Wikipedia gives following circuits of flip flops:

  1. SR latch using NORs

    enter image description here enter image description here

  2. SR latch using NANDs

    enter image description here enter image description here

Note that circuit 1 have R input at top and S below it. Also note that circuit 2 have active LOW inputs, that is inputs with bar above them. The action columns of truth tables have entries in reverse order of each other.

I was wondering: if I flip inputs of circuit, S at top and R below it, will it still certify as valid SR latch, especially because the order of action is changed?
For example, Forbidden is no more adjacent to Set, but to Reset:

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1 Answer 1


All you have really done is swapped the names of the inputs. In your circuit, asserting the R input causes the latch to be set while asserting the S input causes the latch to be reset. You haven't changed the behavior of the circuit at all; it is still a latch...it just has confusing input names.

  • \$\begingroup\$ Right!!! But those swapping does not fit in the definition of latch, thats why in circuit 1, wikipedia have S at bottom against Q' and R at top, against Q, but not S at top and R at bottom, right?? I believe, for action table, only two orders are allowed: Forbidden -> Reset -> Set -> No Change or reverse: No Change -> Set -> Reset -> Forbidden. S at top and R at bottom does not result in any of these two orders. Thats why wikipedia have R at top and S at bottom. Right? (actually I was thinking why wikipedia have R at top and S at bottom and this whole doubt arose) \$\endgroup\$
    – RajS
    Commented Aug 17, 2019 at 16:18
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    \$\begingroup\$ @anir Wikipedia has R on top because reset starts with on R and set starts with a S. Swapping the names does not swap the behaviour of the circuit, like Elliot is answering \$\endgroup\$
    – Huisman
    Commented Aug 17, 2019 at 16:59
  • 2
    \$\begingroup\$ @anir The "definition" of a latch doesn't require any specific names for the inputs. Sometimes you will see "Clear" instead of "Reset" inputs. It's like a water faucet...it doesn't matter if you mark it 'C' for cold or 'C' for caliente as long as the desired water comes out. \$\endgroup\$ Commented Aug 17, 2019 at 19:26
  • \$\begingroup\$ I know names are just for reference. But names surely hold some meaning, Reset does not mean Set & vice versa. Let me ask other way. (1) does the last (unnumbered/3rd) truth table correctly defines behavior of last circuit. (2) If yes, then is that truth table a valid definition of a latch (especially because it follows different order of actions from those I specified in above comment & also because no text has specified such truth table)? (3) If its not valid, then can we call that circuit diagram incorrect? (4) Is there any such thing as valid order of actions as I stated in above comment? \$\endgroup\$
    – RajS
    Commented Aug 17, 2019 at 20:13
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    \$\begingroup\$ Yes, the third circuit is still a latch. The circuit is identical to the first circuit except for the names. The wires don't care what you call them. The truth table is also valid. The order of the rows does not matter. If you swapped the S and R columns you get the same truth table as the first circuit. The truth table does not imply any "order" of actions, it is simply a list of input conditions and output values. \$\endgroup\$ Commented Aug 17, 2019 at 20:33

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