Multiple-mode data acquisition system
TECHNICAL FIELD 
     This invention pertains to electrical data acquisition systems and, more particularly, to systems for acquiring and encoding data from a plurality of local and remote sources for transmission to a distant location. 
     BACKGROUND ART 
     In data acquisition systems, information about a plurality of parameters is often obtained by simultaneously deploying numerous sensors. Various factors such as physical restrictions limiting access to the sensors, their remoteness, and the cost of cable, require acquisition and processing of signals from the sensors at a local unit before transmission to a distant master data processing unit. Moreover, in many applications the acquired signals must be monitored and processed locally to assure timely activation of local process controls and then temporarily stored to facilitate sequential transmission of data signals from all of the local sensors to the master processing unit. 
     The transmission format between the local unit of the acquisition system and the master unit is determined by consideration of such factors as limited frequency responses available through the connecting cables and differences in the data acquisition rates of various sensors. Systems such as that disclosed in Vancsa, U.S. Pat. No. 4,349,821 and in Nelligan, U.S. Pat. No. 4,012,712 suggest various data multiplexing and local signal storage schemes, respectively, in endeavors to enhance transmission resolution of signals acquired over parallel channels from several sensors. The electrical links providing data transmission channels between a local unit and the sensors used in monitoring process controls in such fields as submerged petroleum production facilities, for example, often require different signal output formats to accommodate for differences in the relative proximity of sensors to the local data acquisition station (e.g., signals transmitted over longer electrical links are more susceptible to picking up noise and are influenced by the greater link resistence), and to take into consideration special transmission requirements imposed on interconnections (e.g., inductive couplings) between some of the links and the local acquisition unit. There is a need therefore, for a data acquisition system which accommodates differences in possible operating modes and characteristics of various sensor channels. Moreover, while it may be desirable in particular applications to locally process data signals before transmission to a central distant master processing unit, it is highly desirable that in the event of failure of the processing stage at the local unit, the data signals be reliably transmitted directly to the master unit in a format which can be accurately decoded. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is one object of the present invention to provide an improved data acquisition system. 
     It is another object to provide a system for acquiring and relaying data generated in different signal formats by a plurality of sensors. 
     It is yet another object to provide a system for more accurately acquiring and relaying data generated in different signal formats by a plurality of sensors. 
     It is yet another object to provide a system which, after suffering a partial system failure, is usable for reliably acquiring and relaying data generated in different signal formats by a plurality of sensors. 
     It is still yet another object to provide a data acquisition system which accommodates differences in operating modes and characteristics between incoming data channels. 
     It is another object to provide a method for accurately and reliably encoding data signals from incoming channels having different operating modes and characteristics. 
     These and other objects are achieved with a multiple-mode data acquisition system and method of operation in which a local unit sequentially acquires and transmits to a master data processing unit data signals received in a plurality of formats, during both normal and fallback modes. The local unit includes a multiplexer sequentially receiving incoming data signals and applying those signals via different analog-to-digital converter stages to a micro-processor based telemetry encoding and transmitting logic stage. After processing and storage in binary form, the micro-processor stage addresses a pair of input ports of the multiplexer receiving different reference potentials and uses the binary signal to toggle the multiplexer between those potentials to drive a voltage-to-frequency converter included in one of the analog-to-digital converter stages while enabling a data transmitter, thereby causing the resulting modulated signal within a predetermined frequency band to be relayed to the master unit. In a fallback mode, failure of the microprocessor is accommodated by shifting control of the multiplexer to a preprogrammed mode scanner which sequentially switches the multiplexer between its several input data signal ports while the data transmitter is held in an enabled condition, thereby directly transmitting the incoming data signals within the predetermined frequency band to the master data processing unit. 
