Abstract:
A method for identifying memory modifications includes designating a first portion of a first memory as read-only. An abort condition is identified responsive to receiving a write instruction having a target address within the first portion. In response to the abort condition, a second portion of the first memory including at least the target address is flagged as being modified. The write instruction is executed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates to industrial control systems and, in particular, to a method and apparatus for synchronizing an industrial controller with a redundant controller.  
         [0004]     This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.  
         [0005]     Industrial controllers are special purpose computers used for controlling factory automation and the like. Under the direction of stored programs, a processor of the industrial controller examines a series of inputs reflecting the status of a controlled process and changes outputs affecting control of the controlled process. The stored control programs may be is continuously executed in a series of execution cycles, executed periodically, or executed based on events.  
         [0006]     The inputs received by the industrial controller from the controlled process and the outputs transmitted by the industrial controller to the controlled process are normally passed through one or more input/output (I/O) modules) which serve as an electrical interface between the controller and the controlled process. The inputs and outputs are recorded in an I/O data table in memory. Input values may be asynchronously read from the controlled process by specialized circuitry. Output values are written directly to the I/O data table by the processor, then communicated to the controlled process by the specialized communications circuitry.  
         [0007]     Industrial controllers must often provide uninterrupted and reliable operation for long periods of time. One method of ensuring such operation is by using redundant, secondary controller components (including an independent processor) that may be switched in to replace primary controller components while the industrial controller is running. In the event of a failure of a primary component, or the need for maintenance of the components, for example, the secondary components may be activated to take over control functions. Maintenance or testing of the control program may be performed with the primary processor reserving the possibility of switching to the secondary processor (and a previous version or state of the control program) if problems develop.  
         [0008]     Ideally, the switch-over between controllers or their components should occur without undue disruption of the controlled process. For this to be possible, the secondary processor must be running or waiting to run the same program (and maintaining its current state) and must be working with the same data in its I/O data table as is the primary processor.  
         [0009]     The same control program may be simply pre-stored in each of the primary and secondary processors. The data of the I/O data table, however, cannot be pre-stored but changes continuously during the controlled process. Further, because control processes are I/O intensive, there is typically a large amount of data in the I/O data table. For this reason, transmitting the data to the secondary processor is difficult.  
         [0010]     One technique for reducing the overhead associated with the synchronization of the primary and redundant controllers is to monitor specific changes to the I/O table using custom circuitry, such as an application specific integrated circuit (ASIC), that flags changes in the portion of the memory designated for storing the I/O table. Upon reaching a synchronization point, the flags that have been set are read, the associated data in the flagged locations is collected, and the modified data is sent to the redundant controller. This approach is described in greater detail in U.S. Pat. No. 5,933,347, entitled “Industrial Controller with Program Synchronized Updating of Back-up Controller,” subject to common assignment with the present application, and incorporated herein by reference in its entirety.  
         [0011]     Increasingly, the use of custom processors and associated integrated circuits, such as the monitoring circuit described in the &#39;347 patent, is becoming less feasible. To reduce cost and increase flexibility, industrial control systems are being developed with commercially available microprocessors and supporting circuitry that do not have the same functionality as the previously used customized solutions.  
         [0012]     The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     The present inventors have recognized that memory modifications may be tracked for purposes of synchronizing primary and redundant controllers using commercially available hardware. A memory management unit may be used to set the protection of the monitored memory to read-only. Subsequent writes to the monitored memory are trapped by an abort handler, logged, and subsequently allowed. The logged memory locations may then be communicated between the primary and redundant controllers.  
         [0014]     One aspect of the present invention is seen in a method for identifying memory modifications. The method includes designating a first portion of a first memory as read-only. An abort condition is identified responsive to receiving a write instruction having a target address within the first portion. In response to the abort condition, a second portion of the first memory including at least the target address is flagged as being modified. The write instruction is executed.  
         [0015]     Another aspect of the present invention is seen in an industrial controller including a memory, a memory management unit, and a processor. The memory is operable to store a user program and a data table. The memory management unit is operable control access to the memory and to store protection data associated with the memory. The memory management unit is operable to signal an abort condition responsive to a memory access instruction conflicting with the protection data. The processor is operable to communicate with the memory through the memory management unit, execute the user program to designate in the protection data a first portion of the memory including the data table as being read-only, issue write instructions in accordance with the user program having target addresses targeting the first portion, and execute an abort handler invoked by the signaling of an abort condition by the memory management resulting from a selected one of the write instructions having a target address conflicting with the read-only designation of the first portion. The abort handler is operable to flag a second portion of the memory including at least the target address as being modified and allow execution of the selected write instruction.  
