Abstract:
An industrial control system provides a programmable logic controller communicating with I/O modules which provide an electrical interface to controlled equipment and which support web servers for communication of I/O data directly with the Internet. The programmable logic controller communicates with the I/O modules over a control-network which may be separate from or shared with the Internet communications. A lock-out system prevents collisions between web clients writing to I/O and allows the programmable logic controller to limit web access to critical I/O points.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application No. 60/285,291, filed on Apr. 20, 2001. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The present invention relates to industrial control systems used for real-time control of industrial processes, and in particular to a programmable logic controller with input/output (I/O) modules that can communicate directly with the World Wide Web. 
     Industrial control systems are special purpose computer systems used in controlling industrial processes. Under the direction of a stored control program, a programmable logic controller, being part of the industrial control system, reads inputs from a series of input modules and writes outputs to a series of output modules. The input modules read signals from sensors associated with the industrial process and the output modules provide signals to actuators and the like in the industrial process. The inputs and outputs may be binary, that is on or off or analog, providing a value with a continuous range or more complex I/O devices like motor controllers and the like. 
     Industrial control systems differ from conventional computer systems in that they provide highly reliable operation and deterministic real-time control. In part, this requires both that data communicated between the programmable logic controller and the input and output modules be transmitted in a predictable sequence. This may be provided by use of an I/O image table which is populated with complete sets of input or output data before control decisions are made and before outputs are forwarded to the I/O modules. 
     Often it is desired to monitor an industrial process remotely. The development of the Internet and the Web has raised the possibility of using Web servers and browsers to implement a remote human/machine interface that could be used on any browser enabled computer. Current proposals for providing Web access to an industrial control system envision a Web server module that may plug into a common backplane to communicate with the programmable logic controller. Input and output data is communicated between the I/O module and the programmable logic controller and selectively communicated, under the control of the programmable logic controller, between the programmable logic controller and the Web server module. 
     Such a Web server module may be inappropriately expensive for simple control applications, particularly with regard to the necessary reprogramming of the control program executed by the programmable logic controller to coordinate communication with the Web server module. This coordination process can significantly add to the processing burden of the programmable logic controller. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides I/O modules each having integral Web servers that may communicate directly with the Web without the intervention of the programmable logic controller. Web access can thus be provided, efficiently, for even a few I/O points. In addition, extremely rapid Web access is possible by working directly with the I/O data rather than indirectly with I/O data as stored in the I/O image table normally supervised by the programmable logic controller. 
     As an additional benefit, access to the I/O data can be obtained before the programmable logic controller is operational for troubleshooting and set-up of the industrial process, or in the event of failure of the programmable logic controller. 
     Potential conflicts between these two sources of direct communication with I/O (the programmable logic controller and the Web-based HMI) are avoided by a write disable command that may be sent to the Web server interface of the I/O modules from the programmable logic controller, after the latter is enabled, to selectively prevent output data from coming from the Web. 
     Specifically, then, the present invention provides an I/O module for use with an industrial control system having a programmable logic controller executing an industrial control program and communicating over a control-network with the I/O module. The I/O module includes: an I/O interface communicating electrical signals between the I/O interface and an industrial process; an Internet interface communicating electrical signals between the Internet interface and the Internet; and a network interface communicating electrical signals between the network interface and the control-network. A processing unit connected to the I/O interface, the Internet interface and the network interface and executing a stored program, passes data both between the Internet and the I/O interface and between the network interface and the I/O interface. 
     It is thus one object of the invention to permit Internet based communication with the industrial process directly through the I/O module without intervention of the programmable logic controller. 
     The programmable logic controller may include an Internet/control-network bridge and the network interface and the Internet interface may both include a media connection to the control-network and the Internet interface may communicate with the Internet by way of the control-network and the bridge. The control-network may, but need not be, an Ethernet network. 
     Thus it is another object of the invention to permit direct Internet communication with I/O modules without the need for multiple network media. An Ethernet network, like other networks, can support control protocols and TCP/IP needed for Web communication. 
     Alternatively, the Internet interface may include a connector for attaching the Internet interface directly to a communications medium other than the control-network. 
     Thus it is another object of the invention to permit direct Web to I/O communication in systems where Web messages are not easily or economically handled by the control-network. 
     The processing unit of the I/O module may also execute its stored program to accept a write disable command from the programmable logic controller causing the stored program to controllably block data from passing between the Internet interface and the I/O interface. 
     Thus it is another object of the invention to allow communication between the I/O modules and both the Web and the programmable logic controller without conflicts. The programmable logic controller once on line, can block Web access to critical outputs or the like. 
