Abstract:
A load management system which includes a central message generator and a plurality of addressable remote load controllers which selectively connect and disconnect high power deferrable loads to and from a power source in response to transmitted messages. The load controllers include means for translating coded tone pair inputs into digital data. Tones selected from three such tone pairs are used in one scheme, where a tone selected from the first tone pair is used for the initial bit of a message, and subsequent tones are alternately selected from the remaining two tone pairs or the remaining bits. One of the tones of the first tone pair is utilized as a test tone which initiates a test routine sequence. The test tone can be transmitted by a portable, low power transmitter to test the functioning of the remote units. A message format includes two code sets, a zone code set and a command/address code set. Each load controller has a preprogrammed zone identifier and a preprogrammed address identifier, and is responsive to a command/address code message only when the last received zone code message has identified the preprogrammed zone identifier of that load controller and the command/address message indicates the preprogrammed address identifier of that load controller. All load controllers having a common zone identifier are responsive to a scram instruction message which identifies that zone.

Description:
This application is a division of application Ser. No. 125,285, filed Feb. 27, 1980, and issued into U.S. Pat. No. 4,352,992 on Oct. 5, 1982. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the invention relates to apparatus for addressably controlling remote units. The preferred embodiment is an apparatus which is useful in an electric power company load management system which includes a central message generator, and a plurality of addressable load controllers which selectively connect and disconnect high power deferrable loads to and from a power source in response to transmitted messages. 
     2. Description of the Prior Art 
     The power systems of electric power companies regularly encounter periods of peak load demand. During these periods the power drain placed upon the power system is significantly higher than average. The peak load periods vary according to the time of day and the seasons of the year. It is excessively costly to maintain the supply of power during these peak load periods, but if the power demand is not met the result can be power failure or blackout. 
     In a load management system, the power supply and demand are monitored, and certain high power deferrable loads, such as water heaters and air conditioners, are selectively turned off to evenly distribute the power demand over time, thereby alleviating peak load periods. The result is a more efficient and less costly power system. It is also desirable for a load management system to be able to scram, or disconnect, a large number of deferrable loads in a short period of time to avoid the occurrence of a blackout. 
     Apparatus for addressably controlling remote units have been used in other fields, such as for telephone paging, burglar alarm systems, remote control for repeater systems, etc. However, these units operate with schemes which often require relatively long message times, or expensive receiving equipment, or a small number of addressable units. Moreover, many of these systems have a significant problem with noise interference. 
     SUMMARY OF THE INVENTION 
     To obtain an accurate understanding of the scope of the invention, reference should be made to the claims of this application. In certain embodiments, there is a load management system which includes a central message generator and a plurality of addressable remote load controllers which selectively connect and disconnect high power deferrable loads to and from a power source in response to transmitted messages. The load controllers include means for translating coded tone input into digital data. Three tone pairs are used in one scheme, where a first tone pair is used for the initial bit of a message, and the remaining two tone pairs alternate for the remaining bits. One of the tones of the first tone pair is utilized as a test tone which initiates a test routine sequence. The test tone can be transmitted by a portable, low power transmitter to test the functioning of the remote units. A message format includes two code sets, a zone code set and a command/address code set. Each load controller has a preprogrammed zone identifier and a preprogrammed address identifier, and is responsive to a command/address code message only when the last received zone code message has identified the preprogrammed zone identifier of that load controller and the command/address message indicates the preprogrammed address identifier of that load controller. All load controllers having a common zone identifier are responsive to a scram instruction message which identifies that zone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a load management system embodying the present invention. 
     FIG. 2 is a schematic representation of the addressing system of the load controllers of FIG. 1. 
     FIG. 3 is a schematic diagram of a load controller of FIG. 1. 
     FIGS. 4, 5 and 6 are graphs, illustrating a base period of a window input sequencer 25 of FIG. 3, the base period being divided up into eighteen time windows. FIG. 4 shows a phase relationship of an input frequency with respect to the base period and the eighteen time windows, where the input frequency has a period which is one-half of the base period of the window input sequencer (twice the frequency). FIG. 5 illustrates the phase relationship where the input frequency has three times the frequency of the window input sequencer. FIG. 6 illustrates the phase relationship of a non-multiple frequency. 
