SR Latch: Why reverse S and R in NAND and NOR if it reverses the outputs too?

Question

Here is a NOR based SR latch: And here is a NAND based SR latch:

So, basically first we flipped the orientation of R and S and then declare that in our NAND latch, the output would be flipped i.e. 0 for set and 1 for reset which defies the meaning of set and reset.

Why in the first place did we change the names of the input corresponding to Q only to find out it reverses our output ?
Had we kept the RS naming in both the latches, we would be getting a sensical output that would be 1 for Set regardless of NAND or NOR gates used.

PS: I don't know if this question belongs to this stackexchange as it focuses on why a decision was made. Please move to proper site if required.

Answer 1

Choice of what represents SET or RST in either circuit is really somewhat arbitrary and and up to the designer to choose.

It is a relatively good idea to use the strategy that you suggested and have the SET input be the one that makes the Q output go to a '1'. I can see no reason why you would do it differently unless that was some design choice in a particular circuit to do so.

In the end I would not spend too much time trying to figure out the original motivation as long as you understand how both latches work.

On a final note you may want to consider drawing the dual NAND gate SR Latch with the DeMorgan equivalent gate symbols in that it makes it a lot easier to visualize the logic operation.

Answer 2

Here is the truth tables

SR NOR latch
S  R      Q Q'
0  0      no change from previous values
0  1      0 1
1  0      1 0
1  1      undefined behavior


SR NAND latch
S' R'     Q Q'
0  0      undefined behavior
0  1      1 0
1  0      0 1
1  1      no change from previous values

"New latch"
SR NAND latch with S' replaced with R and R' replaced with S
S  R      Q Q'
0  0      undefined behavior
1  0      1 0
0  1      0 1
1  1      no change from previous values

So, with reference to the last truth table you can see how it is different from SR NOR latch. In particular, if I'm designing with your new latch, how should i have the latch keep its previous value? Intuitively, I would want to keep Set and Rst low. But with your Latch, this gives me undefined behavior.