         [0016]     Still another aspect of the present invention is seen in an industrial control system including a primary industrial controller and a secondary industrial controller. The primary industrial controller includes a first memory and is operable to control a process in accordance with a user program and a data table stored in the first memory. The primary industrial controller is operable to designate a first portion of the first memory including the data table as read-only, and identify abort conditions responsive to receiving write instructions from the user program having target addresses within the first portion. In response to the abort conditions, the primary industrial controller is operable to flag subsequent portions of the first memory including at least the target addresses as being modified, execute the write instructions, and periodically generate update data based on the portions of the first memory flagged as being modified. The secondary industrial controller includes a second memory operable to store a copy of the user program and a copy of the data table. The secondary industrial controller is operable to receive the update data and update its copy of the data table based on the update data.  
         [0017]     These and other objects, advantages and aspects of the invention will become apparent from the following description. The particular objects and advantages described herein may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made, therefore, to the claims herein for interpreting the scope of the invention.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0018]     The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and:  
         [0019]      FIG. 1  is a simplified diagram of an industrial control system in accordance with one embodiment of the present invention;  
         [0020]      FIG. 2  is a simplified block diagram of an industrial controller in the system of  FIG. 1 ; and  
         [0021]      FIGS. 3 and 4  are simplified flow diagrams of methods used by the industrial controller of  FIG. 2  to track memory modifications for synchronizing with a redundant controller. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     One or more specific embodiments of the present invention will be described below. It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the present invention unless explicitly indicated as being “critical” or “essential.” 
         [0023]     Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to  FIG. 1 , the present invention shall be described in the context of an industrial control system  10 . The industrial control system  10  includes a primary controller  12   a  and secondary controller  12   b  housed in separate racks  14 . Each rack  14  holds processor modules  16   a  and  16   b , respectively, to be described in greater detail below.  
         [0024]     Within the racks  14  of primary controller  12   a  are I/O modules  18  having I/O lines  20  communicating with a controlled process (not shown) for transferring input and output signals between the controllers  12   a  and the controlled process. In addition, both the racks  14  include communication modules  22  connecting the controllers  12   a  and  12   b  to a common general purpose link  24  and communication modules  26  connecting controllers  12   a  and  12 ( b ) to a special dedicated communication link  28 . The general purpose communication link  24  may also connect to an I/O rack  30  having additional I/O modules  18  and I/O lines  20 . The industrial control system  10  may implement a standard and commonly available high-speed serial network including but not limited to: Ethernet, DeviceNet, ControlNet, Firewire or FieldBus and may optionally include one or more bridges for translating between different of the above standard or other protocols.  
         [0025]     The dedicated communication link  28  is used for the communication of I/O data between the processor modules  16   a  and  16   b  and the communication of information coordinating a switch-over between the operation of the primary and secondary controllers  12   a  and  12   b.    
         [0026]     Referring now to  FIGS. 1 and 2 , a simplified block diagram of the primary controller  12   a  is provided. The primary controller  12   a  includes a microprocessor  32  communicating with a memory  34  through a memory management unit  36 . The primary controller  12   a  also includes a plurality of ports  38  for communicating with the communication modules  22 ,  26  and other associated peripherals, such as a human interface module (not shown). In the illustrated embodiment, the microprocessor  32 , memory  34 , and memory management unit  36  are commercially available hardware components, standard in the computer industry. The memory  34  may include volatile or nonvolatile memory types (i.e., or a combination of both) well known in the art. For example, the memory  34  may include flash memory, dynamic memory, a hard disk, etc.  
         [0027]     The memory  34  is operable to store a user program  40  and an I/O data table  42 . Generally, the user program  40  may include relay ladder logic frequently used in an industrial control environment, as well as general purpose arithmetic and logical instructions. The user program  40  comprises a sequence of instructions that are executed in repeated execution cycles at a scan point scanning through the user program and writing data to the I/O data table  42 . At the same time, the I/O data table  42  is asynchronously updated over the link  24  with current input values from the I/O modules  18 . Output values in the I/O data table  42  may be transmitted to the I/O modules  18  synchronously to the execution cycles of the user program  40 . The operation of the microprocessor  32 , memory  34 , and memory management unit  36  is also affected by executive code  43 , which includes the operating system and all supervisory mode code that manages the execution of the user program  40 .  
         [0028]     In general, the primary controller  12   a  tracks modified memory locations and, when the user program  40  reaches a synchronization point (e.g., at the end of a scan), the modified data is sent to the secondary controller  12   b  to update its corresponding memory. The primary controller  12   a  pauses and waits for confirmation of the successful transfer before beginning a subsequent scan. Hence, if control is transferred from the primary controller  12   a  to the secondary controller  12   b , the secondary controller  12   b  will operate on the same I/O data. The portion of the memory  34  designated for monitoring may vary. In one embodiment, the portion of the memory  34  associated with the I/O data table  42  may be designated for monitoring. In other embodiments, other portions, such as the user program  40 , may also be monitored.  
