An SR Latch is a basic type of
latch that has
set and
reset inputs. One can
construct this latch using 4
NAND gates, two of which are
contained in the
device assembled in
bistable multivibrator:
+----+ C S R | Y
S(H)---|NAND| +---------------+ ------+-------
| |o--o|S T|---Y(H) 0 - - | rest
+-| | | | 1 0 0 | rest
| +----+ | bistable | 1 1 0 | 1
| +----+ | multivibrator | 1 0 1 | 0
R(H)---|NAND| | | 1 1 1 | *bad*
| | |o--o|R B|o--Y(L)
+-| | +---------------+
| +----+
|
C(H)
This behaves pretty much the same as the bistable multivibrator circuit, but now the inputs (S and R) are high asserting, and there is an additional clock (C) input. When the clock input is asserted, the latch "opens", and allows the values of S and R to affect the state of the device. Otherwise, if C=0, the latch is "closed", and it merely rests at whatever state it was in prior to the latch closing (it "latches in" the value).
See also SR Flip-Flop, D Latch.