     FIGS. 7, 8, 9, 10 and 11 are graphical illustrations of examples of tones and derived digital data which may be accumulated in windows 1-18 of FIG. 3. FIG. 7 illustrates the accumulated data for a tone which has a base period which is one-half the base period of the window input sequencer (twice the frequency). FIG. 8 illustrates the digital data accumulated where the input frequency is three times the frequency of the window input sequencer. FIG. 9 is an example of a double frequency where there is a crossover point which is at the threshold between two windows. FIG. 10 illustrates a triple frequency input with outside noise interference. FIG. 11 is illustrative of the digital data accumulated where the input frequency is not one of the frequencies which are being searched for. 
     FIG. 12 illustrates the command/address code format utilized in the preferred embodiment of the present invention. FIG. 13 illustrates the zone code format which is also utilized in the preferred embodiment. 
     FIG. 14 is schematical illustration of the digital controller 40 of FIG. 3 with relays 65 and 66 and test light 64. 
     FIG. 15 illustrates the load controller 20 of FIG. 3 with relays 65 and 66, and with portable test transmitter 99 transmitting a test tone in proximity to the load controller. Also shown in FIG. 15 is the message generator 11 and message transmitter 13 of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic illustration of a load management system embodying the present invention. Such a system functions to selectively control the load for a power system, such as for an electric power company system of a metropolitan area. By selectively controlling the loads, such a load management system can prevent and diminish the occurrences of peak load demands which are both costly and a potential cause of blackouts. The power system includes a power generating souce 15, connected to a series of loads L1A, L1B, L2A, L2B, . . . LNA and LNB through power distribution lines 16. A power load monitor 17 monitors the power supply by power generating source 15 and the load demand of loads L1A through LNB through line 18. Message generator 11 is responsive to power load monitor 17. During various power-load conditions message generator 11 transmits messages to control the overall load which is being placed upon the power generating source 15. Such messages are transmitted through telephone transmission line 12. Effective transmission over telephone lines is restricted to tone frequencies between 40 Hz and 1300 Hz. Messages generated by message generator 11 are received by message transmitter 13 through telephone line 12. Message transmitter 13 transmits the messages on an FM frequency. These messages are transmitted from antenna 14 and are received by each of the load controllers LC1 through LCN. In response to appropriate received messages, a load controller, such as load controller LC1, selectively connects and disconnects loads L1A and L1B to and from power generating source 15 through switch means S1A and S1B respectively. 
     Each of the load controllers is responsive to appropriate messages only when an appropriate identification has also been transmitted. FIG. 2 is illustrative of the identification scheme which is used in the preferred embodiment. Each load controller has two preprogrammable identification numbers. The first identification number is a zone identifier which is a six bit binary number. One of the zone identifier numbers is not used for identification purposes, but rather is used to disable future messages. Therefore there are sixty-three zone identification numbers which are available, enabling the power system to be broken down into up to sixty-three zones, such as zone Z1, zone Z2 . . . zone Z63. 
     The address identifier is a five bit binary number, thereby allowing up to thirty-two addresses within each zone. Utilizing the full available sixty-three zones, and all available addresses, a power system can have as many as two thousand sixteen individual unique identification numbers for the load controllers. In a further expanded system, a number of load controllers can each have the same zone and address identification. Such an expanded system would be most effective where the load controllers are evenly divided between the number of available zones, and between the number of available addresses within each zone. 
     FIG. 3 schematically illustrates a load controller 20 of FIG. 1. FM receiver 21 receives radio frequency transmissions from message transmitter 13 through antenna 22. The output of FM receiver 21 are the audio tone signals which were initially generated by message generator 11. This output is transmitted through line 22 to crossover detector 23. The function of crossover detector 23 is to generate an output in response to the detection of a particular phase in the cycle of the incoming tone through line 22. In the preferred embodiment, the phase of the cycle that is detected is the crossover point from a positive state to a negative state in the cycle. Alternatively, both positive to negative and negative to positive may be searched. As an additional alternative, positive and/or negative peaks may be sensed. These alternatives, however, add to the complexity of the crossover detector. 
