Detection system with randomized transmissions
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention in general relates to detection systems and in particular to detection systems having a plurality of detector/sending units for reporting the existence of a condition to a central receiving unit. 
     2. Description of the Prior Art 
     Detection systems which include a plurality of remote sending units which transmit coded signals to a central receiving unit which decodes the signals to produce an alarm or other indication of a condition at the remote location are well known. The conditions may be the existence of a fire, an intrusion, an emergency or other condition desired to be monitored. Or the condition may be the status of the sending unit, such as the condition of its battery or other sensor status. Systems in which such conditions are reported at periodic intervals are generally known as supervised systems. Because the sending units act independently, two or more transmissions will occasionally overlap, a situation referred to as collision or clash. When a clash occurs, information from the clashing transmissions is lost at the receiving unit. If clash occurs in a supervised transmission, the sending unit appears to be missing or not functioning for that supervisory cycle. The sending unit is then erroneously reported as missing or not functioning. If the two clashing transmitters have identical or very close reporting cycles, their transmissions may become synchronized, resulting in multiple successive clashes. 
     Prior art systems have attempted to solve the problem of clash by requiring the transmissions from an individual sending unit to be missing for a time equal to several supervisory cycles and by having loose tolerances on the transmitter electronics. The loose tolerances decreases the probability that two or more transmitters in a system will have supervisory cycles that are close enough to cause multiple successive clashes. However, this approach is effective only when the duration of the transmissions are very short relative to the supervisory period. Further, a detection system must operate continuously for years, and in a large system with, say, thirty or more transmitters installed over a wide area with varying ambient conditions (which can change the cycle periods) the probability is unacceptably high that two or more transmitters will at some time have reporting cycles that are sufficiently close to cause synchronized clashing. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a detection system in which the periods between transmissions of individual sending units are randomized, thus markedly decreasing the probability of synchronized clashing. 
     The invention provides a detection system comprising a plurality of sending units, each of the units including a sensing means for sensing a condition, a means responsive to the sensing means for sending a data signal representative of the condition at randomized time intervals, and receiving means for receiving the data signals and producing an output indicative of the condition. Preferably the means for sending includes a means for generating a pseudo-random number, and a means for delaying the sending of the data signal for a time period related to the pseudo-random number. 
     The inventoion also provides a method of providing an indication of a condition at a remote location comprising the steps of sensing the condition, waiting for a randomized time interval, sending a data signal representative of the condition, and receiving the data signal and utilizing it to provide an indication of the condition. Preferably, the step of waiting comprises generating a pseudo-random number and waiting for a time interval related to the pseudo-random number. In the preferred embodiment, the step of waiting for a time interval related to the pseudo-random number comprises cycling through a timing loop for a number of times equal to the pseudo-random number. The method may also include the step of waiting for an additional predetermined time interval. 
     Numerous other features, objects and advantages of the invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a schematic illustration of an exemplary detection system according to the invention; 
     FIG. 2 is a detailed circuit diagram of an exemplary sending unit according to the invention; and 
     FIG. 3 is a flow chart showing the steps of the preferred embodiment of the microcomputer program according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Directing attention to FIG. 1, an exemplary embodiment of the detection system according to the invention is shown. This embodiment is generally referred to as a security system. The embodiment includes three remote sending units 10, 11 and 12 and a receiving unit 18. The sending units include an intrusion detector 10 on a door, a panic button unit 11, and fire detector unit 12, each of which produces a signal when the particular condition they are designed to detect occurs. Each remote detector unit 10, 11 and 12 has a radio frequency (r-f) transmitter 14, 15 and 16 respectively, associated with it which transmits an r-f signal at randomized time intervals which signal is received by the receiving unit 18. The receiving unit 18 decodes the signals and provides outputs, such as flashing lights 20, a siren 21, or a signal 22 over a telephone line 23 to a monitoring station (not shown), which indicate the conditions detected. 
     Turning now to a more detailed description of the invention, the preferred embodiment of the detection system shown in FIG. 1 includes an intrusion detector unit 10, a panic button unit 11 and a fire detector unit 12. It is understood that the three remote units shown are exemplary. An embodiment may have two such remote units or it may have hundreds. Other types of detectors than intrusion, panic and fire may also be included. Remote unit 10 includes a magnetic contact device 31 on a door which is connected via wire 32 to a signal processing circuit 33. The processing circuit 33 is connected to r-f transmitter 14 which transmits a signal to receiving unit 18 via antenna 34. Similarly, panic unit 11 comprises a panic button 35 which is connected to signal processing circuit 36, which is connected to transmitter 15, having antenna 37, and fire unit 12 comprises fire detector 38 which is connected to signal processor 39, which is connected to transmitter 16, having antenna 40. Receiving unit 18 includes antenna 42 which is connected to a receiver and signal processing circuitry within its chassis 43. The signal processing circuitry is connected to annunciator lights 20, siren 21, and a telephone line 23. It is understood that the outputs 20, 21 and 23 are exemplary only. In some embodiments, only one such output may be used or a variety of others. It is also understood that a wide variety of other signals, such as battery status signals, supervision signals, etc. may be transmitted between sending units 10, 11 and 12 and receiving unit 18. 