     The master processing unit includes a detector which signals a central microprocessor based telemetry receiver upon reception of a signal within the predetermined frequency band and a signal decoder which applies data signals extracted from that frequency band directly to the microprocessor. In a fallback mode, the master unit accommodates direct transmission of data signals in the predetermined frequency band by sequentially converting those signals via a frequency digitizer into binary data which is then applied to the microprocessor based telemetry receiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of this invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 is a single line block diagram schematic of an embodiment of a data acquisition system. 
     FIG. 2 is a single line block diagram showing the local portion of the system in one phase of a normal data acquisition mode. 
     FIG. 3 is a single line block diagram of the local portion of the system in another phase of a normal data acquisition mode. 
     FIG. 4 is a single line block diagram of the system in a normal transmit mode. 
     FIG. 5 is a single line block diagram of the system in one phase of a fallback mode. 
     FIG. 6 is a single line block diagram of the system in another phase of a second fallback mode. 
    
    
     DETAILED DESCRIPTION 
     Refer now to FIG. 1 where a local unit 10 of a data acquisition system is connected via a single pair cable 12 to a distantly located master data processing unit 14. Local unit 10 includes an analog multiplexer 16 having a plurality of input ports 18 coupled to receive incoming data signals from groups of numerous local sensor channels 20, only one of which is shown, and remote sensor channels 22, only one of which is shown. Two groups of data signal links are represented by the two channels shown. The first channel, 20, couples a sensor 24 which is disposed in relative proximity to the network, and connected through its local connecting link 25 via a dedicated amplifier 26 to one of the input ports 18. In the second type, a sensor 28 is disposed relatively remotely from local unit 10 and is coupled to multiplexer 16 via a remote connecting link 29 which includes a voltage-to-frequency encoder 30, an inductive coupler 32, a decoder 34, a voltage normalizer 36 and one of the remote data signal channels 22. Voltage normalizer 36 is coupled to receive a low reference potential 38 and a high reference potential 40, and serves to convert digital data signals in a variable frequency low signal band (e.g., less than 150 Hertz) established by the encoder and decoders of connecting link 29 to either a low reference level or a high reference level, thereby normalizing the voltage amplitudes of data signals emanating from the remote sensor decoders. Data signals arriving at multiplexer 16 via local signal channels 20 are in an analog voltage format while data signals arriving via remote signal channels 22 are normalized square wave signals alternating between the reference voltage amplitudes at a rate within the variable frequency low band at a frequency proportional to the value of the analog signal emanating from sensor 28. 
     Periodically, a microprocessor based telemetry encoding and transmitting logic unit 42 sequentially scans the input channels of multiplexer 16 via a multiplexer control 44, thereby sequentially connecting input ports 18 to an output port 46 of the multiplexer. The microprocessor based logic unit 42 is a microprocessing stage provided by a commercially available unit such as a model NSC 800 available from National Semiconductor. Output port 46 is coupled to a voltage-to-frequency converter 48 operating in a variable frequency high band (e.g., 3000 to 5000 Hertz) and a voltage level comparator 50. Converter 48 is coupled to both the input port of a data transmitter 52 and a waveform shaper 54. The waveform shaper is in turn coupled to one input port 56 of a frequency digitizer 58. The output of comparator 50 is coupled to a second input port 60 of frequency digitizer 58. The input of signals via ports 56, 60 into frequency ditigizer 58 is controlled by microprocessor stage 42 via control leads 62 to control admission of input data via either port 56 or 60 to the exclusion of input data arriving via the other of those two ports. Frequency digitizer 58 converts frequency domain variable frequency high and low band signals into binary format in a gated reference frequency counting measurement technique using an external crystal 64 as an accurate frequency reference. In combination, waveform shaper 54 and frequency digitizer 58 serve to provide an analog-to-digital conversion of data signals arriving from multiplexer 16 via voltage-to-frequency converter 48 while signals arriving at input port 60 are normalized by comparator 50 to voltage amplitudes conforming to input specification levels of digitizer 58 and microprocessor stage 42. As signals arriving via input port 60 are in a variable frequency low signal band while those arriving via port 56 are in a variable frequency high signal band, microprocessor stage 42 controls the input frequency counting interval utilized by frequency digitizer 58 as a part of the frequency digitizing cycle. Data signals are applied to microprocessor stage 42 by frequency digitizer 58 via data leads 66 as the input data signals are converted into a binary signal format. 
