CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation application of U.S. patent application Ser. No. 09/616,254, filed Jul. 14, 2000, now U.S. Pat. No. 6,745,090, entitled PAGE BACK SYSTEM AND METHOD FOR REMOTE PAGING IN A CONTROL SYSTEM. 


   TECHNICAL FIELD 
   The present invention relates to the art of industrial controllers, and more particularly to a method and apparatus for remote paging in a distributed control system. 
   BACKGROUND OF THE INVENTION 
   Industrial controllers are special purpose computers used for controlling industrial processes, manufacturing equipment, and other factory automation applications. In accordance with a control program, an industrial controller may measure one or more process variables or inputs reflecting the status of a controlled process, and change outputs effecting control of the process. The inputs and outputs may be binary, (e.g., on or off), as well as analog inputs and outputs assuming a continuous range of values. The control program may be executed in a series of execution cycles with batch processing capabilities. 
   The measured inputs received from a controlled process and the outputs transmitted to the process generally pass through one or more input/output (I/O) modules. These I/O modules serve as an electrical interface between the controller and the controlled process, and may be located proximate or remote from the controller. The inputs and outputs are recorded in an I/O table in processor memory. Input values may be asynchronously read from the controlled process by one or more input modules and output values are written directly to the I/O table by the processor for subsequent communication to the process by specialized communications circuitry. An output module may interface directly with a controlled process, by providing an output from an I/O table to an actuator such as a motor, valve, solenoid, and the like. 
   During execution of the control program, values of the inputs and outputs exchanged with the controlled process pass through the I/O table. The values of inputs in the I/O table are asynchronously updated from the controlled process by dedicated scanning circuitry. This scanning circuitry may communicate with input and/or output modules over a bus on a backplane or network communications. The scanning circuitry also asynchronously writes values of the outputs in the I/O table to the controlled process. The output values from the I/O table are then communicated to one or more output modules for interfacing with the process. Thus, the processor may simply access the I/O table rather than needing to communicate directly with the controlled process. 
   An industrial controller may be customized to a particular process by writing control software that may be stored in the controller&#39;s memory and/or by changing the hardware configuration of the controller to match the control task. In distributed control systems, controller hardware configuration is facilitated by separating the industrial controller into a number of control modules, each of which performs a different function. Particular control modules needed for the control task may then be connected together on a common backplane within a rack and/or through a network or other communications medium. The control modules may include processors, power supplies, network communication modules, and I/O modules exchanging input and output signals directly with the controlled process. Data may be exchanged between modules using a backplane communications bus, which may be serial or parallel, or via a network. In addition to performing I/O operations based solely on network communications, smart modules exist which may execute autonomous logical or other programs. 
   Various control modules of a distributed industrial control system may be spatially distributed along a common communication link in several racks. Certain I/O modules may thus be located in close proximity to a portion of the control equipment, and away from the remainder of the controller. Data is communicated with these remote modules over a common communication link, or network, wherein all modules on the network communicate using a standard communications protocol. 
   In a typical distributed control system, one or more I/O modules are provided for interfacing with a process. The outputs derive their control or output values in the form of a message from a master or peer device over a network or a backplane. For example, an output module may receive an output value from a processor, such as a programmable logic controller (PLC), via a communications network or a backplane communications bus. The desired output value is generally sent to the output module in a message, such as an I/O message. The output module receiving such a message will provide a corresponding output (analog or digital) to the controlled process. Input modules measure a value of a process variable and report the input values to a master or peer device over a network or backplane. The input values may be used by a processor (e.g., a PLC) for performing control computations. 
   Conventional control devices typically provide a run mode wherein a module executes a control program and a configure mode wherein the control program execution is suspended. As control systems become more widely distributed, the logic or control program associated with a particular process or system may be executed on a large number of modules or devices. In this way, individual processors in the devices execute a program autonomously from the rest of the system components. Smart devices, such as I/O modules, transducers, sensors, valves, and the like may thus be programmed to execute certain logical or other programs or operations independently from other such devices. 