     The I/O module may be used in conjunction with an Internet filter module positioned between the I/O module and the Internet. The Internet filter module may have a processing unit executing a stored program to accept a write disable command from the programmable logic controller causing the stored program to controllably block data from passing between the Internet and the I/O interface. The Internet filter may be shared with multiple Internet interfaces for multiple I/O modules and the write disable command from the programmable logic controller may cause the stored program to controllably block data from passing between the Internet and the specific ones of I/O interfaces including the I/O interface. 
     Thus it is another object of the invention to permit a centralized blocking of Web access to Internet ready I/O modules. 
     The I/O interface may communicate with multiple independent I/O points and the processor of the Internet filter module may further execute the stored program to accept a write disable command from the programmable logic controller causing the stored program to controllably block data from passing between the Internet and the I/O interface with respect to selected I/O points. 
     Thus it is another object of the invention to permit selective blocking of Internet control of the I/O points, for example, to allow reading but not writing of I/O points. 
     The foregoing objects and advantages may not apply to all embodiments of the inventions and are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of a prior art control system showing the relationship between a programmable logic controller and various I/O modules communicating with an industrial process; 
         FIG. 2  is a schematic representation of a first embodiment of the present invention in which a programmable logic controller communicates over a control-network with internal or external I/O modules each of which may support a web server communicating over a second network with the Internet and showing the use of an optional in-line Internet filter to prevent conflicting I/O commands; 
         FIG. 3  is a figure similar to that of  FIG. 2  showing an alternative embodiment in which a single network supports both communications between the programmable logic controller and the I/O modules and communication between the Internet and the I/O modules; 
         FIG. 4  is a symbolic representation of a write token/lockout flag table used for a simple I/O module to facilitate combined web and programmable logic control access; 
         FIG. 5  is a schematic block diagram of an I/O module according to the present invention showing the multiple network interfaces which may communicate with one or different media and showing a processor and embedded web server suitable for one embodiment of the invention; and 
         FIG. 6  is a symbolic representation of a filter table suitable for the optional in-line Internet filter of  FIG. 2  in controlling access to various I/O points. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a conventional industrial control system  10  provides a programmable logic controller  12  connected to interface terminal  14 . The interface terminal  14  may be a standard desktop computer and is used to develop programs for the programmable logic controller and to provide interface for monitoring its operation. 
     The programmable logic controller  12  also includes a network interface (not shown) to communicate over a control-network  16  with one or more I/O modules  18 . The I/O modules may be in the same housing as the programmable logic controller  12  in which case the control-network is a portion of a backplane (not shown) or may be remote from the programmable logic controller  12 . 
     The I/O modules  18  provide an interface between the control-network  16  and output electrical signals  20  that may be written to actuators  22  of the controlled process  24 . The I/O modules  18  also receive input electrical signals  26  from sensors  28  of the control process  24 . Generally as is understood in the art, the programmable logic controller reads input electrical signals  26  from sensors  28  via input I/O modules  18  and based on those input signals, and the logic of the control program in the programmable logic controller  12 , provides output signals to other I/O modules  18  which become output electrical signals  20  to actuators  22 . 
     Referring now to  FIG. 2 , I/O modules  18 , remote from the programmable logic controller  12 , communicate over a high speed serial control-network  16 , and I/O modules  18 ′, contained within a rack  30  holding the programmable logic controller, communicate via a backplane portion of the control network  16 ′. In the embodiment of  FIG. 2 , each of the I/O modules  18  and  18 ′ provide a connection to a second Internet-network  32  such as may support the TCP/IP protocol used by the Internet. The Internet network  32  may be, for example, conductive metal cable, a radio link, an infrared link, or an optical fiber and connects in a daisy chain fashion among the I/O modules  18  and  18 ′, however, other connection topologies well known in the art may be used. The control network  16  can be, instead of a high speed serial network, any of a variety of other types of networks known in the art (e.g., a parallel network rather than a serial network). 
     Referring now momentarily to  FIG. 5 , the I/O modules  18  and  18 ′ each incorporate two communications interfaces  34  and  36 . The first communication interface is network interface  34  communicating with the control-network  16 . As is known in the art, the network interface  16  may provide a media access layer implemented in hardware together with firmware executing appropriate, higher level protocols for the control-network  16 . The second communication interface is Internet interface  36  for communicating with the Internet  54  either directly or through an access device  62  such as a modem. The Internet interface  36  provides for a media access layer typically implemented in hardware and the higher-level protocols, for example TCP/IP, necessary for Internet communication. Alternatively for both communication interfaces  34  and  36 , higher-level protocols may be implemented by one or more processors within the I/O module  18 . Also, in alternate embodiments, only one communications interface for communication with one or both of the control network and Internet is used. Further, the Internet  54  and the Internet interface  36  are intended to broadly cover any communications system and interface for communicating with such communication system that operate by way of the same or similar protocols and formats as the World Wide Web including, for example, an intranet. 