     In addition to providing sequenced addressing of input ports 18 for multiplexer 16 and input ports 56, 60 for digitizer 58, the microprocessor based telemetry encoding and transmitting logic unit 42 performs processing and temporary storage for the data signals in their binary format. 
     As a regular operational function, microprocessor unit 42 conducts a self-testing routine to discover the occurrence of internal failure. Upon successful conclusion of that routine, microprocessor stage 42 generates a watchdog timing pulse via lead 68 to watchdog timer 70. Unsatisfactory completion of the self-testing routine prevents generation of the watchdog timing pulse. The omission of the timing pulse during a subsequent interval of predetermined length triggers a shift in the operational mode of network 10 from its normal data acquisition and transmitting mode to a fallback mode and allows watchdog timer 70 to generate a mode-shifting signal via leads 72 to control operation of multiplexer control 44, fallback mode scanner 74 and local power switch 76. Fallback mode scanner 74 is preprogrammed by means such as mode patch jumpers 78, to sequentially address, via multiplexer control 44, and connect input channels 18 to output channel 46 in multiplexer 16. Reception of the mode shifting signal from timer 70 causes multiplexer control 44 to allow input port address signals from mode scanner 74 to address input ports 18 in multiplexer 16. To assure deactivation of the microprocessor unit, timer 70 also applies the mode-shifting signal to local power switch 76, thereby removing power to microprocesser stage 42 to assure its complete disablement. The absence of power to the microprocessor stage 42 removes its ability to inhibit the activation of data transmitter 52, which is the mode of control used, thereby enabling direct transmission of data signals to master unit 14 via voltage-to-frequency converter 48. 
     Master data processing unit 14 includes a microprocessor based telemetry receiver and data processor 80. Voltage-to-frequency converter 48 converts signals to be transmitted via data transmitter 52 and cable pair 12 to variable frequency signals within the variable frequency high band. A detector 82 responds to reception of signals in the variable frequency high band by providing a carrier detect indication signal which causes telemetry receiver 80 to either receive transmission via frequency shift key decoder 84 or to enable frequency digitizer 86 to alternatively convert direct transmission of data into binary data via frequency shaper 88 or decoder 84 during a fallback mode transmission. Frequency digitizer 86 is a gated reference frequency counting stage using an external crystal 90 to provide accurate binary conversion of data signals in either the variable frequency high or variable frequency low bands. Telemetry receiver data processor 80 controls operation of decoder 84, digitizer 86, and frequency shaper 88 to permit reception of data signals from network 10 during its normal transmission mode and during both phases of its fallback transmission mode. Binary data received and processed by receiver 80 is then applied via output port 92 to such types of external networks as data interfaces or central control systems (not shown). 
     FIG. 2, which shows local network 10 in the first phase of a normal data acquisition mode, uses (as do FIGS. 3 through 6) conventional single lines interconnecting the several elements to indicate the transmission of control signals over those lines and chickening of single lines to indicate transmission of data. In the mode shown, microprocessor stage 42 controls addressing of the input ports of multiplexer 16 via control 44. Control signals passing between microprocessor 42, multiplexer control 44 and multiplexer 16 sequentially address the input ports 18. Microprocessor unit 42 is shown in FIG. 2 addressing one of the several input ports 18 conveying low level data signals emanating from a locally deployed sensor 24. Simultaneously, microprocessor unit 42 controls frequency digitizer 58 to accept signals via input port 56. The low level analog signals connected by multiplexer 16 to output port 46 are changed by voltage-to-frequency converter 48 into variable frequency signals within the variable frequency high band. In the mode shown, data transmitter 52 is not enabled and therefore serves to isolate master data processing unit 14 from reception of variable frequency signals emanating from converter 48. The variable frequency signals within the high band are converted by the conjugate action of wave shaper 54 and frequency digitizer 48 into binary signals which are received, processed and temporarily stored by microprocessor stage 42. 