         [0029]     Although the invention is described as it may be implemented for identifying changes to the I/O data table  42 , it may be applied generally to any application in which a memory or a portion of a memory of any device is to be monitored to identify changes to allow synchronization.  
         [0030]     Still referring to  FIG. 2 , as is known in the art, the memory management unit  36  is typically used to translate virtual memory addresses to actual physical memory addresses in the memory  34 . A sequential group of virtual addresses may not be stored in contiguous fashion within the memory  34 . The executive code  43  defines and stores a translation table  44  that the memory management unit  36  uses to define the correspondence relationship between virtual addresses and physical memory addresses. When a virtual address is received by the memory management unit  36 , it consults the translation table  44  to identify the actual physical memory address that is being referenced. Typically, memory tracked by the memory management unit  36  is grouped into pages of configurable size (e.g., typically between 1 k and 4 k bytes). The most significant bits of a virtual address refer to the page, and the least significant bits refer to an index within the page. To increase performance, the memory management unit  36  includes a translation lookaside buffer (TLB)  46  that stores information regarding recent memory transactions. For example, the TLB  46  may store information from the translation table  44  for the last 32 pages accessed.  
         [0031]     Another function of the memory management unit  36  is to allow different protection schemes to be applied to regions of the memory  34 . For example, a block may be designated as read/write or read-only, depending on its particular use. The protection information is also stored in the translation table  44  for each page, along with the virtual to physical translation information. The protection information for recently accessed pages is also stored in the TLB  46 . If a read command is received that addresses a memory location that is designated as read-only, an abort is issued and the microprocessor  32  executes an abort handler  48  that includes instructions for handling the error condition.  
         [0032]     The primary controller  12   a  uses the protection functions of the memory management unit  36  to track modifications to the memory  34 , as described in greater detail below. Generally, the memory locations designated for monitoring (e.g., the I/O data table  42 ) are designated as read-only in the translation table  44 . Any attempted write instructions targeting those locations generate an abort that is addressed by the abort handler  48 . The abort handler  48  logs the memory location of the attempted write in a modified memory log  50 , changes the protection of the target address to allow the write, and sets the program counter back to the instruction in the user program  40  that generated the write instruction. The re-execution of the write then succeeds, as the memory protection has been changed to read/write.  
         [0033]     In a first embodiment, described in reference to the simplified flow diagram of  FIG. 3 , the primary controller  12   a  tracks memory access by page. In block  52  a program scan is started, and the monitored memory locations are designated as read-only in block  54 . The modified memory log  50  is also cleared in block  54  to remove records of the previous scan. Again, the specific portions of the memory  34  designated for monitoring may vary. For purposes of the following examples, the memory locations reserved for the I/O data table  42  are designated for monitoring. The executive code  43  sets the protection status by writing to the translation table  44  and flushing or invalidating the entries in the TLB  46  (i.e., to prevent the TLB  46  from providing state protection data from the previous scan).  
         [0034]     In block  56  a write instruction is received. The protection status of the target address of the write instruction is checked in block  58  by the memory management unit  36 . If the page including the memory location is read-only, the write instruction is rejected and the abort handler  48  is invoked through an interrupt in block  60 . In block  62  the protection state of the page including the target address is set by the abort handler  48  to read/write by modifying the translation table  44  and invalidating or flushing the TLB  46 . The page is logged into the modified memory log  50  in block  64 . The most significant bits of the target addresses specify the page and the least significant bits indicate an index within the page. For example, using a 32 bit address and a page size of 1 k bytes the 19 most significant bits specify the page, and the 13 least significant bits specify the index. As the primary controller  12   a  tracks modifications by page in this embodiment, the index bits may be ignored and only the page bits are stored in the modified memory log  50 . Alternatively, the pages boundaries may be predetermined, and the modified memory log  50  may include a series of flag bits each associated with one of the pages. In such a case, the abort handler  48  would set the flag associated with the page including the target address as opposed to storing the page bits.  
         [0035]     In block  66 , the abort handler  48  returns control to the user program  40  by setting the program counter to the instruction that initiated the write request. When the user program  40  re-executes the write instruction, the instruction is received in block  56 , the protection check passes in block  58  because the page is now designated as read/write, and the write instruction is executed in block  68 . Subsequent write instructions to the same page, (i.e., with different indexes) will also pass the protection check and be written without any subsequent logging or intervention by the abort handler  48 .  