   In distributed control systems, it may be desirable to notify remote systems or personnel of certain process conditions. Telephone lines and modems may be used to provide communications between controllers in a distributed control system and other devices. In addition, modems and pagers may be used to generate and transmit messages across telephone lines. Conventional modems communicate across telephone lines via standard protocols, such as the Telocator Alphanumeric Paging Protocol (TAP). The modem may communicate with a host device, such as a personal computer, via a standard communications or COMM port using standard communications protocols. The universal applicability of conventional modems is achieved in part through the use of standard communications with personal computers and the like. However, the messaging capabilities of these standard communications protocols is limited with respect to communicating with industrial control devices. 
   Industrial control devices typically are provided with communications interfaces allowing communication with other devices across a bus or network. Proprietary or specialized communications protocols and messaging are used in such control system communications architectures. Previously, industrial control devices had to be customized in order to communicate with conventional modem devices using standard communications protocols. This increased the cost of the control device, and sacrificed many advantageous messaging capabilities not supported by the standard protocol of prior modems or pagers. Consequently, the application of conventional modems and pagers to industrial control system devices has heretofore failed to adequately provide full communications messaging capabilities between distributed control system devices and remote devices and/or personnel at minimal cost, and without modification of the control system devices. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, there is provided a modem apparatus adapted to provide full messaging and communications interface between a control device and a remote device via a telephone line or other communications medium. The modem includes an interface adapted to communicate directly with a control system device, such as a programmable logic controller (PLC), using a communications protocol compatible with the normal network or backplane communications used in a distributed control system. This apparatus advantageously interfaces directly with unmodified control system devices, providing the ability to send and receive messages from remote devices or personnel via telephone lines or other suitable communications mediums (e.g., Internet, wireless). The employment in the modem of a communications protocol adapted for communications with a control device allows full messaging capabilities between the control device and remote personnel and/or devices. 
   The apparatus may further be operative to monitor certain control conditions via communications with the control device, and to create and transmit one or more pager messages across telephone lines according to the monitored control conditions. In addition, the apparatus may provide data values in the pager messages to apprise the recipient of process related information relevant to the control condition which initiated the pager message transmission. In addition to the communications protocol compatible with the normal network communications used in a distributed control system, the apparatus may also support standard communications protocols used in conventional modems or pagers. In this regard, a switch or other selection mechanism may be included for user selection of the desired communication protocol. Thus, the modem apparatus may communicate directly with a personal computer or other standard device, whereby the apparatus may be conveniently configured by a user. 
   In accordance with one aspect of the present invention, there is provided a communications apparatus for communicating with an industrial control device and a remote device via a communications medium. The apparatus includes a first communications interface operative to communicate with the industrial control device using a first communications protocol, a second communications interface operative to communicate with the remote device via the communications medium using a second communications protocol, and a processor in electrical communication with the first and second communications interfaces and operative to exchange messages with the industrial control device and the communications medium via the first and second communications interfaces. 
   The processor may receive trigger information from the industrial control device via the first communications interface, and selectively construct and transmit one or more pager messages via the second communications interface according to the trigger information. 
   The communications apparatus may advantageously be programmed or adapted to page remote devices and/or personnel, for example, when a process variable associated with a controlled process exceeds a certain value. According to another aspect of the invention, the first communications protocol may be the Allen-Bradley DF1 protocol and the second communications protocol may be the Telocator Alphanumeric Paging Protocol (TAP). The apparatus thus provides a pager interface with full messaging capabilities for interacting with standard control devices, for example, Allen-Bradley PLC, SLC, Contrologix, and Micrologix processors. 
   In accordance with another aspect of the present invention, the communications apparatus may further comprise a switch in electrical communication with the processor. The switch may be used to select a communications protocol for the first communications interface. This allows the device to selectively communicate with a control device using one communications protocol (e.g., Allen-Bradley DF1 protocol), or with a programming device (e.g., a personal computer) using another protocol (e.g., ASCII). A user may accordingly program the communications apparatus using a computer, then change the switch position and connect the apparatus to a control device. This aspect of the invention allows the use of the pager or modem communications apparatus with devices supporting disparate communications protocols (e.g., a personal computer and a control device), without the need to modify or customize one of the devices. 
