. The following description will explain a display device  131  capable of automatically selecting a dedicated protocol, while referring to  FIGS. 15 through 17 . In the following description, for conveniences&#39; sake, a case where dedicated protocols of PLCs that can possibly be connected with the display device are four kinds, “α, α2, β, and γ” is taken as an example, but the numbers can be appropriately increased or decreased, needless to say. 
   More specifically, as shown in  FIG. 15 , a display device  151  in accordance with the an aspect of the present invention is provided with a protocol determining data storing section  183  for storing a correspondence table for dedicated protocol determination use, in addition to the arrangement of the display device  103  shown in  FIG. 10 . 
   The correspondence table is a table in which response codes upon issuance of predetermined protocol-determination-use commands (for instance, “00”) are listed up with respect to dedicated protocols, as shown in  FIG. 16A . Selected as the protocol-determination-use commands are commands such that response data as different as possible from each other can be obtained so as to correspond to differences between the communication protocols. Here, as shown in the correspondence table of  FIG. 16A , the same response codes are returned like the cases of the dedicated protocols α and α2. In this case, another determination-use command (for instance, “01”) is set, and a table of correspondences between the dedicated protocols and response data is prepared as shown in  FIG. 16B , so that determination of a dedicated protocol should be ensured according to a combination of a response code with each protocol-determination-use command. 
   In the foregoing arrangement, actuation of the system at an operation  1  (ST 1 ) in the flowchart shown in  FIG. 17  is followed by a communication protocol determination process that starts at operation  2 . In this process, at operation  2 , a protocol determination command like “00” is fed from the display device  131  to the PLC  2   c.    
   At operation  3 , in the case where there is no response from the PLC, it is determined that data communication per se with the PLC is not established. Then, in the case where it is determined that all the protocols have been checked at operation  4 , it is determined that the PLC is not connected or that its protocol is a protocol other than the prepared ones, and the operation proceeds to operation  5 , where the determining operation ends. On the other hand, in the case where it is determined at the operation  4  that there remains a protocol to be checked next, a protocol applied at a operation  6  is changed, and the flow returns to the operation  2  so that the aforementioned processing operation should be repeated. Incidentally, each protocol is selected in correspondence to the PLC that can possibly be connected with the display device, and it is prepared beforehand in a form like protocol information stored in the protocol information storing section  18 . The protocol information is, for instance, downloaded at the initial setting stage, from a protocol data base stored in the data processing device  7 , or the like. 
   Furthermore, confirmation of a response from the PLC at the operation  3  is followed by the proceeding to a step  7  where a correspondence table regarding to a protocol determination command sent, among the correspondence tables stored in the protocol information storing section  18 , is referred to, and a protocol corresponding to the response code is determined. For instance, in the correspondence table shown in  FIG. 16A , the communication protocol is determined to be “β” in the case where the response code is “20”, while the protocol is determined to be “γ” in the case where the response code is “30”. Therefore, after the communication protocol to be used is confirmed at the operation  8 , a normal data processing operation at operation  10  is maintained. 
   In the case where the response code confirmed at the operation  7  is “10”, the communication protocol can possibly be or “α” or “α2”, and hence cannot be determined. Therefore, this is followed by return to the operation  1 , where the determination command is changed to “01” and an identical determining operation is carried out. In this case, as shown in  FIG. 16B , the protocol is determined to be “α” in the case where the response code is “01”, while the protocol is determined to be “α2” in the case where the response code is “02”. In the case where the response code is anything other than those, the communication protocol is determined to be something other than those prepared. 
   Furthermore, in the case where a communication error occurs at operation  9  during a normal data processing operation, it is presumed that a changing operation such that the PLC connected to the display device  131  is changed to another took place. In such a case, the flow returns to the communication protocol determining process that starts with operation  2 , in which a communication protocol suitable to the PLC is automatically set. This enables to complete the communication protocol setting operation, without an initial setting operation in a state in which the display device is suspended. 