     A circuit diagram of a processing circuit, such as 36 of an exemplary sending unit, such as 11, is shown in FIG. 2. In this drawing, the numbers on the lines into the microcomputer 50, such as the &#34;1&#34; at the upper-left of the microcomputer 50, refer to the pin numbers of this component. The labels within the microcomputer next to the pins, such as &#34;OSC1&#34; next to pin 1, refer to the internal signals of the computing unit. The pin numbers and other details of the other components, such as EE Prom 51, transmitter 15, and timer 53 are not shown as details of such components are well known in the art. 
     The particular embodiment of the processing unit and transmitter shown in FIG. 2 is a multipurpose one to which a number of different sending devices, such as the panic button 35, fire detector 38, intrusion detector 31 or other devices may be connected. The sensing devices 31, 35 and 38 as well as the interface will not be described in detail as these are well known in the art. Any combination of sensing device and interface which upon triggering of the device places a low signal on line 56 for a time sufficient to activate microcomputer 50 and also on one of the input lines 57, 58 and 59 for a time sufficient to be read by microcomputer 50 may be used in this embodiment. 
     The processing circuit, such as 36, includes microcomputer 50, EE Prom 51, timer 53, inverter 54, ceramic resonator 62, resistors 63 through 66, capacitor 68 and diodes 70, 71 and 72. The processing circuit 36 also includes a power supply (not shown) which provides the voltage source required to use the circuitry, such as Vdd (75) and the ground, such as 76. Finally, the processor 36 also includes a battery status circuit (not shown) which provides a low signal on line 60 when the battery voltage drops below a certain level. The power supply and battery status circuits are known in the art and thus will not be described in detail herein. 
     The number 1 pin of microcomputer 50 is connected to ground through resonator 62 and the Vdd voltage through resistor 63. The number 2 pin is connected to the Vdd voltage. The number 3 pin is connected to the number 26 pin. The number 28 pin is connected to the output of inverter 54 through resistor 64. The input of inverter 54 is connected to input line 56. The number 28 pin is also connected to the number 27 pin through resistor 65 and diode 70 in parallel, with the cathode of the diode toward the number 28 pin. The number 27 pin is also connected to ground through capacitor 68. The number 6 through 9 pins are connected to inputs 57 through 60. The number 24 pin is connected to the output of timer 53. The output of timer 53 is also connected to the input of inverter 54 through diode 71, with the cathode of the diode toward the timer. The number 25 pin is connected to the data output of EE Prom 51. The number 4 and 6 pins are connected to the system ground. The number 16 pin of the microcomputer 50 is connected to the (MR) input of timer 53 and to ground through resistor 66. The number 14 pin is connected to the input of inverter 54 through diode 72 with the cathode of the diode toward the microcomputer. The number 13 pin is connected to the power on input of the transmitter 15 and the number 17 pin is connected to the data input of the transmitter. The number 15 pin is connected to the power on input to the EE Prom 51. Pins 10, 11 and 12 are connected to the data input, chip select, and clock inputs, respectively, of EE Prom 51. 
     In the preferred embodiment of the invention, the parts of the circuits of FIG. 2 are as follows: microcomputer 50 is a PIC 16C58, EE Prom 51 includes either an ER59256 or NMC9306N chip plus a FET and related circuitry as known in the art to power the chip. Transmitter 15 is preferably a transmitter as is described in U.S. patent application Ser. No. 06/765,280 plus associated buffers, transistors, etc. as known in the art to turn on and off the transmitter and to shape the data prior to transmitting it. Timer 53 includes a 4541 programmable timer and its associated components, inverter 54 is one of a Schmitt trigger hex inverter package type 40106 (the other inverters of the package are used in the sensing device interface in this embodiment), resonator 62 is a 2M hertz ceramic resonator, resistors 63, 64, 65 and 66 are 2.2M ohm, 4.7K ohm, 82K ohm and 100K ohm respectively, capacitor 68 is 0.1M farad, and diodes 70, 71 and 72 are type 1N4148. The electronic parts may be replaced by equivalent parts. In particular, transmitter 15 and receiver 18 may be any conventional transmitter/receiver pair, provided an appropriate data signal level is input to transmitter 15. 
