Hi I am from computer science background and hence lacking solid foundation of electronics. I am learning SR latches . Below is SR latch with NOR gates: enter image description here

And below is enable SR latch with NOR gates: enter image description here

I dont get why in the first image, S corresponds to Q and R to Q', whereas in second figure S corresponds to Q' and R to Q. Why the polarity of output is switched in enable SR latch. These diagrams are from this page.

  • \$\begingroup\$ the upper one is not corresponding to the truth table. \$\endgroup\$ – Eugene Sh. Jan 11 '16 at 18:00

The upper circuit is misleading. Ordinarily, applying a high SET input should give a high Q output - to set conventions otherwise is to risk confusing folks, and it has confused you. Note that in the truth table, with SET high and RESET low, Q is low. While this is indeed how the circuit works, from a semantic point of view it makes no sense. A more "normal" description would swap the Q and Q* labels, and at the same time swap 0 and 1 in the output section of the truth table.

It's a pity the page you've linked did not include the truth table for the second circuit. If you find one, you'll see that a SET high produces a Q high, which is the opposite of the truth table for the first circuit.


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