     Window input sequencer 25 repetitively and sequentially applies the output (line 24) of crossover detector 23 to WINDOW 1 (W1) through WINDOW 18 (W18) in data accumulator 26. Included in window input sequencer 25 is means for determining the number of windows which are to be sequenced, and means for determining the time length duration of each window. Window input sequencer 25 possesses the capability of making time length determinations by being connected to clock 27 (line 28). By so selecting the number of windows to be sequenced and the time length of each window, window input sequencer 25 determines a base period or cycle length (and its associated repetition frequency). When referred to at later times in this application, such a base period shall mean the length of time required for window input sequencer 25 to cycle through one complete cycle of sequenced windows. 
     The accumulated data in data accumulator 26 is analyzed by data analyzer 30. The specific data analysis whch is performed by data analyzer will be discussed in detail later in this specification. Generally, data analyzer 30 examines the accumulated data in data accumulator 26 for appropriate repetitious occurrences in the windows, WINDOW 1 through WINDOW 18, to determine whether one of two searched-for tones has been received by FM receiver 21. This analysis is initialized when window input sequencer has received an appropriate amount of data (line 29). When one of the two searched for tones has been found, data analyzer 30 transmits a digital bit to digital controller 40. A bit may consist of either a logic high (line 33) or a logic low (line 32). Digital analyzer 30 then reinitializes window input sequencer 25 for the collection of data for the next sequential bit in a message. Data analyzer 30 is also time sensitive, being connected to clock 27 (line 31). If no appropriate search for a tone is detected within a specific period of time, data analyzer 30 signals window input sequencer 25 to be reinitialized to gather data relating to searching for an initializing bit (line 29). 
     Digital controller 40 includes means for decoding digital messages and for responding to appropriate message transmissions. Digital controller 40 includes a preprogrammable six bit zone identifier, which consists of jumper wire connections 41 through 46, and also includes a preprogrammable five bit address identifier, which consists of jumper wire connections 51 through 55. These jumper wire connections may be readily manually disconnected or reconnected by an operator. Digital controller 40 is also time sensitive, being connected to clock 27 (line 36). In response to appropriate message transmissions, digital controller 40 has the capacity to selectively control the opening and closing of relay 1 (65) and relay 2 (66) through lines 62 and 63, respectively. In response to an appropriate test signal, digital controller 40 also has the capability of turning on test light 64 (line 61) when a test routine has indicated that the system is working properly. The internal functioning of digital controller 40 is disclosed in more precise detail in FIG. 12, and will be discussed in a later portion of this application. 
     FIGS. 4, 5 and 6 illustrate the general means by which detection of one of two tones is accomplished. Each of these figures show a base period BP for the window input sequencer (25) of FIG. 3. Base period BP is divided into eighteen time sequential windows, W1 through W18. FIG. 4 illustrates the wave pattern of an input frequency of a tone which has a period which is one-half the base period BP of the window input sequencer (twice the frequency). The wave of this input tone repetitively forms the same pattern for each time window of the base period. The crossover point from positive to negative for each cycle occurs at the threshold between windows W5 and W6, and at the threshold between windows W14 and W15 during each cycle of the window input sequencer. 
     FIG. 5 illustrates the resulting wave form of a triple frequency input. The crossover point for such a tone repetitively occurs at windows W4, W10, and W16. FIG. 6 illustrates a non-multiple frequency. The crossover points for such a frequency tend to occur with an even distribution over all of the windows W1 through W18. 
     During the specific tone detection procedure in the preferred embodiment, window input sequencer 25 inputs 16 detections by crossover detector 23 into data accumulator 26. FIGS. 7-11 illustrate various examples of data inputs which may be accumulated by data accumulator 26 during this period. In each of FIGS. 7-11, the numbers from left to right represent each of the eighteen windows W1-W18 in data accumulator 26. The boxes above each number indicate data inputs, and represent the crossover points from a positive state to a negative state which have been detected by crossover detector 23. In the preferred embodiment, sixteen such data inputs are accumulated before the data is analyzed by data analyzer 30. 
     FIG. 7 is an example of a double frequency input data (frequency of the input tone is twice the frequency of the window input sequencer 25). These sixteen bits of input are evenly divided between windows three and 12, with eight bits being in window three and eight bits being in window 12. FIG. 8 is an example of a triple frequency input. There are six bits in window 2, and five bits each in windows 8 and 14. FIG. 11 illustrates the relative even distribution which occurs when a non-multiple frequency input is received. 
     By checking for repetition occurring in each ninth window, or occurring in each sixth window, data analyzer 30 can determine the existence of a double frequency input or the existence of a triple frequency input respectively. This repetition can be detected by accumulating the number of bits in each ninth window, and in each sixth window respectively. Chart 1 is an example of repetition detection of the double frequency input of FIG. 7. 
     
