Circuit for detecting word sequences in a modem

Abstract

A device for identifying a determined repetitive sequence of predetermined signals arriving on a modem. The device includes a delay circuit so that all the words of a sequence are simultaneously present; a combination circuit for providing a combined word; a circuit for calculating the modulus of each combined word and for comparing this modulus with a threshold; a circuit for counting clock pulses corresponding to the rate at which words arrive; a circuit for inhibiting the counting circuit when the modulus of the combined word is lower than the threshold; and a circuit for providing an identification signal when a predetermined number of clock signals is counted.

What is claimed is:  
     
       1. A process for identifying a predetermined repetitive pattern of signals in a sequence of words arriving on a modem, each signal being digitized as a word having a first portion corresponding to a real value and a second portion corresponding to an imaginary value, the process including the steps of: 
       A. delaying at least one word of the sequence so that a plurality of words is simultaneously present;  
       B. linearly combining the plurality of words to provide a combined word;  
       C. determining a first magnitude of the combined word;  
       D. comparing the first magnitude with a threshold;  
       E. when the first magnitude is lower than the threshold, counting clock pulses corresponding to a rate at which words arrive on the modem; and  
       F. providing an identification signal when a predetermined number of continuous clock pulses is counted.  
     
     
       2. The process of  claim 1 , wherein the predetermined repetitive pattern includes a first word and a second word, the first word being the inverse of the second word, and wherein the step of linearly combining comprises mathematically adding. 
     
     
       3. The process of  claim 1 , wherein the predetermined repetitive pattern includes a first word and a second word, the first word being identical to the second word, and wherein the step of linearly combining comprises mathematically subtracting. 
     
     
       4. The process of  claim 1 , wherein each of the words of the predetermined repetitive pattern has a predetermined magnitude, and wherein the process further includes the steps of: 
       G. determining a second magnitude of each word arriving on the modem; and  
       H. inhibiting the counting of clock pulses in step E when the magnitude for any word deviates from the predetermined magnitude by more than a predetermined threshold.  
     
     
       5. A device for identifying a predetermined repetitive pattern of signals in a sequence of words arriving on a modem, the signals being digitized as words, each word having a first portion corresponding to a real value and a second portion corresponding to an imaginary value, the words arriving on the modem in a sequence, the device comprising: 
       first means, electrically connectable to the modem, for delaying at least one word of the sequence so that a plurality of words is simultaneously present in the device;  
       second means, connected to the first means, for linearly combining the words to provide a combined word;  
       third means, connected to the second means, for determining a first magnitude of the combined word  
       fourth means, connected to the third means, for comparing the first magnitude with a threshold;  
       fifth means, connected to the fourth means, for, when the first magnitude of the combined word is lower than the threshold, counting clock pulses corresponding to a rate at which the words arrive on the modem; and  
       sixth means, connected to the fifth means, for providing an identification signal when a predetermined number of continuous clock signals has been counted.  
     
     
       6. The device of  claim 5 , wherein each of the words of the predetermined repetitive pattern of signals has a predetermined magnitude, and wherein the device further comprises: 
       seventh means, for determining a magnitude of each word arriving on the modem; and  
       eighth means, connected to the fifth means and the seventh means, for inhibiting counting of the clock pulses when the magnitude of a word arriving on the modem deviates from the predetermined magnitude by more than a predetermined threshold.  
     
     
       7. The device of  claim 5 , wherein the predetermined repetitive pattern includes a first word and a second word, the first word being the inverse of the second word, and wherein the second means includes means for performing a mathematical addition. 
     
     
       8. The device of  claim 5 , wherein the predetermined repetitive pattern includes a first word and a second word, the first word being identical to the second word, and wherein the second means includes means for performing a mathematical subtraction. 
     
     
       9. The device of  claim 5 , wherein said second means includes means for combining two consecutive words so that the combined word has a null value when the predetermined repetitive pattern is present in the two consecutive words. 
     
     
       10. A method for detecting a predetermined pattern of digital words within a continuous sequence of digital words, comprising the steps of: 
       A. combining consecutive pairs of digital words in the continuous sequence to form a series of combined words;  
       B. setting a count indicator to zero when a magnitude of a combined word from the series of combined words is greater than a first threshold;  
       C. incrementing the count indicator once for each combined word having a magnitude greater  less than the first threshold; and  
       D. indicating detection of the predetermined pattern of digital words when the count indicator is incremented beyond a predetermined count value.  
     
     
       11. The method of  claim 10 , wherein step A includes mathematically adding consecutive pairs of digital words to create a sum for each consecutive pair. 
     
     
       12. The method of  claim 11 , wherein the continuous sequence of digital words is received on a calling modem, and detection of the predetermined pattern of digital words represents detection of a rate negotiation signal. 
     
     
       13. The method of  claim 10 , wherein each consecutive pair includes a first word and a second word, and wherein step A includes mathematically subtracting the first word from the other second word for each consecutive pair to create a difference for each consecutive pair. 
     