     In the second phase of the normal data acquisition mode, shown in FIG. 3, microprocessor stage 42 simultaneously addresses those input ports 18 receiving frequency shift keyed data signals in the variable frequency low band via channels 22 and enables frequency digitizer 58 to receive signals via input port 60. As the variable frequency low band input channels 22 are sequentially addressed and connected to output port 46, comparator 50 normalizes the voltage amplitudes while frequency digitizer 58 converts the normalized signals into binary data which is then received, processed and temporarily stored in an internal memory of microprocessor stage 42. It may be noted that the high and low reference levels applied by normalizer 36 to the variable frequency low band signals differs from the input specification tolerances of frequency digitizer 58 and microprocessor unit 42, thereby necessitating the further normalization step with comparator 50. It is noted that in the modes shown in FIGS. 2 and 3, microprocessor stage 42 controls the input frequency counting interval utilized by frequency digitizer 58 for digitizing the data signals arriving via input ports 56, 60. This assures that the data signals supplied by frequency digitizer 58 have compatable ranges in the binary format when they are received by microprocessor stage 42. 
     In its normal data transmission mode, shown in FIG. 4, microprocessor stage 42 continually applies an address control signal via multiplexer control 44 to multiplexer 16. Simultaneously, microprocessor unit 42 applies the binary data signals via control 44 to multiplexer 16 which, in conjunction with the address control signal, allow the binary data signals to alternately address input ports 94, 96 coupled to the low 38 and high 40 reference potentials, respectively. This conjugate addressing of the input ports receiving the low and high reference potentials causes a binary data stream alternating between those potentials to be applied to converter 48, thereby producing a frequency shifted data signal within the variable frequency high band. While conjugately addressing multiplexer 16, microprocessor stage 42 controls local power switch 76 to continuously enable data transmitter 52, thereby causing the variable frequency high band encoded signals to be transmitted to master data processing unit 14. 
     Receiver and data processor 80 is programmed to receive incoming data signals via frequency shift decoder 84 during normal mode data transmission. Consequently, when carrier detector 82 senses the tone of the variable frequency high band signal, its application of a carrier detect signal to processor 80 alerts the processor to begin receiving and processing data received via decoder 84. In effect, decoder 84 converts the frequency shift keyed data signals within the variable frequency high band to binary data in a form acceptable to processor 80. Processor 80 is programmed to receive data signals via decoder 84 in the same sequence as those signals are transmitted by microprocessor stage 42. To enable processor 80 to distinguish between signals originating from different sensors, microprocessing stage 42 addresses the ZERO, or off, input port 98 of multiplexer 16 at the conclusion of transmission of each data signal, thereby disconnecting output port 46 from all of the input ports and interrupting the transmission of data and carrier signals to converter 48. Consequently, the output of converter 48 and transmission from transmitter 52 is temporarily blanked, thereby alerting processor 80 via carrier detector 82 to the completion of transmission of one data signal and the eminent transmission of the next signal within the transmission sequence. At the conclusion of the blanking interval, microprocessor stage 42 again begins addressing the low and high reference potentials applied to input ports 94, 96 with the conjugate action of its address control signal and the next binary data signal in the transmission sequence. Upon resumption of transmission, detector 82 again alerts processor 80 with a carrier detect signal, thereby causing processor 80 to resume reception of the binary data signal converted by decoder 84. 
