SR Latch

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.