     Both communication interfaces  34  and  36  also communicate over an internal bus  38  with a processor  40  and a memory  42  the latter implemented typically as a combination of volatile and nonvolatile solid state memory devices. The memory  42  holds the operating program  43  for the I/O module such as may handle the logic implemented by the I/O module. The memory  42  may also hold a web server program and one or more web pages or applets to be served. Web server programs may be obtained from such open source suppliers as the Apache Foundation. Alternatively, the web server function and the served content may be handled by an embedded web server  45  being a processor and software combination of a type known in the art, also connected to the internal bus  38 . 
     The processor  40  executing the operating program  43  handles communication between the programmable logic controller  12  and the I/O module  18  according to prior art techniques to read and write from and to I/O interfaces  46  which serve to provide electrical interface between the internal bus  38  and output and input electrical signals  20  or  26  as have been described above. In this regard, memory  42  may also include a local I/O image table  50  storing the values of the input electrical signals  26  prior to transfer over the industrial control-network  16  or the Internet  32  and input electrical signals  20  prior to transfer to the industrial process  24 . Generally, the I/O image table  50  serves as an intermediary to all data transfer between the programmable logic controller  12  and the controlled process  24 . 
     In addition, the memory  42  provides for a write token/lockout flag table  52  providing control of access to the I/O image table  50  according to the source of the read or write signals from a specific one of the network interface  34  or Internet interface  36 . Referring to  FIG. 4 , the write token/lockout flag table  52  in its simplest form includes a write token  56  being a flag whose state moderates control of the writing to the I/O image table  50  and in this way controls which of multiple competing clients communicating over the Internet may, for example, write an output electrical signal  20 . According to one procedure, a particular Internet client must obtain the write token (e.g. by resetting a bit in the write token  56 ) prior to making a write to the I/O table  50 . This obtaining of the token requires the client to read the flag if it is set, to take possession of the token by resetting the flag. If the flag is already reset, the client must wait until the bit is set again prior to attempting to reset it. A successful resetting of the flag, thus taking possession of the token, allows that client to write an output electrical signal  20  and blocks other clients from simultaneously affecting these outputs to possibly undesirable consequences. 
     The write token/lockout flag table  52  also includes a lockout  58  that may only be set by a command received over the control-network  16  from the programmable logic controller  12 . The purpose of the lockout  58  is to prevent critical outputs of the controlled process  24  from being changed by other than the programmable logic controller  12  once the programmable logic controller  12  has been fully programmed and is running. This lockout  58  provides for a high level of integrity of outputs necessary in the industrial control environment on control of its setting and resetting and the blocking of the writing to outputs of the controlled process is performed by the operating program  43 . 
     It is anticipated that the write token/lockout flag table  52  may incorporate multiple flags depending on the type of I/O module  18  and the number of input and output signals  20  and  26  handled by the I/O module  18 . In this way, different individual output signals  20  may be written to by different clients writing from the Internet. Further the programmable logic controller may lockout certain outputs and leave others unlocked. 
     Referring now to  FIGS. 2 and 3 , in an alternative embodiment, the write token and/or lockout  58  may be implemented by a filter  60  shared by a number of I/O modules  18 . The filter  60  is positioned so that signals between the Internet  32  and the I/O modules  18  must pass prior through it. The filter  60  executes an internal program to monitor all read or write commands to I/O modules  18  and to block those that have been locked out by the programmable logic controller  12  according to an write token and/or lockout  58 . The filter also implements write tokens  56  that may be queried by Internet clients. 
     Thus for example, referring to  FIG. 6 , a write token/lockout flag table  52  implementing write tokens  56  may provide for a set of write tokens having one bit for each output implemented by a particular I/O module  18  that may be set or reset. The client programs may seek the write token  56  in the filter  60  rather than in the particular I/O modules  18  simply by convention. A similar structure (not shown) may be used to implement lockouts for the various I/O modules. In this case, however, the filter may operate to block messages according to whether the particular lockout bit is set or reset within its structure. 
     Referring now to  FIGS. 3 and 5 , in an alternative embodiment, the two network interfaces  34  and  36  may share common media through jumper  64  thereby imposing the possibly two different high level protocols of the control-network  16  and the Internet  32  on common combination media  68 . In this case, the programmable logic controller  12  monitors the combination media  68  to extract messages that have the protocol of the control-network  16  and a bridge device  66  monitors the combination media  68  to extract messages that have the protocol of the Internet  54 . The low level protocol of the common media  68  may be an Ethernet protocol to provide the functionality of both a control-network executing higher level control net protocols and TCP/IP used on the Internet. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but that modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments also be included as come within the scope of the following claims.