   According to another aspect of the invention, the communications apparatus may be further adapted to receive programming messages from the communications medium via the second communications interface. This allows the apparatus to be programmed via a programming message from a programming device connected to the communications medium. Thus, the communication device&#39;s program may be changed by remote devices or personnel without interrupting the connection between the apparatus and a control device. This is useful, for example, where a process condition has been reported via the communications apparatus (e.g. by transmitting a pager message), such as a high temperature condition, and a remote user wishes to change the value at which the apparatus will provide subsequent pager messages, or to disable future pages altogether. 
   According to yet another aspect of the invention, the apparatus may be adapted to receive a programming message from the communications medium via the second communications interface and to transmit the programming message to the industrial control device via the first communications interface. This feature allows a remote user to program the control device from a telephone connection. 
   According to still another aspect of the invention, the communications apparatus may receive a first message from a first device connected to the communications medium via the second communications interface and transmit or forward the first message to the industrial control device via the first communications interface. In addition, the apparatus may receive a second message from the industrial control device via the first communications interface and transmit or forward the second message to the first device connected to the communications medium via the second communications interface. The apparatus may thus operate as a telephone interface between a remote user and the control device. This is useful, for example, where a remote user or device has received a pager message indicating a particular process variable or condition, and wishes to further interrogate and/or reprogram the control device. 
   In accordance with another aspect of the invention, an industrial control system is provided for controlling a process. The system comprises a modem or pager communications device having a first communications interface operative to communicate using a first communications protocol, and a second communications interface operative to communicate using a second communications protocol. The first communications protocol may be, for example, the Allen-Bradley DF1 protocol and the second communications protocol may be the Telocator Alphanumeric Paging Protocol (TAP). 
   The system further includes a control device having a third communications interface adapted for removable connection with the first communications interface and operative to communicate with the modem device using the first communications protocol (e.g., the Allen-Bradley DF1 protocol), and an output operative to interface with a controlled process. A communications medium maybe operatively connected to the second communications interface to communicate with the modem device using the second communications protocol (e.g., TAP). The control device may be adapted to send trigger information to the modem device, and the modem device is adapted to selectively construct and transmit a pager message via the second communications interface according to the trigger information. 
   According to yet another aspect of the invention a method is provided for sending a message from a control device to a remote device via a communications medium. The method includes obtaining trigger information via a first communications interface from the control device and determining if a trigger condition exists according to the trigger information. If a trigger condition exists, the trigger condition is correlated with a data variable, which is obtained from the control device via the first communications interface. The method further includes creating a pager message including a text string and the data variable, and transmitting the pager message to a remote device via a second communications interface and the communications medium using a second communications protocol. 
