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 .