                       CHART 1______________________________________Window         DataCombination    Accumulation______________________________________1 and 10       02 and 11       03 and 12       164 and 13       05 and 14       06 and 15       07 and 16       08 and 17       09 and 18       0______________________________________ 
    
     The existence of all sixteen bits in windows three and twelve indicates the presence of a double frequency. After this detection of a search for a frequency, data analyzer 30 transmits the appropriate logic signal to digital controller 40. Chart 2 illustrates the data analysis detection of the triple frequency input of FIG. 8. 
     
                       CHART 2______________________________________Window         DataCombination    Accumulation______________________________________1, 7 and 13    02, 8 and 14    163, 9 and 15    04, 10 and 16   05, 11 and 17   06, 12 and 18   0______________________________________ 
    
     There is a possibility that the input frequency may be in a phase relationship with respect to the cycle of the window input sequencer 25 such that the crossover point from a positive state to a negative state occurs at the threshold between adjacent windows. When this occurs, the data input may be distributed between adjacent windows. To detect double and triple frequencies when this type of event occurs, the data analyzer 30 of the preferred embodiment checks for repetition occurrences which include the checking of adjacent windows. The following chart 3 illustrates the window combinations which are checked by data analyzer 30, and also illustrates the results which would occur for a double frequency detection with the data input of FIG. 9. FIG. 9 illustrates the data input of a double frequency tone where the crossover point occurs at a threshold between windows 4 and 5, and windows 13 and 14. 
     
                       CHART 3______________________________________Window         DataCombination    Accumulation______________________________________1, 2, 9 and 10 02, 3, 10 and 11          03, 4, 12 and 13          84, 5, 13 and 14          165, 6, 14 and 15          86, 7, 15 and 16          07, 8, 16 and 17          08, 9, 17 and 18          09, 10, 18 and 1          0______________________________________ 
    
     Owing to radio interference and other signal distorting causes such as noise, sometimes the input signal may not be precisely received by FM receiver 21. To account for this, and to allow for detection of a bandwidth of frequencies, the data analyzer 30 of the preferred embodiment does not require that all sixteen bits be found in the appropriate windows. It has been found that for the frequencies which are used in the preferred embodiment, that the detection of eleven or more bits in an appropriate window combination is indicative of a searched for tone. This represents slightly more than 65% of the theoretical number of 16 bits which should be in the appropriate windows. It is desirable for a practical system that this percentage be at least as low as 90% but not as low as 50%. Chart 4 illustrates a triple frequency input detection of data input illustrated in FIG. 10. This data input includes various erroneous signals, yet the appropriate triple frequency signal is detected. 
     
                       CHART 4______________________________________Window           DataCombination      Accumulation______________________________________1, 2, 7, 8, 13 and 14            22, 3, 8, 9, 14 and 15            13, 4, 9, 10, 15 and 16            34, 5, 10, 11, 16 and 17            125, 6, 11, 12, 17 and 18            11______________________________________ 
    
     The following chart 5 illustrates the double frequency detection for the data input of FIG. 10, and shows that even though the signal is distorted, a false double frequency tone is not detected. 
     
                       CHART 5______________________________________Window         DataCombination    Accumulation______________________________________1, 2, 9 and 10 42, 3, 10 and 11          33, 4, 12 and 13          24, 5, 13 and 14          5______________________________________ 
    
     There being no combination producing an accumulation of eleven bits or more, a double frequency is not detected. The following two charts illustrate the non-detection of a double frequency (chart 6) and the non-detection of a triple frequency (chart 7) from the data input which is illustrated in FIG. 11. 
     
                       CHART 6______________________________________Window         DataCombination    Accumulation______________________________________1, 2, 9 and 10 22, 3, 10 and 11          13, 4, 12 and 13          14, 5, 13 and 14          25, 6, 14 and 15          26, 7, 15 and 16          27, 8, 16 and 17          28, 9, 17 and 18          29, 10, 18 and 1          2______________________________________ 
    
     
                       CHART 7______________________________________Window           DataCombination      Accumulation______________________________________1, 2, 7, 8, 13 and 14            62, 3, 8, 9, 14 and 15            53, 4, 9, 10, 15 and 16            54, 5, 10, 11, 16 and 17            55, 6, 11, 12, 17 and 18            6______________________________________ 
    
