Here is the initial state of SR latch both reset and set inputs are zero.

enter image description here

Here we apply a pulse to the reset input and it shows it changes in this way first - enter image description here

and then - enter image description here

The thing that I don't understand how R pulse can change Q state without receiving feedback signal from second NOR gate, and second NOR gate must receive feedback signal from first NOR gate which one of them happens first?


1 Answer 1


The truth table of a NOR gate tells you that the output will only be 1, if both inputs are 0. As soon as one input gets to logic 1, the output switches to a logic 0.

When R is set to 1, the condition of both inputs being zero is no longer fulfilled -> The output goes immediately low to 0. At this time the lower NOR gate still has to wait for the logic 0 to appear at its upper input - until than it will stay low. But as soon as the higher input sees the 0 from Q, the gate has two low 0 as input and will switch the output to 1.

  • \$\begingroup\$ thank you very much. Oh it's very simple .it's really frustrating not to understand something because of very simple thing. but it happens. \$\endgroup\$ Commented Jan 24, 2021 at 12:23
  • \$\begingroup\$ Happens to anyone! \$\endgroup\$
    – jusaca
    Commented Jan 24, 2021 at 13:26

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