   [Seventh Aspect] 
   The following description of an aspect of the present invention will explain an arrangement of a host computer of a control system  187  in more detail, while referring to  FIG. 18 . A host computer  87  for control use includes a common protocol interface section  51  for communication with the common network  6 , various processing sections  52 , and a server section  53 . The processing sections  52  include user processing sections  521  through  523  for display or control of the state of the target system  8  of the host computer  87 , and a setting section  524  for executing the setting of the whole control system  187 . The server section  53  is provided between the processing section  52  and the common protocol interface section  51 , so as to control the common protocol interface section in response to a request from the processing sections or the like. Furthermore, in the server section  53 , in addition to a data processing section  55  that controls the common protocol interface section  51  so as to cyclically store data from each PLC to a storing section  54 , there are provided a DDE server section  56 , a function processing section  57 , and a function simple-processing section  58 , so that the data processing section  55  and the various processing section  52  communicate each other through a plurality of procedures. 
   The foregoing DDE server section  56  functions as a server for dynamic data exchange (DDE) defined by an operating system. Therefore, in the case where the processing section  52  is a user processing section  521  capable of operating as a DDE client, the user processing section  521  and the DDE server section  56  can transmit data mutually without troubles. 
   Thus, since the server section  53  in accordance with an aspect of the invention is provided with the DDE server  56 , the processing sections  52  previously described are applicable for the foregoing purpose as long as the processing sections  52  are capable of functioning as clients in DDE. Incidentally, in the case where the control system  187  has been used since before, the processing sections  52  are specialized for each control system  187 , and it is often bothersome to produce new processing sections  52 . Besides, the processing sections  52  are often arranged so as to transmit data through a procedure defined by the operating system of the host computer  87 , such as DDE, so as to be associated with another application that operates on the operating system of the host computer  87 . Therefore, in many cases, the processing sections  52  can be applied without troubles. 
   Furthermore, since the foregoing DDE is defined by the operating system, relatively many applications that are available in the market can operate as DDE clients. Therefore, even in the case where the processing sections  52  are not used, a user processing section  521  can be relatively easily prepared by using these applications. 
   Furthermore, as described above, each display device  318 , including data processing section and the host computer  87  communicate mutually according to the common protocol, irrespective of a type of the PLC connected with the display device  318 . Therefore, when the PLC connected with the display device is changed or when a newly developed PLC is connected thereto, the foregoing DDE server section  56  need not change its operation. Consequently, time and labor for developing programs of the host computer  87  can be drastically saved as compared with the case where different DDE server sections  56  are prepared for each PLC as conventionally, and therefore, time and labor can be saved upon incorporation of a new PLC to the control system  187 . 
   Incidentally, the foregoing DDE is a procedure defined by the operating system for dynamic data exchange between various applications, and is not necessarily optimized for control of the control-target devices  9 . In result, in the case where the data exchange with the processing sections  52  is limited to DDE, the processing rate can possibly decrease, and some processing operations cannot be executed. Consequently, the server section  53  in accordance with the present embodiment is provided with the function processing section  57  and the function simple-processing section  58 , for exchange of data with the processing sections  52  by a procedure other than DDE. 
   The foregoing function processing sections  57  and  58  are interfaces between the foregoing data processing section  55  and the processing sections  52  that require a processing operation that cannot be executed or whose processing rate is insufficient in the case of data exchange by DDE, such as the user processing section  522  or  523  that executes higher-level processing operations as compared with the user processing section  521 , or the setting section  524  that executes the setting of the whole control system  187 . For instance, the function processing sections  57  and  58  can be realized as DLL (dynamic link library) in Windows as the operating system of Microsoft Co. Among the function processing sections  57  and  58 , the function processing section  57  includes as recallable functions all the functions that the data processing section  55  can possibly be requested to do by the processing sections  52 , such as all the settings conducted by the setting section  524 , display and control of data from the control-target devices  9 , the PLCs, and the display devices  318 , etc. Incidentally, the foregoing settings include the setting upon incorporation of a display device  318  into the common network  6 , the setting of each PLC connected with each display device  318 , and the setting of relationship between the control-target devices  9  and device addresses. Thus, the processing sections  52  can control all the functions of the data processing section  55  by recalling each function of the function processing section  57 . 