     
       14. The method of  claim 13 , wherein the continuous sequence of digital words is received on a called modem, and detection of the predetermined pattern of digital words represents detection of a rate negotiation signal. 
     
     
       15. The method of  claim 10 , further comprising the steps of: 
       E. calculating a magnitude of each word in the continuous sequence of digital words; and  
       F. setting the count indicator to zero when the magnitude for any word exceeds a second threshold.  
     
     
       16. The method of  claim 10 , wherein the predetermined count value is 32. 
     
     
       17. An apparatus for detecting a predetermined pattern of digital words from a continuous sequence of digital words, comprising: 
       means for combining consecutive pairs of digital words in the continuous sequence of digital words to form a series of combined words;  
       means for setting a count indicator to zero when a magnitude of a combined word from the series of combined words is greater than a first threshold;  
       means for incrementing the count indicator once for each combined word having a magnitude greater  less than the first threshold and  
       means for indicating detection of the predetermined pattern when the count indicator is incremented beyond a predetermined count value.  
     
     
       18. The apparatus of  claim 17 , wherein the means for combining includes means for mathematically adding pairs of consecutive digital words to create a sum for each consecutive pair. 
     
     
       19. The apparatus of  claim 18 , wherein the continuous sequence of digital words is received on a calling modem, and detection of the predetermined pattern represents detection of a rate negotiation signal. 
     
     
       20. The apparatus of  claim 17 , wherein each pair of concentric words includes a first word and a second word, and wherein the means for combining includes means for mathematically subtracting the first word from the second word to create a difference for each consecutive pair. 
     
     
       21. The apparatus of  claim 20 , wherein the continuous sequence of digital words is received on a called modem, and detection of the predetermined pattern represents detection of a rate negotiation signal. 
     
     
       22. The apparatus of  claim 17 , further comprising: 
       means for calculating a magnitude of each word in the continuous sequence of digital words; and  
       means for setting the count indicator to zero when the magnitude of any word exceeds a second threshold.  
     
     
       23. The apparatus of  claim 17 , wherein the predetermined count value is 32. 
     
     
       24. An apparatus for detecting a predetermined pattern of digital signals from a continuous sequence of digital words arriving at the apparatus at a baud rate, comprising: 
       a counting circuit having a reset input and an output indicating a number of times counted, the counting circuit incrementing at the baud rate;  
       a combinatorial logic circuit having an output, coupled to the reset input of the counting circuit, which resets the counting circuit when a mathematical combination of two successive words arriving at the apparatus exceeds a first threshold; and  
       an indicator circuit, coupled to the output of the counting circuit, which asserts a detection signal when the output of the counting circuit exceeds a predetermined count value.  
     
     
       25. The apparatus of  claim 24 , wherein the counting circuit includes a counter having a reset terminal connected to the output of the combinatorial logic circuit, an input terminal connected to a signal representing the baud rate, and an output terminal connected to the indicator circuit. 
     
     
       26. The apparatus of  claim 24 , wherein the combinatorial logic circuit comprises: 
       a delay circuit, coupled to a source of the continuous sequence of digital words, which delays the continuous sequence of digital words by one word;  
       a mathematical calculator, coupled to the delay circuit and to the source of the continuous sequence of digital words, having an output which provides a linear combination of each pair of consecutive digital words;  
       a complex conjugate magnitude calculator with an output and an input connected to the output of the mathematical calculator; and  
       a comparator, having a first input connected to a signal representing a  the first threshold, a second input connected to the output of the complex conjugate magnitude calculator, and an output, coupled to the reset input of the counting circuit, which is activated whenever the output of the complex conjugate magnitude calculator is greater than the first threshold input.  
     
     
       27. The apparatus of  claim 26 , wherein the mathematical calculator comprises: 
       an adder/subtractor that generates a mathematical combination of each pair of consecutive digital words, the mathematical combination being one of a sum of the pair of consecutive digital words and a difference between the pair of consecutive digital words.  
     
     
       28. The apparatus of  claim 24 , wherein the indicator circuit comprises a comparator having a first input connected to a signal representing the predetermined count value, a second input connected to output of the counting circuit, and an output that is activated whenever the output of the counting count is greater then the predetermined count value. 
     
     
       29. The apparatus of  claim 24 , further comprising: 
       a magnitude circuit having an input connected to a source of the continuous sequence of digital words, and an output, connected to the counting circuit reset input, that asserts a reset signal when the magnitude of a word from the continuous sequence of words exceeds a second threshold.  
     
     
       30. The apparatus of  claim 29 , wherein the magnitude circuit comprises: 
       a complex conjugate magnitude calculator with an output and an input connected to the source of the continuous stream  sequence of digital words; and  
       a comparator having a first input connected to a signal representing the second threshold, a second input connected to the output of the complex conjugate magnitude calculator, and an output that is activated whenever the output of the complex conjugate magnitude calculator exceeds the second threshold.  
     
     
       31. The apparatus of  claim 24 , wherein the continuous sequence of digital words is received on a modem, and assertion of the detection signal indicates detection of a rate negotiation signal.