JPS6449352A - Input turn-off detecting circuit - Google Patents

Abstract

PURPOSE:To correctly detect two input signals and the output turn-off of a CLK with a simple circuit constitution by providing a circuit reproducing means and an alarm signal generating means, which displays the abnormality of the input signal with the change of the signal on an output side when a prescribed time set beforehand or more passes and the change point of the input signal is detected. CONSTITUTION:When a signal P and a signal N are mutually inputted in a 'high' condition, these level conditions of output terminals Q and *Q, when a clock CLK is at the 'high' level, are inverted and outputted to the S terminal of RS-F.F circuits 21 and 22. For the RS-F.F terminals 21 and 22, when the S terminal is at the 'high' level and an R terminal is at the 'low' level, the output terminal Q goes to the 'high' level, and in a reverse case, the output terminal Q goes to the 'low' level. Thus, when the signal to be inputted to a monostable multi-vibrator circuit 20a is that of an RZ type with the change point like a signal 13', and it is not inputted in a prescribed time T, an alarm signal 6, which is an input cut-off signal, is generated. Thus, the cutoff of two input signal clocks can be detected with the simple circuit constitution and without fail.