   For instance, in the case of a function for reading data from a control-target device  9 , an IP address of a display device  318 , a name of the control-target device  9 , etc. are given as arguments, while success/failure of the reading, data read out, etc. are returned as returned values, like in the DDE case. The function is optimized for data reading from the control-target device  9 , unlike the DDE case. Therefore, an amount of data transmitted from the processing sections  52  to the data processing section  55  or processed therein upon reading of data can be decreased. Furthermore, the link setting is unnecessary. Therefore, in the case of an identical processing operation, the processing rate can be improved, as compared with the DDE case. Furthermore, in the case where an optimal procedure is different even with respect to a similar processing operation, functions are prepared for respective processing operations so that the processing operations can be carried out through the optimal procedures, respectively. Therefore, by selecting and using an optimal function for each processing operation, the processing rate can be improved as compared with the later-described case where the function simple-processing section  78  is utilized. Furthermore, processing operations such as settings of various kinds and transmission of data at specified timings, which cannot be defined by DDE, can be realized. 
   On the other hand, in the function simple-processing section  58 , there are provided, among the functions prepared by the function processing section  57 , only functions that can be used relatively easily and that cannot be executed at a sufficient speed by DDE. Functions satisfying the foregoing requirements include a function for reading data from a PLC via a display device  318 , and a function for writing data from a PLC via a display device  318 . These functions do not provide return values before the writing and reading operations are completed, whereby control is not returned to the processing sections  52 . Incidentally, in the present embodiment, the functions are realized by recall of functions in the function processing section  57  by the function simple-processing section  58 , but the function simple-processing section  58  may control the data processing section  55  directly. 
   Thus, as to the function simple-processing section  58 , the number of functions prepared, the number of arguments, and the timings for return of control to the processing sections  52  as recalled are further limited, as compared with the case of the function processing section  57 . Therefore, use of only the functions of the function simple-processing section  58  in producing the user processing section  522  enables to save time and labor for selecting optimal functions from among similar functions, time and labor for conducting the setting of arguments and the initial setting in order to use optimal functions, and time and labor for considering timings of control. Consequently, though the processing rate is higher than in the DDE case, the user processing section  522  can be created relatively easily. 
   Here, since the host computer  187  communicates according to the common protocol, the foregoing function processing section  57  or the function simple-processing section  58  need not change its operation, even in the case where the PLC connected to the display device is changed or in the case where a newly developed PLC is connected thereto. Consequently, time and labor for developing a program of the host computer  87  can be drastically saved, as compared with the case where different function processing sections  57  and different function simple-processing sections  58  are prepared for each PLC. Therefore, time and labor required upon incorporation of a new PLC in the control system  187  can be saved. 
   Additionally, the server section  53  in accordance with the present aspect is equipped with the two function processing sections  57  and  58  whose degrees of difficulty in recalling differ from each other. Therefore, the developer of the server section  53  is allowed to recommend use of the function processing section  57  or use of the function simple-processing section  58  depending on respective expertise of the developers of the processing sections  52  and respective necessary functions of the processing sections  52 . Consequently, errors of the control system  187  caused by a mistake of the developers of the processing sections  52  can be prevented more surely, as compared with the case where only the function processing section  57  is provided, in spite of that more complex or higher-speed processing is possible as compared with the case where only the function simple-processing section  58  is provided. Incidentally, the present embodiment is explained by taking as an example the case where there are two different degrees of difficulty of the function processing sections, but a plurality of function processing sections with different degrees of difficulty may be provided so that the same effect can be achieved. 
