An S-R latch (or S-R flip-flop) is a
bistable multivibrator circuit made by
cross-connecting two
NAND gates.
______
S ---| |
| |O-+------>Q
+-|_____| |
| |
+------------+
| |
+----------+ |
| ______ |
+-| | |
| |O---+----Q'
R ---|_____|
If S=0 and R=1, the latch resets, and Q=0. If S=1 and R=0, the latch sets, and Q=1. If S=0 and R=0, the latch holds the present value of Q. In there cases Q' (Q
not) is the opposite of Q. However, if S and R are both 1, Q and Q'
output the same
value, meaning that Q = Q'. This is called the
forbidden case.
If some poor
EE student puts an S-R latch into the
forbidden state, then Q=Q', and all the basic assumptions of logic fail. All the laws of
mathematics and
physics break down, 0 equals
Infinity, atomic
nucleii repel their
electrons,
matter ceases to exist, and the
universe disappears in
a puff of logic.