Circuit for recognizing cyclically occurring pulse sequences
This invention relates to a circuit for recognising cyclically occurring pulse sequences, and finds application, for example, in an internal combustion engine ignition system test apparatus. In such an apparatus it is desirable to be able to distinguish between a cyclic misfire condition in which a misfire occurs at a specific point in each engine cycle and a condition in which misfires occur randomly. The present invention has for its object to provide a circuit for this purpose. 
     A recognition circuit in accordance with the invention includes a counter and a control logic network therefor, said counter counting the pulses in a pulse sequence following a specific event in said pulse sequence, said logic circuit acting to stop said counter at the next occurrence of said specific event and comparison means for comparing the number of pulses counted with a preset number. 
    
    
     An example of the invention is shown in the accompanying drawings in which: 
     FIG. 1 is a circuit diagram of the recognition circuit and 
     FIGS. 2a to 2c is graphical representation of the waveforms at various points in FIG. 1. 
    
    
     Referring firstly to FIG. 1 the circuit shown receives inputs at terminals 10 and 11 from a missing pulse detector circuit (not shown) which forms the subject of copending application Ser. No. 152,127 of even data (based on U.K. Patent Application No. 7918385). At terminal 10 a train of pulses is received-one pulse for each pulse supplied to the missing pulse detector circuit and shown in trace A in FIGS. 2a to 2c. In this trace a single occurrence of a missing pulse is shown. At terminal 11, a single pulse is received occurring immediately before the pulse following the missing pulse as shown in trace B of FIGS. 2a to 2c. 
     Terminal 11 is connected to the CLOCK terminal of an integrated circuit D-type flip-flop circuit (type 4013) 12. The SET terminal of this circuit is connected to earth and its Q output is connected to its D input. 
     The Q output of the flip-flop circuit 12 is also connected to the CARRY IN input of a CMOS integrated circuit 4-bit binary counter (type 4516) 13 and to one input of a NAND gate 14. The CLOCK input of counter 13 and the other input of gate 14 are both connected to the terminal 10. The output of NAND gate 14 is connected via an inverter 15 to the RESET input of counter 13. The PRESET ENABLE and the PRESET DATA inputs of counter 13 are all earthed and its UP/DOWN input is connected to a +ve supply. The outputs Q 1  to Q 4  of the counter 13 are connected to the inputs A 0  to A 3  of a 4-bit digital comparator 16, the B inputs of which are connected to earth by respective resistors 17, 18 19 and 20. A selector switch 21 has a common pole 21a connected to the +ve supply, a first contact 21b connected to the B1 input of comparator 16, a second contact 21c connected to input B 2  of the comparator 16, a third contact 21a connected by diodes 22 and 23 to the B 2  and B 1  inputs of the comparator, and a fourth contact connected directly to the B 3  input and by a diode 24 to the B 1  input of comparator 16. The B 0  input of comparator 16 is connected to the +ve supply. 
     The A&gt;B output of the comparator 16 is connected to the RESET input of the flip-flop circuit 12. The A=B output of comparator 16 is connected to one input of a NAND gate 26 and the A&lt;B output is connected to one input of a NAND gate 25. The other input of each of gates 25 and 26 is connected to the output of an inverter 27, the input of which is connected by a resistor 28 to the +ve supply and by a capacitor 29 to the output of a NAND gate 30. One input of NAND gate 30 is connected to the Q output of flip-flop circuit 12 and its other input is connected to earth by a capacitor 31 and to the terminal 11 by a resistor 32. 
     In use until a pulse has been received at the CLOCK terminal of circuit 12, the Q output of the circuit 12 is high so that the signal at the output of inverter 15 is a pulse train in phase with the pulse train at terminal 10, (see trace A, FIG. 2). This pulse train maintains counter 13 reset to zero. When the Q output of circuit 12 goes low as a result of receiving a pulse at terminal 11, these resetting pulses are blocked and the signal at the CARRY IN terminal of counter 13 goes low, so that the pulses at terminal 10, including that immediately following the pulse at terminal 11, are counted. In the arrangement and circumstances actually illustrated in FIGS. 1 and 2a, no further pulse is missing so that when four pulses have been received by counter 13, the comparator A&gt;B output goes high (switch 21 being in the 21b position so that the preset B input to comparator 16 is 0011, and the count of counter 13 then being 0100). This causes flip-flop circuit 12 to be reset and await the next pulse at terminal 11. The output of inverter 27 goes high momentarily only if the Q output of flip-flop circuit 12 is high when the input at terminal 11 goes low. In the case of an isolated missing pulse these circumstances do not arise. 
     In the situation shown in FIG. 2b a cyclic fault exists, i.e. every fourth pulse is missing. In these circumstances only three pulses are admitted to counter 13 before the next pulse arrives at terminal 11. As a result flip-flop circuit 12 is clocked so that the Q output thereof go high and this prevents further counting up. The output of inverter 27 does now goes high momentarily so that the A=B output of the comparator 16 is strobed through the gate 26. 
     In the situation shown in FIG. 2c there are two missing pulses within a four pulse cycle so that when the Q output of flip-flop circuit 12 goes high the A&lt;B output of comparator 16 is strobed through gate 25. 
     The recognition circuit described herein forms part of an internal combustion engine spark ignition system test apparatus described in co-pending application Ser. No. 151,929 (based on U.K. Patent Application No. 7918389).