S-R latch made with NOR gates are shown in Figure 1:
The following is the truth table for S-R latch made with NOR gates.
If the S-R latch made with NOR gates in Figure 1, the forbidden state is because if we get simultaneous pulses at S and R , both outputs go to 0, which does not hurt anything, but when go to an indeterminate state when the inputs return to 0. This is undesirable becouse it leads to an unpredictable result; hence, this state is called FORBIDDEN.
When SET=1, RESET=1; this condition tries to SET and RESET the latch at the same time, and it produces . If the inputs are returned to zero simultaneously, the resulting output state is unpredictable. This input condition should not be used.
Hence, in the S-R latch made with NOR gates the forbidden state is .