     To avoid false detection of a subharmonic tone, a timer may be checked after the 16 bits have been checked to verify that the proper frequency range was received. The data analysis which has been described above has been illustrated with double frequencies and triple frequencies both for ease of illustration of the concept and because these are the multiples which are utilized in the preferred embodiment. Other multiple frequencies could be utilized and still fall within the spirit of the present invention. For simplification of the analysis in the data analyzer 30, it is desirable that the multiples of the multiples frequencies sensed are primal with respect to each other and are divisible into the number of windows which are sequenced. Also, it should be noted that the above described embodiment could be adopted to include the detection of more than two frequencies for a single bit of information. Where more than two frequencies are used for a single bit, the information contained in a bit depends upon the number of frequencies in the set. For instance, an embodiment using three frequencies could be based upon trinary logic, and an embodiment with eight frequency sets could utilize an octal system. The base period should be an integer fraction times the frequency to be sensed, more preferably an integer times the frequency to be sensed. 
     It is preferred that the sequencing through the windows be at a regular rate, although syncopated or other methods may alternatively be used. 
     By detecting both a logic high frequency and a logic low frequency simultaneously, the load controllers of the present invention can receive digital messages without the need of synchronous timing for the reception of the data bits. After a sensed-for window input sequencer 25 immediately starts collecting data for the reception of the next bit in the message. In order that the continuation of the just detected tone does not cause incorrect detection of a false bit of information, different base frequencies for the window input sequencer 25 are used for any two consecutive bits of information to be received, and thus different tone pairs are detected for consecutive bits. Different base frequencies are selected by altering the base period of time over which window input sequencer 25 sequences the 18 windows. Alternatively, the number of windows sequenced could be altered. It should also be noted that the X and Y multiples could be changed over consecutive bits to achieve sensitivity of different tone pairs. To aid in ensuring the detection of a sensed-for tone, in the preferred embodiment the tones are transmitted for a period of time which is at least twice as long as the period of time which it takes for the transmitted tone to trigger crossover detector 16 times. The length of transmission of each tone input in the preferred embodiment is 47 milliseconds. 
     In the preferred embodiment three sets of tone pairs are utilized. The first set of tone pairs provides for the detection of an initializer bit. The remaining two frequencies alternate for the remaining bits of a message. The following chart 8 illustrates the window input sequencer base frequencies and the sensed frequencies which are used for a ten bit message. 
     
                       CHART 8______________________________________  Window Input  Sequencer    Double       TripleBit    Frequency (Hz)               Frequency (Hz)                            Frequency (Hz)______________________________________1      414.30       828.60       1242.902 4 6 8 10  382.43       764.86       1147.293 5 7 9  355.11       710.22       1065.34______________________________________ 
    
     One restriction in the preferred embodiment is that the sensed tones be in the range of 400 to 1300 Hz, so that they may be easily transmitted through telephone lines without significant distortion. Other embodiments of the present invention need not comply with this restriction, depending upon the particular application. 
     The particular frequencies which are used were determined to some extent by the base internal clock mechanism of the preferred embodiment which is a crystal which operates at 3,579,545 Hz. This is a commonly used crystal and is readily available at a relatively inexpensive price. The frequency of this crystal is prescaled to achieve the time duration of each of the windows in the data accumulator 29 by so prescaling, and by selecting the number of windows which are used, the window input sequencer 25 can select its base frequency. 
     There are two message formats which are both used in the preferred embodiment. A first format shall be referred to as a command/address code set and consists of a ten bit binary message. This format is illustrated in FIG. 12. The first bit is start/test bit. The reception of a logic low for bit 1 starts the message reception sequence. The reception of a logic high for bit 1 initializes a test routine. The second bit is a code indicator bit, and indicates that the remaining bits of the message transmitted will be either in the command/address format or in the zone code format. Bits 3 through 5 indicate a command. Bits 6 through 10 indicate a specific address. 
     The second message format shall be referred to as a zone code format, and is illustrated in FIG. 13. Bits 1 and 2 of the zone code format are the same as bits 1 and 2 of the command/address format. Bit 3 of the zone code format is a scram/enable and disable bit. This bit indictes whether a scram message is being sent or whether the zone code message transmitted is for the purpose of enabling the load controllers of a particular zone to be receptive to subsequential command/address messages. Bits 4 and 5 of the zone code format identify one of up to sixty-three zones. In the preferred embodiment of the present invention, a sixty-four zone code is transmittable in two bits by preprogramming the load controllers for the sequential reception of one of eight frequencies for two sequential bits. The transmission from central message generator unit 11 transmits up to sixty-four combinations over for the two bits by transmitting one of eight frequencies each bit. Therefore effectively an octal format is achieved, however the individual controllers need only be sensitive to one frequency for a bit, therefore the reception is binary. The eight frequencies which ae used by message generator unit 11 are shown in chart 9. 
     
