TM9-4935-481-14-1
Therefore the TTL outputs cannot be wired AND.
e. Flip-Flops. A flip-flop (also called a bistable element) may assume one of the two stable states depending upon the
pulse history of one or more input points. It has one or more output points.
(1) R-S flip-flop. An R-S flip-flop consists of two cross-coupled NAND gates as shown in figure 3-17. The flip-flop is
in "SET" or "1" state when its Q output is low and its Q output is high. The flip-flop is in "RESET" or "0" state when its Q
output is low and its Q output is high.
(a) The "S" in "S" stands for "SET". The bar over S indicates that the set input is activated by the low level and
is insensitive to the high level. The "R" in "R" stands for "RESET". The bar over R indicates that the reset input is activated
by the low level, and is insensitive to the high level. To summarize, a R-S flip-flop consisting of two NAND gates, responds
to low levels and is insensitive to high levels.
(b) When a low level is applied to the S input while the R input is high, the Q output will be high and the Q output
will be low. Thus, the flip-flop will be in set state. If the S input is now changed to high level while the R input is still at high
level, the flip-flop will remain in the set state, "remembering" the last activating input (low at S). If now the R input is
changed from high to low while the S input remains high (inactive), the flip-flop will be reset. That is, the Q output will
become low and the Q output will become high. If now the R input is changed to high while S input remains high, the flip-flop
will remain in the reset state, "remembering" the last active input (low at R-). To summarize, if one of the two inputs is low
(active), the flip-flop will respond to that input, and if both inputs are high (inactive) the flip-flop will remain in the state
determined by the last low (active) input.
(c) If both inputs of the R-S flip-flop are low (active), both outputs (Q and Q) are high, and the flip-flop is said to
be in an ambiguous state. If later both inputs change simultaneously to high, the state of the flip-flop cannot be predicted.
That is, it will be known that one output will be high, and the other output will be low, but it will be impossible to predict which
output (Q or Q) will be high and which low. There are two ways to avoid this unpredictable state. One is to avoid applying a
low simultaneously to S and R. The second is to remove low levels sequentially. The flip-flop will respond to the input to
which the low level was removed last.
MS161766
Figure 3-17. R-S flip-flop
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