machine circuitry as1 provided for indicating the setting of weft thread monitor sw comprises a series connection of rs flip flop 16 integrator 17 comparator or threshold stage 18 pulse stretching stage 19 and indication device 20 the circuits 16 18 designed for producing a timing pulse 18 39 provide a timing circuitry the set input of rs flipflop 16 is connected to the output of rs flipflop 13 of trigger circuitry 6 13 whereas the reset input is connected to the output of pulse former 5 of signal channel 1 5 in case delay circuit 12 is set such that controlling pulse k 39 ends prior to thread travel pulse f 39 rs flipflop 16 of indicator circuitry as1 furnishes a short difference pulse 16 39 of duration t for each weft insertion now indicator circuitry as1 is designed in such a manner that only difference pulses 16 39 are indicated whose duration t surmounts the duration ts of the safety interval of e g three milliseconds for this purpose difference pulse 16 39 is transformed into a triangle pulse 17 39 by integrator 17 the threshold of comparator 18 following integrator 17 is set such that the latter generates a timing pulse 18 39 whose duration is shorter than the duration t of difference pulse 16 39 by the amount ts of the safety interval thus when.