                       CHART 9______________________________________Two Tone Sequential Frequencies (Hz)______________________________________721.68745.74799.00860.47894.88932.17972.701016.72______________________________________ 
    
     FIG. 14 is a schematic illustration of the message decoding which is accomplished by digital controller 40, and also illustrates relays 65 and 66, and test light 64. Bits 1 through 10 are generally illustrated in a time sequence from the top to the bottom of the figure. 
     Bit 1 indicates either that a test routine is being requested or acts as an initializer (box 110). If a test is indicated, then test routine is activated (box 115) in which the relay activation circuits are tested and, if found to be functioning properly, test light 64 is turned on to indicate that the unit is in working order. 
     For the zone identification portion of the message, load controller 20 is preprogrammed to be sensitive to two sequential frequencies selected from the above eight, depending upon the combination of jumper wires 41 through 46 which are connected on the unit. Sixteen iuputs are used, as is the case for the other bits, however only eight windows are sequenced, and the load controller is sensitive to only one frequency for each bit. If the first bit is a message initializer, then bit 2 is analyzed for code identification (box 120). If a command/address code format is indicated (FIG. 13) then the subsequent transmitted command (box 130) and address (box 160) are analyzed and responded to. However, this is only performed if the digital controller 40 has been previously been enabled to receive and respond to command/address messages (box 225). When a command has been given and an appropriate address identification transmitted, then relays 65 and 66 are selectively controlled by relay controller 300. In one embodiment of the present invention, the command signals available are: 
     
                       CHART 10______________________________________Three BitSignal    Command______________________________________000       Activate relay 1 with time out of 7.5 Min.001       Activate relay 1 with time out of 15 Min.010       Activate relay 2 with time out of 7.5 Min.011       Activate relay 2 with time out of 15 Min.100       Restore load on relay 1101       Restore load on relay 2110       Activate both relays with time out of 7.5 Min.111       Activate both relays with time out of 15 Min.______________________________________ 
    
     When a zone code transmission is indicated (box 120) bit 3 is analyzed to determine whether the message transmission is a scram transmission or an enable/disable transmission (box 230). Bits 4 and 5 are then analyzed (boxes 240 and 250) to determine whether the zone code transmitted matches the preprogrammed zone identifier of load controller 20 (jumpers 41 through 46). If the proper signals have been received and decoded (box 245) then subsequent address commands are enabled (box 225). If a scram indication has been sent (box 230) and proper zone identification signals decoded (box 245), then load controller 20 (of FIG. 3) activates relays 65 and 66 to remove its loads from the power system (box) 255. Whenever an inappropriate message is sent, then the further response to address commands are disabled (box 225) until a zone code message is later sent which successfully identifies the zone identifier of the load controller 20. 
     As has been previously stated, the first tone pair is utilized to either initialize a message transmission or to initialize a test routine sequence. To initialize a test routine sequence in the preferred embodiment, a portable, low power (less than 1 watt) test tone transmitter 99 is used. Such a transmitter can be easily carried by power company personnel while taking meter readings, and used to test the operation of the load controllers at their remote locations. 
     The reception of a test tone by load controller 20 activates a test routine (box 115). Such a test routine checks the operability of the relays in an unenergized state without causing the relay output controls to change state by momentarily energizing the relays (box 116) and by simultaneously sensing the relay current (box 117). When the relays are found to be in working order, test light 64, a light emitting diode, is lit. 
     Suitable apparatus for use of the invention may include an F-8 microprocessor, such as 3870 manufactured by Mostek. 
     While there have been described above the principles of this invention in connection with specific apparatus and techniques, it is to be clearly understood that this description is made only by way of an example and not as a limitation to the scope of the invention. As an example, with certain definitions set forth in the claims, a paging system incorporating the principles of this invention could be within the scope of the invention.