   Furthermore, in the foregoing embodiment, there is provided the DDE server section  56  that is capable of recalling data through a simpler procedure than those of the function processing sections  57  and  58 , and data can be exchanged to some extent without development of the processing sections  52 . Therefore, errors upon development of the processing sections  52  can be further reduced, whereby errors in operations of the control system  187  can be prevented. 
   [Eighth Aspect] 
   Incidentally, the foregoing first through seventh aspects of the present invention are described by taking as an example the case where data transmitted through the common communication line  6  are mainly data for control of the control unit  2 , but identical effects such as reduction of time and labor in development, management, and maintenance of the control system  1  can be achieved in the case where other data such as image data like background images, initial values, or application programs are transmitted. 
   The following description will explain, as an embodiment of the present invention, the case where characters and image data to be used in information display concerning PLC control are transmitted, while referring to  FIGS. 19 through 21 . 
     FIG. 19  is a block diagram schematically illustrating a control system  191  in accordance with an aspect of the present invention. PLCs  2   c α,  2   c β, and  2   c γ control control-target devices  91  such as a motor and an electro-magnetic valve, among control-target devices  9 , in response to a control output of a host computer  197 . Further, the PLCs  2   c α,  2   c β, and  2   c γ detect controlled states of the foregoing control-target devices  91 , by means of sensors  199  among the control-target devices  9 , and transmit the detected results to the host computer  197 , so that the results are used in display and calculation, and further, analysis, of the foregoing control output, as well as they are subjected to image display by means of corresponding display devices  3   g α,  3   g β, and  3   g γ. Thus, the foregoing control device controls manufacturing devices, manufacturing lines, etc. 
   Incidentally, in the example shown in  FIG. 19 , three PLCs with reference codes  2   c α,  2   c β, and  2   c γ are provided, but needless to say, one, or two, more PLCs may be provided, and a plurality of the host computers  197  may be provided. In the example shown in  FIG. 19 , control-target devices  91  and sensors  199  corresponding to the PLCs  2   c β and  2   c γ are omitted for simplification of the drawing. 
   The PLCs  22 α,  22 β, and  22 γ are connected to the display devices  193 α,  193 β, and  193 γ corresponding thereto with use of connection cables  194 α,  194 β, and  194 γ as the foregoing dedicated communication lines, respectively, so that data transmission should be carried out according to dedicated protocols α, β, and γ, that are predetermined according to manufacturers and versions of the PLCs  22 α,  22 β, and  22 γ. 
   What should be noted is that the display devices  193 α,  193 β, and  193 γ in accordance with the present aspect are provided between the PLCs  22 α,  22 β, and  22 γ and the host computer  197 , respectively, unlike in the conventional cases. The display devices  193 α,  193 β, and  193 γ have been developed as personal-computer-applied devices that are therefore capable of easily executing communication with general-use personal computers according to a general-use protocol δ. On the other hand, the dedicated protocols α, β, and γ have been developed with respect to the corresponding to the PLCs  22 α,  22 β, and  22 γ, respectively, to which the display devices are necessarily connected. 
   Therefore, the display devices  193 α,  193 β, and  193 γ that are connected with the host computer  197  via the common communication line  6  realized by a network circuit such as an Ethernet (registered trademark), and a common protocol, such as a protocol for the TCP/IP, is used as the protocol for the above communication. Thus, protocol conversion between the protocol δ and the protocols α, β, and γ is performed by the display devices  193 α,  193 β, and  193 γ, respectively. 
   The common communication line  6  is connectable, for maintenance or the like, with an external host  10  provided at a distance, for example, at manufacturers of the display devices  193 α,  193 β, and  193 γ, respectively, via routers or public telephone lines. 