   To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described. The following description and the annexed drawings set forth in detail certain illustrative applications and aspects of the invention. However, these aspects are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view illustrating a distributed industrial control system having multiple functional modules contained in several racks joined by communication links; 
       FIG. 2  is a schematic block diagram of a single functional module of  FIG. 1  illustrating its connection to a common backplane and communication links to communicate with other modules; 
       FIG. 3A  is a schematic block diagram illustrating an exemplary control system having a communications apparatus in accordance with an aspect of the present invention; 
       FIG. 3B  is a schematic block diagram illustrating an exemplary memory organization according to an aspect of the invention; 
       FIG. 3C  is a schematic illustration of an exemplary message string definition in accordance with an aspect of the invention; 
       FIG. 4A  is a schematic illustration of an exemplary trigger information request message from a communications apparatus to a control device according to an aspect of the invention; 
       FIG. 4B  is a schematic illustration of an exemplary trigger information response message from a control device to a communications apparatus according to another aspect of the invention; 
       FIG. 4C  is a schematic illustration of an exemplary status information request message from a communications apparatus to a control device according to an aspect of the invention; 
       FIG. 4D  is a schematic illustration of an exemplary status information response message from a control device to a communications apparatus according to another aspect of the invention; 
       FIG. 4E  is a schematic illustration of an exemplary variable data request message from a communications apparatus to a control device according to an aspect of the invention; 
       FIG. 4F  is a schematic illustration of an exemplary variable data response message from a control device to a communications apparatus according to another aspect of the invention; 
       FIG. 4G  is a schematic illustration of an exemplary pager message from a communications apparatus according to an aspect of the invention; 
       FIG. 5A  is a flow diagram illustrating an exemplary method for sending a message from a control device to a remote device via a communications medium in accordance with another aspect of the invention; 
       FIG. 5B  is a flow diagram further illustrating the method of  FIG. 5A ; 
       FIG. 6A  is a flow diagram illustrating another exemplary method for sending a message from a control device to a remote device via a communications medium in accordance with another aspect of the invention; 
       FIG. 6B  is a flow diagram further illustrating the method of  FIG. 6A ; 
       FIG. 6C  is a flow diagram further illustrating the method of  FIGS. 6A and 6B ; and 
       FIG. 6D  is a flow diagram further illustrating the method of  FIGS. 6A–6C . 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   The various aspects of the present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. The invention provides a system and method for creating and sending a pager message from a communications device to a remote device via a communications medium, according to a control condition in a control device which triggers the pager message construction and transmission. The communications device communicates with the control device using a communications protocol, for example, the Allen-Bradley DF1 protocol, to allow use with standard control devices. 
   Referring to  FIG. 1 , a distributed industrial control system  10  suitable for use with the present invention provides a first and second rack  12 A and  12 B for holding a number of functional modules  14  electrically interconnected by backplanes  16 A and  16 B running along the rear of the racks  12 A and  12 B respectively. Each module  14  may be individually removed from the rack  12 A or  12 B thereby disconnecting it from its respective backplane  16  as will be described below for repair or replacement and to allow custom configuration of the distributed system  10 . 
   The modules  14  within the rack  12 A may include, for example, a power supply module  18 , a processor module  26 , two communication modules  24 A and  24 B and two I/O modules  20 . A power supply module  18  receives an external source of power (not shown) and provides regulated voltages to the other modules  14  by means of conductors on the backplane  16 A. 
   The I/O modules  20  provide an interface between inputs from, and outputs to external equipment (not shown) via cabling  22  attached to the I/O modules  20  at terminals on their front panels. The I/O modules  20  convert input signals on the cables  22  into digital words for transmission on the backplane  16 A. The I/O modules  20  also convert other digital words from the backplane  16 A to the necessary signal levels for control of equipment. 
   The communication modules  24 A and  24 B provide a similar interface between the backplane  16 A and one of two external high speed communication networks  27 A and  27 B. The high speed communication networks  27 A and  27 B may connect with other modules  14  or with remote racks of I/O modules  20  or the like. In the example illustrated in  FIG. 1 , the high speed communication network  27 A connects with backplane  16 A via the communication module  24 A, whereas the high speed communication network  27 B connects the communication module  24 B with communication modules  24 C and  24 D in rack  12 B. The processor module  26  processes information provided by the communication modules  24 A and  24 B and the I/O modules  20  according to a stored program and provides output information to the communication module  24  and the I/O modules  20  in response to that stored program and received input messages. 
   Referring also to  FIG. 2 , the functional modules  14  are attached to the backplane  16  by means of a separable electrical connector  30  that permits the removal of the module  14  from the backplane  16  so that it may be replaced or repaired without disturbing the other modules  14 . The backplane  16  provides the module  14  with both power and a communication channel to the other modules  14 . 
   Local communication with the other modules  14  through the backplane  16  is accomplished by means of a backplane interface  32  which electrically connects the backplane  16  through connector  30 . The backplane interface  32  monitors messages on the backplane  16  to identify those messages intended for the particular module  14 , based on a message address being part of the message and indicating the message&#39;s destination. Messages received by the backplane interface  32  are conveyed to an internal bus  34  in the module  14 . 