     FIG. 3 shows a flow chart of the program according to the invention with which the microcomputer is programmed. 
     The invention functions as follows. Microcomputer 50 reads the condition signals input on the pins 6, 7, 8 and 9, encodes them, calculates a randomized time delay, waits for the calculated time, and then turns on the transmitter 15 by a signal on output pin 13, and modulates the transmitter 15 via a data signal output on pin 17 to send a signal representative of the condition to the receiving unit 18, which decodes the signal and provides an indication of the condition on annunciator 20, alarm 21, or telephone line 23. 
     Turning now to a more detailed discussion of the operation, to conserve battery power microcomputer 50 is normally held in stand-by by a low signal on pin 28. The timer 53, however, operates continuously as long as a battery with sufficient charge is connected to the system. The timer 53 is programmed to change its output (connected to pin 24 of the microcomputer 50) from high to low at appropriate times when it is desired to make a supervisory report. This low signal is applied to the input of inverter 54 which causes its output to go high, placing a high signal on pin 28 of microcomputer 50 to turn it on. Or, a low signal on the input 56 will also place a high signal on microcomputer input pin 28 to turn it on. A short time after pin 28 goes high, pin 27 also will go high (with a delay determined by resistor 65 and capacitor 68) and clears the microcomputer. Once turned on, the microcomputer drives its number 14 pin low to keep itself on. It then initializes the software, turns on the EE Prom by placing a high signal on pin 15, and enables the EE Prom 51 by placing a high signal on pin 11 (chip select), reads the sending unit identification data from EE Prom 51 on pin 25 while clocking the EE Prom with a signal output on pin 12 and sending the address from which the data is to be read via pin 10. The identification data consists of a preamble, system identification number, and transmitter identification number. The microcomputer 50 adds the current status (as defined by its input pins 6 through 8) to the identification data to complete a data signal to be transmitted. The microcomputer 50 then computes a 4-bit pseudo-random number (0 through 15) as follows: a 15-bit shift register is initialized with a non-zero value. The contents of the register are shifted left, with the right-most bit (bit 1) replaced by the exclusive-OR of bits 14 and 15 (the two left-most bits). This new number in the register is the pseudo-random number which is used to determine the number of 20 millisecond delay loops to be executed by the microcomputer. This randomized delay may be from 0 to 300 milliseconds (15×20 milliseconds) and will average 150 milliseconds. Each successive shift of the 15-bit register will generate a new 15-bit number in a pseudo-random sequence. The sequence repeats after 32,767 numbers have been generated. Only 4 bits from the 15-bit number are used to determine the randomized delay. 
     The microcomputer 50 waits through the number of loop time periods determined by the pseudo-random number, then applies a high signal on pin 13. This high signal turns on the transmitter 15 and battery level indicator circuit (not shown). The preamble, system identification number, transmitter identification number and status are then output on pin 17. The battery status is then read on line 9 (a low signal indicates a low battery) and transmitted while a polynominal for checking the data (the CRC) is calculated. The CRC and an end of transmission signal (EOT) are then transmitted and the transmitter is turned off. After a supervisory transmission (activated by timer 53), the microcomputer then resets the timer by a high signal on pin 16 and returns itself to stand-by. Non-supervisory transmissions, however, are repeated with a predetermined fixed delay plus a pseudo-random delay before the microcomputer resets the timer and returns to standby. If the condition to be reported is on pins 6 or 7, the transmission is repeated nine times with a 100 millisecond predetermined fixed delay plus the random delay. If the condition to be reported is on input 8 (the panic button input), the transmitter will typically be in a portable unit. Because the transmitter location is not fixed, the signal strength may be marginal, so the transmission is repeated thirty times with an 850 millisecond fixed delay plus the random delay. In the preferred embodiment, the transmitt4ed data word lasts 18 milliseconds. Supervisory transmission reporting is set to about 60 seconds by conventional RC tuning and programming of timer 53. The preferred computer program for determining the random delay and the CRC is provided at the end of the description just prior to the claims. 
     The EE prom may be programmed with the identification data in any conventional manner. In the preferred embodiment, a separate port is provided (not shown) which connects to the system ground, the Vdd line, and pins 25, 11, 12, 15 and 10 of microcomputer 50, and which shunts pin 28 of the microcomputer to ground. The ground (low) signal on pin 28 holds the microcomputer in standby and the connections to pins 25, 11, 12, 15 and 10 via the port may then ##SPC1##