   In the host computer  197 , a network file  62  composed of node information concerning which node is connected with what type of a display device, symbol names of control-target devices and sensors connected with the PLCs  22 α,  22 β, and  22 γ, is formed beforehand by a setting section  61 , and respective control outputs to the PLCs  22 α,  22 β, and  22 γ are transmitted to the common communication line  6  via a common protocol interface section  64  from an application section  63 , with reference to the network file  62 . The control outputs are transmitted to the PLCs  22 α,  22 β, and  22 γ via the display devices  193 α,  193 β, and  193 γ, respectively, so as to cause the control-target apparatus  9   a  to be subjected to control in response to the control outputs. Further, when a control state such as a detection result of the sensor  199  is sent out from each display device, the data are returned to the application section  63  via the common protocol interface section  64 , so as to be used in the display, calculation and analysis of control outputs. 
   Furthermore, in the host computer  197 , image data of images displayed on display screens of the display devices  193 α,  193 β, and  193 γ, conversion data for conversion between the protocols α, β, and γ and the common protocol δ, etc. are registered beforehand in a data file  65 . The image data are fed to a display device necessitating the same via the common protocol interface section  64 , upon maintenance such as a change to a product to be processed or an order of processing. The conversion data is fed to a display device necessitating the same via the common protocol interface section  64 , upon maintenance such as a change to a PLC to be connected with the common communication line  6 . 
   In response to this, each display device  193 α,  193 β, and  193 γ has a dedicated protocol interface section  11  connected with the PLC, a common protocol interface section  12  connected with the host computer  197 , a protocol conversion circuit  139  for mutual conversion of the protocols, a display panel  194  as the display section, an input section  15 , and an image memory  19 . Command data is attached to the foregoing control output or image data as will be described later, so that the common protocol interface section  12  along with the protocol conversion section  139  executes protocol conversion in the case where the command data are a control output, or writes the data into the image memory  19  in the case where the data are image data. Upon completion of updating of the image memory  19 , displayed contents on the display panel  194  are updated. The image memory  19  stores, for example, invariable characters and image data such as a background screen illustrating manufacturing lines with names of control-target devices and units attached thereto. Variable data representing a control state inputted from the protocol conversion circuit  139  are synthesized with the foregoing character and image data, and an image of the same is displayed by the display panel  194 . 
   The display devices  193 α,  193 β, and  193 γ are integrally incorporated along with the corresponding PLCs  22 α,  22 β, and  22 γ in an operation desk of manufacturing lines or the like, or are independently provided, so as to be used as a control panel or the like. On a front side of the display panel  194 , an input section  15  such as a touch panel is provided, and data input, indirect control of a control-target device  91 , etc. can be executed in response to an inputted result. The display devices  193 α,  193 β, and  193 γ are arranged substantially identical to a generally-used personal computer as described above, and in each, a central processing unit, memories like ROM and RAM, an external memory device such as a hard disk device, a graphic controller, and a communication controller are connected mutually. 
     FIG. 20  is a view of an example of a data format, illustrating a data format Dδ according to the common protocol δ on the common communication line  6 , and data formats Dα through Dγ according to the protocols dedicated for the PLCs  22 α to  22 γ, respectively. The figure exemplifies a case where the foregoing common communication line  6  is an Ethernet. 
   More specifically, the protocol δ on the common communication line  6  side is arranged so that normal main data following to header data D 1  and D 2  are divided into command data D 31  and main data D 32 . The command data D 31  represent a command such as writing or reading, and at the same time, in the present embodiment, they also indicate whether it is normal data communication between the PLCs  22 α,  22 β, and  22 γ and the host computer, or it is a downloading operation of image data from the host computer to the image memories  19  in the display devices  193 α,  193 β, and  193 γ. 
   Each of the protocols α, β, and γ on the PLCs  22 α,  22 β, and  22 γ sides is composed of, following to a header d 1 , command data d 2 , address data d 3 , main data d 4 , and end data d 5 . The main data D 32  are composed of address data d 3  of the foregoing control-target devices  91  and sensors  199 , etc., and main data d 4  representing numerical values and ON/OFF information. 