     Operational failure of either microprocessor stage 42 or its control program will be detected by its periodic self-checking routine. Upon its conclusion, the self-checking routine emits a watchdog time pulse via lead 68, to timer 70. Detection of any failure within processor stage 42 prevents the self-checking routine from running to its conclusion. Consequently, the occurrence of a partial failure of either stage 42 or its control program will prevent the self-checking program from running to its conclusion and its generation of a watchdog time pulse. Timer 70 is cyclically reset by the watchdog time pulse; its absence will cause timer 70 to generate the mode shifting signal via leads 72 to deactivate microprocessor stage 42 by toggling switch 76 to remove the local power source from processor stage 42. Loss of power to microprocessor stage 42 removes its ability to inhibit the activation of data transmitter 52, which is therefore enabled for fallback mode operation. The mode shifting signal also causes control 44 to shift control of the multiplexer addressing function from microprocessor stage 42 to fallback mode scanner 74. When enabled by the mode shifting signal, scanner 74 addresses input ports 18 in fixed, predetermined sequence. 
     As shown in FIG. 5, scanner 74 is addressing one of the input ports 18 receiving an analog signal emanating from a local sensor 24. This signal is coupled to output port 46 and changed to a variable frequency within the variable high frequency band by converter 48, amplified by transmitter 52 and sensed by carrier detector 82. Initially, telemetry receiver 80 attempts to receive these data signals via frequency shift decoder 84. The absence of frequency shift however causes the output from decoder 84 to remain in a single binary condition, an event which is subsequently detected by processor 80 during generation of the carrier detect signal. Ultimately, processor 80 reverts to a fallback reception mode patterned upon the same transmission sequence as that employed by scanner 74. In this mode, processor 80 controls frequency digitizer 86 to sequentially receive through its variable frequency high terminal 100, a preset number of incoming data signals via frequency shaper 88. In combination, frequency shaper 88 and digitizer 86 convert the variable frequency high band signals into binary data which is then applied directly to processor 80. Scanner 74 addresses ZERO input port 98 at the conclusion of acquisition and direct transmission of each data signal, thereby signalling processor 80 via detector 82 of the eminent transmission of the next data signal within the sequence. 
     Refer now to FIG. 6. After detecting a preset number of blanking intervals during a sequence while in the fallback mode, processor 80 is alerted to the eminence of a shift in the format of data signals being acquired from the remote channels 22 and directly transmitted by local unit 10. Then, processor 80 enables frequency digitizer 86 to receive and convert incoming data signals via decoder 84 and variable frequency low input terminal 102. Simultaneously, scanner 74 continues through its data acquisition sequence by next addressing each input port 18 coupled to a normalized variable frequency low band signal. The scanner 74 also has the ability to lengthen the time during which each channel of the variable frequency low band signals is connected to the voltage to frequency converter 48. The normalization of these variable frequency low band signals by normalizer 36, in combination with the frequency conversion of those signals by converter 48 causes those signals to be amplified and transmitted by transmitter 52 as frequency shift keyed signals in the variable frequency high band. After decoding by shift key decoder 84 however, the resulting data signals must be converted at a different gating rate by digitizer 86 before application to processor 80. Consequently, the data signals are applied to digitizer 86 via input port 102 for binary conversion. After sequentially coupling all input ports 18 receiving both low level analog signals emanating from locally disposed sensors 24 and normalized variable frequency low band signals derived from remote sensors 28, scanner 74 precipitates a final blanking interval before re-initiating its preset scanning sequence. In this manner, network 10 assures continuous, uninterrupted data transmission from both the local and remote sensors even though the network has suffered a partial failure in its microprocessing stage. Data acquisition and transmission continues despite the differences between the signal formats produced by the local and remote sensor units. 
     It is apparent from the foregoing paragraphs that the invention disclosed provides a data acquisition system and method for accurate local acquisition, processing and transmission of data signals emanating from multiple variable frequency encoded data sources over a plurality of distinct and widely separated frequency bands. Moreover, reliable acquisition and transmission of data emanating from all of the data sources is accurately continued even after occurrence of failure of the microprocessor functions, through direct transmission of data signals in unprocessed form with direct variable frequency encoding of data signals acquired on higher frequency channels and frequency shift keying in the same variable frequency band of data signals acquired over low frequency channel.