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' (Qnot) 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.
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