   The common protocol interface section  12  and the protocol conversion section  139  constitute a 2-way driver that executes communication with the host computer  197  as well as communication with the PLCs  22 α,  22 β, and  22 γ, and conducts protocol conversion of data inputted/outputted by using protocol conversion data supplied from the foregoing data file  65  at real time. 
   Such protocol conversion can be conducted by using conversion tables for conversion between the foregoing protocol δ common on the common communication line  6  and the respective protocols α, β, and γ dedicated for the PLCs  22 α,  22 β, and  22 γ, conversion functions, etc., and such tables or functions are stored beforehand in the aforementioned data file  25 , so as to be selected upon creation of the network file  62  for the setting of types of the display devices  193 α,  193 β, and  193 γ that are respectively connected with the aforementioned nodes, and to be set in the protocol conversion section  139 . 
     FIG. 21  is a timing chart for explaining downloading operation of image data from the host computer  197  or the external host  10  to the image memories  19  of the display devices  193 α,  193 β, and  193 γ. At operation s 1 , the host side conducts a node search for searching which node is connected with what type of a display device, and at operation s 2  the display devices  193 α,  193 β, and  193 γ successively respond thereto, along with status information including indication of the nodes connected therewith, as well as their types. 
   At operation s 3 , image-data-transmission start data are fed from the host side, and at operation s 4 , acknowledge is returned from the display side. Then, at operation s 5 , actual transmission of image data is started. Upon completion of transmission of one packet of image data at operation s 5 , acknowledge is returned from the display side as shown at a step s 6  in the figure. Then, upon completion of downloading of a predetermined amount of display data, that is, an entirety or a part of the image data of one display screen, image-data-transmission end data are supplied from the host side as shown by operation s 7 . Then, upon return of acknowledge from the display side at operation s 8 , transmission of image data ends. 
   As described above, in the present embodiment, unlike in conventional cases, display devices  193 α,  193 β, and  193 γ that excel in computation and that have high applicability to data communication are provided between the PLCs  22 α,  22 β, and  22 γ and the host computer  197  or the external host  10 . Therefore, it is possible to transmit control outputs from the host computer  197 , control state data from the PLCs  22 α,  22 β, and  22 γ, etc. without difficulties via the display devices to the PLCs  22 α,  22 β, and  22 γ and the host computer  197 . On the other hand, since it is possible to download image data directly to the image memory  19  via the common communication line  6  from the host computer  197  or the external host  10 , there is no need to carry out complex operations each time a product or a producing method is changed, such as an operation of taking a personal computer or the like for the downloading use into a manufacturing site, changing the wire connection, and downloading the data. Thus, it is possible to extremely improve the workability. 
   Furthermore, there is no need to temporarily stop the functions of the display devices  193 α,  193 β, and  193 γ upon downloading, for example, to once turn the display devices  193 α,  193 β, and  193 γ into offline states, then download data, and thereafter again turn them into online states. Moreover, since it is possible to successively download data to the display devices  193 α,  193 β, and  193 γ without changing the wire connection as described above, a time while the line is stopped can be extremely decreased. 
   Furthermore, since protocol conversion between the dedicated protocols α, β, and γ and the common protocol δ is performed by the display devices  193 α,  193 β, and  193 γ, respective programs with different protocols dedicated for the PLCs  22 α,  22 β, and  22 γ need not be provided on the host computer  197  side, and a program can be produced with the common protocol δ. Thus, steps for producing a program can be drastically simplified. 
   [Ninth Aspect] 
   The following description of an embodiment of the present invention will explain another example of data transmitted through the common communication line  6 , referring to  FIGS. 22 and 23A  through  23 G. In the example, preset data that are set beforehand in the control unit  2  and/or the display devices  3 , like initial values or application programs, are transmitted. 