   The internal bus  34  joins the backplane interface  32  with a memory  36 , a microprocessor  28 , front panel circuitry  38 , I/O interface circuitry  39  (if the module is an I/O module  20 ) and communication network interface circuitry  41  (if the module is a communication module  24 ). The microprocessor  28  may be a general purpose microprocessor providing for the sequential execution of instructions contained in memory  36  and the reading and writing of data to and from the memory  36  and the other devices associated with the internal bus  34 . 
   The microprocessor  28  includes an internal clock circuit (not shown) providing the timing of the microprocessor  28  but may also communicate with an external precision clock  43  of improved precision. This clock  43  may be a crystal controlled oscillator or other time standard including a radio link to an NBS time standard. The precision of the clock  43  is recorded in the memory  36  as a quality factor. The panel circuitry  38  includes status indication lights such as are well known in the art and manually operable switches such as for locking the module  14  in the off state. 
   The memory  36  holds programs executed by the microprocessor  28  to provide the functions as will be described and also variables and data necessary for the execution of those programs. For I/O modules  20 , the memory  36  also includes an I/O table holding the current state of inputs and outputs received from and transmitted to the industrial controller  10  via the I/O modules  20 . 
   Referring now to  FIG. 3A , an exemplary distributed control system  100  is illustrated in which various aspects of the present invention may be implemented. The system includes a communications device or apparatus  102  operatively connected between a telephone line  104  and one of a personal computer (PC)  106  and a control device (PLC)  108  for communication therewith. The communications device  102  includes a modem  110  providing a communications interface between a processor  112  and a remote device (not shown) via the telephone line  104 . The modem  110  may communicate via the telephone line  104  using a standard pager protocol, for example the Telocator Alphanumeric Paging Protocol (TAP). While the exemplary communications device  102  is illustrated as communicating via the telephone line  104 , the invention finds application in association with other communications mediums, for example, wireless and the Internet. 
   The processor  112  also connects to a configuration/communication port  114 , which provides a communications interface with one of the PC  106  and the PLC  108 , as described in greater detail infra. The port  114  may be, for example, an RS-232 or RS-485 compatible serial port. A configuration switch  116  and a configuration mode indicator LED  118  are operatively connected to the processor  112  to allow a user to select one of two communications protocols (not shown) used in communicating via port  114 . For example, a control device protocol (e.g., Allen-Bradley DF1 protocol) may be used for communicating with the PLC  108  when the switch  116  is in one switch state, and another protocol (e.g., ASCII) may be used in communicating with the PC  106  when the switch  116  is in another switch state. The LED  118  provides a user with an indication of which state the switch  116  is in. 
   The provision of the switch  116  and the support for a plurality of communications protocols allows a user to configure or otherwise communicate with the device  102  by connecting a COMM port  107  of the PC  106  to the port  114  of device  102 , and sending programming or other messages (e.g., using ASCII programming instructions) to the device  102  in a configuration mode with the switch  116  in one state. Thereafter, the user may change the switch state, and connect the port  114  to the PLC control device  108  for paging operation (e.g., using DF1 protocol), as described in greater detail infra. The PLC control device  108  may be operatively connected to interface with a process  120  by reading one or more inputs  122  (e.g., process variables) and/or providing one or more outputs  124  (e.g., control signals). 
   Referring also to  FIG. 3B , the device  102  further includes a memory  130  having stored therein pager company data  132 , a trigger list  134 , a pager message list  136 , and variable data  138 . It will be appreciated that the memory  130  may further include program instructions (not shown), which may be executed by the processor  112  to implement the various functions associated with operation of the device  102 . The pager company data  132  may comprise one or more entries in the memory  130  (e.g., PAGER COMPANY DATA #  1 , PAGER COMPANY DATA #  2 , . . . , PAGER COMPANY DATA # X, where X is an integer), whereby pager messages may be sent via one or more pager service provider companies. The device  102  may accordingly be programmed to attempt to send a pager message via a first pager company, and if unsuccessful, to make subsequent attempts using a different pager company. The pager company data may include, for example, telephone access numbers, PIN numbers, and the like, to enable proper connection and transmission of pager messages via the pager company service providers. 
