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Starting electronics
SR-type NOR bistable
The simplest bistable or latch is the SR-type (sometimes
called RS-type) bistable, the operation of which acts like two
gates connected together as in Figure 11.3. The circuit is constructed
so that the two gates are cross-coupled — the output
of the first is connected to the input of the second, and the
output of the second is connected to the input of the first.
Figure 11.3
gates
An SR-type bistable circuit, comprising two cross-coupled NOR
There are two inputs to the circuit, and also two outputs. The
inputs are labelled S and R (hence, the reason why the circuits
are called SR-type…), which stand for ‘Set’ and ‘Reset’. Now, it
so happens that one output is, in most instances, the inverse
of the other, so for convenience one output (by convention)
is labelled Q and the other output (also by convention) is
labelled (called bar-Q), to show this.
Both inputs to the SR-type NOR gate bistable should normally
be at logic 0. As one input (either S or R) changes to logic 1,
the output of that gate goes to logic 0. As the gates are crosscoupled,
this is fed back to the second input of the other
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