     FIG. 22  is a block diagram illustrating a functional arrangement of a control device in accordance with another aspect of the present invention that is similar to the arrangement shown in  FIG. 19  described above, in which the members having a similar structure (function) as those in the above-mentioned aspects will be designated by the same reference numerals and their description will be omitted. In the present aspect, a memory  20  is provided in each of the display devices  322 α,  322 β, and  322 γ. In the memory  20 , like the image memory  19 , data is installed by downloading the same from the host computer  227 . The data thus installed are preset data such as application programs or initial values, for example, and may be preset data for the display devices  322 α,  322 β, and  322 γ, or may be preset data for the PLCs  22 α,  22 β, and  22 γ. 
   In the case where preset data is downloaded, preset data is transmitted according to the common protocol δvia the common communication line  6  from the host computer  7   g , and the common protocol interface section  12  determines based on the aforementioned command data that the data are preset data to be installed, and causes the data to be stored in the memory  20 . In the case where the preset data thus stored is data for the PLCs  22 α,  22 β, and  22 γ, the display devices  322 α,  322 β, and  322 γ output commands for stop of operations to the PLCs  22 α,  22 β, and  22 γ, and thereafter, the preset data thus stored are successively subjected to protocol conversion by the common protocol interface section  12  and the protocol conversion section  13   h , and are transmitted to the PLCs  22 α,  22 β, and  22 γ. Upon completion of the transmission, a command for resumption of operations is outputted thereto. 
     FIGS. 23A through 23G  are views illustrating an example of a data format of data to be downloaded to the display devices  322 α,  322 β, and  322 γ.  FIG. 23A  illustrates the main data D 32  in  FIG. 20 , which in the present embodiment are further divided into a header D 321  and data D 322 . The header D 321  is composed of type data D 3211  and number data D 3212 . The foregoing type data D 3211  represent a rough classification regarding which type of data the data D 322  following thereto are, the foregoing image data, the system setting data, or other data. The number data D 3212  represent a specific classification regarding where the data D 322  following thereto are ranked, among the foregoing types of data. 
   For example, in  FIG. 23C , “B” as the type data D 3211  indicates that data are image data, and “000” as the number data D 3212  indicates that the data are image data upon processing a product A. In  FIG. 23D , the data are also image data, and “001” as the number data D 3212  indicates that the data are image data upon processing a product B. 
   In  FIG. 23E , “S” as the type data D 3211  indicates that the data are system setting data for the display devices  322 α,  322 β, and  322 γ, and “100” as the number data D 3212  indicates that the data are data for conversion between the dedicated protocols α, β, and γ and the common protocol δ. In  FIG. 23F , the data are also system setting data, and “201” as the number data D 3212  indicates that the data are a data providing time table that specifies timings of data transmission from the display devices  322 α,  322 β, and  322 γ to the host computer  7   g . In  FIG. 23G , the data is also system setting data, and “202” as the number data D 3212  indicates that the data is a status format such as a bit length, presence/absence of codes, etc. upon data transmission from the display devices  322 α,  322 β, and  322 γ to the host computer  227 . 
   Incidentally, such classification as above of data is performed along with the classification according to the foregoing command data D 31 , or alternatively, either of them may be performed. 
   The foregoing operation for downloading preset data can be carried out in a manner similar to that shown in  FIG. 21 , by transmitting preset-data-transmission start data at the operation s 3  and transmitting preset-data transmission end data at the step s 7  from the host side in  FIG. 21 . 
   Thus, not only the foregoing image data, but also a relatively large amount of preset data produced on the host computer  227  side with use of various application programs upon a change of a product produced or a drastic change in processing data can be downloaded to the display devices  322 α,  322 β, and  322 γ and/or the PLCs  22 α,  22 β, and  22 γ. In the downloading of data to the PLCs  22 α,  22 β, and  22 γ particularly, complex works such as connection of a personal computer become unnecessary, and the workability can be extremely improved, while a time while a line is stopped can be shortened. 