   The trigger list  134  may comprise one or more individual trigger lists, such as a binary trigger list  134   a  and a status trigger list  134   b . This allows pager message construction and transmission by the device  102  to be triggered by binary information and/or status information received from the PLC control device  108 . Transmitted pager messages (as illustrated and described in greater detail with respect to  FIG. 4G  infra) may include a text string field obtained from the pager message list  136 , and a process variable or value obtained from the control device  108  according to the variable data list  138 . In this regard, a user may define N pager messages and N process variables, where N is an integer, which may be selectively transmitted by the communications device  102  to a remote device (not shown) via the modem interface  110  and the telephone line  104 . 
   Referring also to  FIG. 3C , an exemplary message string definition  140  is illustrated. A user may define pager messages via PC  106  or another programming device connected to the port  114 . The defined information may be stored by the processor  112  in one or more tables or lists of the memory  130 . The pager message definition includes the provision of a message or text string  142 , for example, “Pump over temp, temperature is &lt;var&gt;” as illustrated in  FIG. 3C . Also included in the definition  140 , are a trigger address  144  (e.g., B 3 : 0 / 0 ), a data variable address  146  (e.g., N 7 : 0 ), and a trigger on definition (e.g., a low to high transition). 
   In the example of  FIG. 140 , the device  102  may periodically interrogate the control device  108  by sensing a low to high transition on the binary data (e.g., bit) at address B 3 : 0 / 0  of the device  108 . If such a transition is sensed, the communications device  102  obtains the data variable at control device address N 7 : 0 , and constructs a pager message including the text string and the data variable. The recipient of the pager message will then be provided with useful information comprising a textual indication of the type of control condition which initiated the message, as well as a value indication related thereto. For example, the message may indicate “Pump over temp, temperature is 135”. It will be appreciated that pager messages can be sent including no process variable, and that the user may accordingly program the communications device  102  using a definition (e.g., definition  140 ) to selectively include such variable information, for example, by leaving the variable field  146  empty. 
   Referring now to  FIGS. 4A–4G , several exemplary information request and response messages are illustrated, along with an exemplary pager message. It will be recognized that the system and apparatus of the present invention may employ message types and formats other than those illustrated and described herein.  FIG. 4A  illustrates an exemplary trigger information request message  150  from a communications apparatus (e.g., device  102 ) to a control device (e.g., PLC  108 ) according to an aspect of the invention. The request message  150  may be used by the device  102  to request binary trigger information (e.g., one or more binary values defined as trigger points in the definition  140 ) and includes a header field  152 , a starting address field  154 , a length field  156 , and a trailer field  158 . 
   Referring also to  FIG. 4B , the PLC control device  108  in turn provides a response message  160  to the communications device  102 , which includes a header field  162 , the requested binary trigger information  164 , and a trailer field  166 . It will be appreciated that the header and trailer fields in the illustrated exemplary messages may be advantageously employed to provide for proper message routing, for example, including source and destination addresses, checksum values, and the like. The communications device  102  may evaluate the binary trigger information  164  received from the control device  108  in order to determine whether a trigger condition exists, as described in greater detail infra. 
   Referring also to  FIGS. 4C and 4D , an exemplary status information request message  170  is illustrated from a communications apparatus (e.g., device  102 ) to a control device (e.g., PLC  108 ). Like the binary trigger information request message  150  of  FIG. 4A , the status information request message  170  may be used by the device  102  to request trigger information (e.g., one or more status indications or conditions defined as trigger points in a pager message definition). Message  170  includes a header field  171 , a status code list  172 , and a trailer field  173 . In  FIG. 4D , the PLC control device  108  responds with a status information response message  175  to the communications device  102 , which includes a header field  176 , the requested status code information  177 , and a trailer field  178 . The communications device  102  may evaluate the status code information  177  received from the control device  108  in order to determine whether a trigger condition exists. 