         [0036]     The program scan continues in block  70 . If another write instruction is received, the primary controller  12   a  transitions back to block  56 . If a synchronization point is reached in block  72  (i.e., at the end of the program scan), the modified memory log  50  is read in block  74 , and the pages with modified memory values are transferred to the secondary controller  12   b  in block  76  over the dedicated communication link  28 . The program scan ends in block  78 . The next program scan begins in block  52  so that the protection status can be changed back to read-only and the TLB  46  can be flushed. The modified memory log  50  is also cleared.  
         [0037]     Hence, for every program scan, the pages with modified data values are transferred to the secondary controller  12   b . By setting the page size to a relatively small value (e.g., 1 k bytes), and considering that write instruction are often clustered within a given page, the inefficiency resulting from sending an entire page, as opposed to sending individual words, may be kept within tolerable limits.  
         [0038]     In some cases, the memory management unit  36  may store a “dirty” bit in the TLB  46  for each page to which a write has been made. This dirty bit may be used to indicate that a page has been modified. The dirty bit functionality may be used in conjunction with the modified memory log  50  to track modified pages. If entries are replaced or flushed from the TLB  46  with the dirty bit set, the modified memory log  50  is updated to reflect the pages with modified data.  
         [0039]     In a second embodiment, illustrated by the simplified flow diagram of  FIG. 4 , the primary controller  12   a  tracks the locations of individual write instructions, such that only the modified words within each page are transferred when the synchronization point is reached. This embodiment may be employed where small page sizes are not supported by the memory management unit  36  or where writes tend to be dispersed, rather than clustered. A configuration parameter of the primary controller  12   a  may be used to select the particular management mode (i.e., page or word).  
         [0040]     In block  80  a program scan is started, and the monitored memory locations are designated as read-only in block  81 . The modified memory log  50  is also cleared in block  81  to remove records of the previous scan. In block  82  a write instruction is received. The protection status of the target address of the memory management unit is checked in block  84  by the memory management unit  36 . If the page including the memory location is read-only, the write instruction is rejected and the abort handler  48  is invoked using by interrupt in block  86 . In block  88  the protection state of the page including the target address is set to read/write in the translation table  44  by modifying the translation table  44  and invalidating or flushing the TLB  46 . The target address (i.e., including page and index fields) is logged into the modified memory log  50  in block  90 .  
         [0041]     At this point the abort handler  48  knows the address of the attempted write and the address of the aborted instruction. In block  91 , the abort handler  48  stores the instruction following the aborted instruction in a temporary register or memory location and inserts a “trap” instruction, such as a software interrupt (SWI) in its place. Measures should be taken (e.g., by the operating system) to ensure that the store instruction which caused the abort does not modify the program counter such that it still points to the inserted SWI.  
         [0042]     In block  92  the abort handler  48  returns control to the user program  40  by setting the program counter to the write instruction. When the user program  40  re-executes the write instruction, the instruction is received in block  82 , the protection check passes in block  84  because the page is not designated as read/write, and the write instruction is executed in block  94 .  
         [0043]     The trap instruction is executed in block  96 . The abort handler  48  is invoked by the trap instruction and proceeds to change the protection status back to read-only in block  98 , replace the trap instruction with the displaced instruction in block  100 , and return control to the user program  40  with the program counter pointing to the instruction following the write in block  102 .  
         [0044]     The program scan continues in block  104 . If another write instruction is received, the primary controller  12   a  transitions back to block  82 . If a synchronization point is reached in block  106  (i.e., at the end of the program scan), the modified memory log  50  is read in block  108 , and the specific modified memory values are transferred to the secondary controller  12   b  in block  110  over the dedicated communication link  28 . The program scan ends in block  112 .  
         [0045]     The embodiment of  FIG. 4  may be further modified by counting the number of times each page is accessed and storing the count in the modified memory log  50 . After a predetermined threshold is reached, such as  5  writes to a page, the entire page is logged into the modified memory log  50 , the individual entries are removed, and the page is set to read/write for the remainder of the scan, as described in the embodiment of  FIG. 3 . This modification allows write instructions that are clustered on a give page to proceed without further intervention by the abort handler  48 , thereby increasing efficiency.  
         [0046]     In another embodiment, instead of using a trap instruction, the abort handler  48  may analyze the aborted write instruction and execute the write instruction itself targeting the address while operating in a supervisory mode that overrides the read-only protection. The abort handler  48  would then return control to the user program  40  with the program counter pointing at the next instruction.  
         [0047]     Tracking memory changes using the protection functions of the memory management unit, as described herein, has numerous advantages. For example, the use of conventional hardware reduces the cost, increases the flexibility, and reduces the development time for the industrial control system  10 .  
         [0048]     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.