   Furthermore, since the preset data for each of the PLCs  22 α,  22 β, and  22 γ can be produced according to the common protocol δ, steps required for producing data can be remarkably simplified. Furthermore, in the case where many PLCs are connected to the common communication line  6  in large-scale manufacturing lines or the like, traffic increases and loads on communication also increase. Therefore, if the PLCs are directly connected to the network circuit  503  as conventionally, the foregoing downloading operation is difficult to be carried out. Conversely, it can be carried out without troubles by providing display devices with high data communication applicability as in the present invention. 
   Incidentally, in the foregoing descriptions, the case where protocol information is downloaded from the host computer e.g.,  71 ,  227 , data processing device  7  is explained, but instead of, or in addition to, the downloading of protocol information from the host computer, reading/writing of data with respect to a memory card may be provided so that information may be downloaded from the memory card, or information may be downloaded from a loader at appropriate timings such as production of a display screen, or alternatively, a plurality of kinds of communication protocols may be prepared in a ROM beforehand so that a necessary communication protocol may be selected. However, since new dedicated protocols are often developed, a display device is provided so that protocol in information is downloaded thereto from outside. Incidentally, in the case where a protocol dedicated for a control unit connected to a display device is invariable, it may be fixed in a ROM beforehand. 
   Furthermore, in the foregoing embodiment, the display devices and the host computer are connected mutually via the generally used communication protocol, but alternatively a dedicated communication protocol that is common to all the display devices but is specialized only for these display devices is applicable. Furthermore, each display device may be equipped with data input/output means like a video camera in addition to a PLC, so that various kinds of data such as video data can be transmitted via the display devices and the common communication line. 
   Furthermore, it is possible to arrange that a plurality of PLCs are connectable with the dedicated communication line and data transmission is carried out with one display device involved. Furthermore, shown as an example is a case where a type of a PLC to access is specified and reading/writing of data is carried out with a specific address designated for the PLC, but alternatively the following arrangement may be taken: as to address also, address display common for all the PLCs is set beforehand, and address display expressed in a manner dedicated for each PLC and a conversion table is prepared. In the latter case, it is possible to make a program without a type of PLC to access taken into consideration, whereby the applicability of the program is improved. 
   Incidentally, in the foregoing aspects, a control system including a display device and a control unit is taken as an example in explanation, but the present invention is not limited to this arrangement. Aspects of the present invention are widely applicable to a system including first data processing means e.g. control unit having its own dedicated communication protocol, second data processing means e.g., display device, data processing device having a common communication protocol such as a personal-computer-applied device, and data processing means e.g. display device provided between the foregoing two, making mutual transfer of data possible. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 
   INDUSTRIAL APPLICABILITY 
   As described above, in a control system in accordance with the present invention, a display device, is applied as center of data communication. This allows a control unit that had conventionally been also used in data communication now to be used exclusively for control, that is, I/O control, thereby reducing loads on the control unit. Furthermore, the display device side or the host computer side connected with the display device need not consider innumerable communication, protocols of the control units. This ensures a decrease in the number of operations software necessary for development, management, and maintenance of a control system. 
   Furthermore, according to a data transmission method in accordance with an aspect of the present invention, data is transmitted to a first data processing device like a PLC according to a communication protocol (dedicated protocol) dependent on the first data processing device as conventionally, while data is transmitted to a second data processing device other than the foregoing devices according to a common communication protocol that is common to all the second data processing devices. Furthermore, a communication code (program) dedicated for each device is not provided, but rather a conversion table is provided for each device, so that data to be transmitted to the first data processing device are produced at real time by using the conversion table during an operation of the system. This ensures a decrease in the number of steps of software like the foregoing control system, while it also ensures immediate response to a change of the first data processing device without stopping the system. 
   Furthermore, a dedicated protocol is determined in the following manner: before data transmission with the first data processing device according to a dedicated protocol, predetermined data is sent and the dedicated protocol is determined based on a response to the foregoing predetermined data from the first data processing device. This ensures reduction of time and labor in setting, as compared with the case where the user determines the dedicated protocol, thereby enabling prevention of errors in the setting.