   Referring now to  FIGS. 4E and 4F , if device  102  determines (e.g., from the requested binary and/or status information) that a trigger condition exists, the trigger condition will be correlated with the pager message definitions programmed by the user. The correlation will indicate whether a data variable is needed for inclusion in the pager message. If so, a variable data read message  180  may be sent from the communications device  102  to the control device  108 . The message  180  includes a header field  181 , the address of the appropriate variable data  182  (e.g., variable address  146  of definition  140  in  FIG. 3C ), and a trailer field  183 . In reply, the control device  108  sends a variable data response message  185 , as illustrated in  FIG. 4F . Message  185  includes a header field  186 , the requested variable data value  187 , and a trailer field  188 . 
   Referring now to  FIG. 4G , once the desired data value  187  has been received in the response message  185 , communications device  102  creates a pager message  190  for transmission to a remote device via the modem  110  and the telephone line  104 . The message  190  includes a header field  192 , a text string  194 , a data variable  196 , and a frailer field  198 . The header field may include information obtained from the pager company data  132  of memory  130  of  FIG. 3B . The text string may be a user-defined string obtained from the text string list  136 , and the data variable may be the value  187  obtained in the variable data response message  185  of  FIG. 4F . 
   It will be appreciated that further messages are contemplated as within the scope of the present invention. In particular, the communications device  102  may be programmed through programming messages (not shown) from the PC  106  via port  114 , or alternatively through programming messages from a remote device (not shown) via the telephone line  104 . Moreover, the control device may be programmed using the telephone line  104 , the device  102 , and the port  114 . This enables a pager message recipient to effectuate a telephone link with the communications device  102  remotely using telephone line  104 , and to then communicate with the control device  108  via data request messages and/or programming messages. 
   Referring now to  FIGS. 5A and 5B , an exemplary method  200  for sending a message from a control device to a remote device via a telephone line is illustrated in accordance with another aspect of the invention. The method may be implemented, for example, using the exemplary communications device  102  in the control system  100  of  FIG. 3A . The method  200  begins at step  202 , in which a trigger information request message (e.g., messages  150  and  170  of  FIGS. 4A and 4C ) is sent to the control device (e.g., PLC control device  108  of  FIG. 3A ), after which a trigger information response message (e.g., messages  160  and  175  of  FIGS. 4B and 4D ) is received at step  204  from the control device  108 . 
   Thereafter, the communications device  102  determines whether a trigger condition exists at step  206 . If not, the method continues again at step  202 . In this regard, it will be appreciated that the trigger information request messages may be sent to the control device  108  periodically, and that the user configuration or programming of the communications device  102  may provide the period at which the messages are sent. 
   If a trigger condition is determined to exist at decision step  206 , the trigger condition is correlated with a data variable at step  208 , and a data read request message (e.g., message  180  of  FIG. 4E ) is sent to the control device  108  at step  210 . Thereafter, a data read response message (e.g., message  185  of  FIG. 4F ) is received at step  212 . Referring also to  FIG. 5B , the trigger condition is then correlated with a text string (e.g., from list  136  of  FIGS. 3A and 3B ) at step  214 . 
   The communications device  102  then constructs or creates a pager message (e.g., message  190  of  FIG. 4G ) including the text string and the data variable (e.g., value  187  obtained in the variable data response message  185  of  FIG. 4F ), which is then transmitted to a remote device via a telephone line (e.g. line  104  of  FIG. 3A ) at step  218 . Thereafter, the method returns to step  202 , whereat another trigger information request message may be sent to the control device  108  as described supra. 
