FIELD OF THE INVENTION The present invention relates generally to a distributed processing unit for converting field sensor measurements, such as temperature and velocity of a fluid, from an analog signal to a digital signal and, more particularly, to such a distributed processing unit for converting the field sensor measurements to a digital signal and for attaching a time of occurrence of the particular measurement to the signal representing the sensor measurements.

DESCRIPTION OF THE RELATED ART A process control system typically includes a plurality of process measurements taken from the process in which it is monitoring to enable operators to properly perform the process. For example, one such process, which includes a process control system, is a reactor coolant system of a nuclear power plant. A typical reac¬ tor coolant system includes a nuclear reactor wherein a controlled nuclear reaction, which generates heat, is occurring. Typically, borated water is contained in the reactor for controlling the nuclear reaction process and for passing the heat away from the reactor. A primary loop communicating with the reactor functions to pass the borated water (i.e., the heat) away from the reactor and to transfer the heat to a secondary loop. The secondary loop is isolated from the primary loop and generates

passed from the primary loop. The steam of the secondary loop is used to produce electricity as is well known in the art. The primary loop then returns the borated water back into the reactor where the above described process is repeated.

To properly control the heat production process, a number of process parameters, such as temperature and flow velocity of the borated water, are measured. Typical¬ ly, field sensors, such as resistance temperature detectors (RTDs) and flowmeters, are attached to the primary loop for respectively measuring the temperature and flow of the borated water. Analog signals, representing its respective process measurement, from the field sensors are passed to a distributed processing unit where the signals are digi- tized. The distributed processing unit typically includes an input/output card for receiving the field sensor signals and for converting these input signals to a digital signal . The input/output card also includes a general purpose controller for periodically sending the digital signal to remote locations via a data highway, such as WESNET3 of Westinghouse Electric Corporation, where personal computer workstations and the like monitor the process signals.

It may become necessary to store these process measurements for forming a historical database. Such a database is useful because it allows operators to see a sequence of process events for later analysis, for example, in the unlikely event of an accident. It also allows operators flexibility in their monitoring process because the process measurement does not have to be observed by an operator at the precise time it is received by the worksta¬ tion. However, to achieve such a database, the process measurements should be labeled with its actual time of actual occurrence so that the operators can tell precisely at what time the measurement was taken. Presently known devices for attaching a time of occurrence to process measurements utilize the general purpose controller for achieving this function. In the case of a standard distributed processing unit which does

not stamp its process measurements with a time of occur¬ rence, maintenance personnel should re-program and add additional memory to the general purpose controller, and should re-configure various wire terminations within the controller for converting the distributed processing unit. In the case of manufacturing a new distributed processing unit with time stamping capabilities, the distributed processing unit would still require additional memory and extensive programming. Although the presently known and utilized device is satisfactory, it is not without drawbacks. Such re¬ wiring, re-programming, and adding of memory are laborious and time consuming. In addition, the processing power of the controller is diminished which, in turn, causes slower processing time. Consequently, an improved distributed processing unit is needed for monitoring field sensor inputs and for attaching a time of occurrence to each sensor input.

SUMMARY OF THE INVENTION The present invention provides an improvement designed to satisfy the aforementioned needs. Particular¬ ly, the present invention is directed to a distributed processing unit for monitoring field sensor inputs and for attaching a time of occurrence to the sensor input compris- ing (a) an input/output card for receiving the inputs from the sensors; said input/output card including (i) an analog to digital converter for converting each sensor input to a digital signal; and (ii) a clock operatively associated with said analog to digital converter for stamping a time of occurrence to each sensor input which stamping forms a time stamped datum; and (b) a general purpose controller connected to said input/output card for retrieving the time stamped data from said input/output card and for communi¬ cating the time stamped data. It is an object of the present invention to provide an improved distributed processing unit for attaching a. time of occurrence to signals representing sensor measurements.

It is a feature of the present invention to provide a clock operatively connected to an analog to digital converter of the distributed processing unit for stamping a time of occurrence to the sensor input which stamping forms a time stamped datum.

It is an advantage of the present invention to provide an efficient means for converting a standard distributed processing unit to a distributed processing unit which stamps its process measurements with a time of occurrence.

BRIEF DESCRIPTION OF THE DRAWINGS In the course of the following detailed descrip¬ tion, reference will be made to the attached drawings in which: Fig. 1 is a schematic diagram of a system for measuring process parameters of a primary loop of a nuclear power plant; and

Fig. 2 is a schematic diagram of a distributed processing unit of the present invention for attaching a time of occurrence to signals representative of the process parameters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. Also, in the following description, it is to be understood that such terms as "forward," "left," "right," "upwardly," "downwardly," and the like are words of convenience and are not to be con¬ strued as limiting terms. „ Now referring to Fig. 1, a system for monitoring process parameters, such as temperature and velocity, of the process of a primary loop of a nuclear power plant (not shown) is illustrated, generally referred to as 10. Although the primary loop of a nuclear power plant is illustrated in the preferred embodiment, any process which monitors process parameters may be used with the present invention. Such process parameters are typically needed for proper operation of the particular process.

A plurality of field sensors 20 are connected to the primary loop, although only two sensors are shown in this embodiment, for monitoring the process parameter and for creating an analog signal representative of the partic- ular process parameter. The field sensor may be, for example, a resistance temperature detector (RTD) 20a for measuring the temperature of the primary fluid, or it may be a flowmeter 20b for measuring the flow of the primary fluid. A distributed processing unit 30 (DPU) is connect¬ ed via cables 40 to the field sensors 20 for receiving the analog signal from the field sensors 20 and converting it to a digital signal, as is well known in the art. A data highway 50, such as WESNET3 of Westinghouse Electric Corporation, is connected to the DPU 30 for transmitting the digital signal to a plurality of remote workstations 60 which are connected to the data highway 50. The worksta¬ tions 60 may be IBM workstations which are available from IBM corporation and are well known in the art. The work- stations 60 are operable to receive the digitized signal and to display it in a suitable form, for example a numeri¬ cal value on a display screen or on a printout device (both of which are not shown) . The process measurements allow the operators to properly control the process within the primary loop.

Referring now to Fig. 2, the DPU 30 of the present invention is illustrated for stamping a time of occurrence to the signal representative of the process parameters. Each DPU 30 typically includes a plurality of input/output cards 70 for receiving the analog signal from the field sensors (not shown in Fig. 2) . It is instructive to note that the number of input/output cards is determined by the number of field sensors 20, as is well known in the art. The input/output card 70 includes an analog to digital converter (A/D converter) 80 for receiving the analog signal and converting it to a digital signal. Data storage 90, such as standard memory which is well known in the art, is connected to the A/D converter 80 for temporarily

storing the digital signal received from the A/D converter 90. Processing logic 100, such as an Intel 80C196, is connected to the data storage 90 for controlling the time stamping function within the input/output card 70. A clock 110, such as a crystal control clock, is attached to the processing logic 100 for providing a signal representative of the time of day. The processing logic 110 is programmed to periodically retrieve both the digital signal represen¬ tative of the process parameter and the time of day signal and to combine them to form a time stamped datum.

A general purpose controller 120, typically a microprocessor, is attached to the input/output card 70 for receiving the time stamped data from the input/output 70 and for communicating it to the data highway 50. This data is then passed to the plurality of workstations (not shown in Fig. 2) for displaying the data on a display or on a printout (both of which are not shown) .

It is thought that the present invention and many of its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form, construction and arrange¬ ment thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantag¬ es, the form hereinbefore described merely a preferred or exemplary embodiment thereof.