   Referring now to  FIGS. 6A–6D , another exemplary method  300  is illustrated for sending a message from a control device (e.g., device  108  of  FIG. 3A ) to a remote device (not shown) via a telephone line (e.g., line  104  of  FIG. 3A ) in accordance with another aspect of the invention. Although the methods  200  and  300  illustrated and described herein may be implemented in the communications apparatus  102  of  FIG. 3A , it will be appreciated that the methods may be practiced in other systems and apparatus beyond those illustrated herein, and further that the apparatus and systems of the invention may operate according to methodologies other than the methods  200  and  300  illustrated herein. 
   The method  300  begins at step  302  of  FIG. 6A , after which a binary trigger information request message (e.g., message  150 ) is sent to the control device (e.g., device  108 ) at step  304 . An optional watchdog timer (not shown) may be activated in the communications apparatus (e.g., device  102 ) to provide for situations where a control device (e.g., device  108 ) does not respond to a request message from the communications apparatus or device  102 . If such a watchdog timer is activated or enabled, its timer is started at step  304  to guard for a non-responsive control device. If the control device  108  does not provide a response message prior to the watchdog timer timing out, the method  300  proceeds from decision step  306  to  FIG. 6D , as described in greater detail infra. 
   If a binary trigger information response message (e.g., message  160 ) is timely received, a status information request message is sent to the control device at step  308 , and a watchdog timer is started. If the control device does not respond within the watchdog timer period, the method proceeds from decision step  310  to  FIG. 6D , as described in greater detail infra. Referring also to  FIG. 6B , if a status information response message (e.g., message  175 ) is timely received at step  310 , the binary trigger information from the binary information response message is correlated with one or more entries in a binary trigger list (e.g., list  134   a  of  FIG. 3B ) at step  312 , and the status information from the status information response message is correlated with one or more entries in a status trigger list (e.g., list  134   b ) at step  314 . Based on the correlations in steps  312  and  314 , the communications device determines whether a trigger condition exists at decision step  316 . If no trigger condition exists, the method returns to step  304  of  FIG. 6A . 
   If a trigger condition exists, the trigger condition is correlated with a data variable (e.g., from list  138 ) at step  318 . Referring also to  FIG. 6C , a variable data read request message (e.g., message  180 ) is then sent to the control device at step  320 , after which the communications device determines at step  322  whether a response message (e.g., message  185 ) has been received before a watchdog timer started in step  320 . If so, the trigger condition is correlated with a text string (e.g., from list  136 ) at step  324 . Thereafter, a pager message (e.g., message  190  of  FIG. 4G ) is created at step  326 , which includes the data variable and the text string. The pager message is then sent or transmitted via a telephone line (e.g., line  104 ) to a remote device (not shown) at step  328 . 
   Referring also to  FIG. 6D , if the control device fails to respond to a request message at various steps in the method  200  (e.g., steps  306 ,  310 , and  322 ) before timeout of the watchdog timer, the communications device may create a watchdog timeout pager message (not shown) at step  330 , which is transmitted to a remote device via the telephone line at step  332 . This enables a recipient of the timeout pager message to take some action corresponding to the loss of communications between the control device and the communications device. 
   Although the invention has been shown and described with respect to certain illustrated aspects, it will be appreciated that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure, which performs the function in the herein illustrated exemplary aspects of the invention. In this regard, it will also be recognized that the invention includes a system as well as a computer-readable medium having computer-executable instructions for performing the steps of the various methods of the invention. 
   In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes”, “including”, “has”, “having”, and variants thereof are used in either the detailed description or the claims, these terms are intended to be inclusive in a manner similar to the term “comprising.”
The invention comprises a modem apparatus adapted to provide full messaging and communications interface between a control device and a communications medium such as a telephone line. The modem can comprise an interface adapted to communicate directly with a control system device, such as a programmable logic controller (PLC), using a communications protocol compatible with the normal network communications used in a distributed control system. The apparatus advantageously interfaces directly with unmodified control system devices, providing the ability to send and receive messages from remote devices or personnel via a communications medium. The invention also comprises a control system including a modem device providing full communications between a control device and remote personnel and/or devices. The apparatus can further be operative to monitor certain control conditions via communications with the control device, and to create and transmit messages across a communications medium according to the monitored control conditions.