A latch is an electronic logic circuit that has two inputs and one output. One of the inputs is called the SET input; the other is called the RESET input.
Latch circuits can be either active-high or active-low. The difference is determined by whether the operation of the latch circuit is triggered by HIGH or LOW signals on the inputs.
Active-high circuit: Both inputs are normally tied to ground (LOW), and the latch is triggered by a momentary HIGH signal on either of the inputs.
Active-low circuit: Both inputs are normally HIGH, and the latch is triggered by a momentary LOW signal on either input.
In an active-high latch, both the SET and RESET inputs are connected to ground. When the SET input goes HIGH, the output also goes HIGH. When the SET input returns to LOW, however, the output remains HIGH. The output of the active-high latch stays HIGH until the RESET input goes HIGH. Then, the output returns to LOW and will go HIGH again only when the SET input is triggered once more.
In other words, the latch remembers that the SET input has been activated. If the SET input goes HIGH for even a moment, the output goes HIGH and stays HIGH, even after the SET input returns to LOW. The output returns to LOW only when the RESET input goes HIGH.
On the other hand, in an active-low latch the inputs are normally held at HIGH. When the SET input momentarily goes LOW, the output goes HIGH. The output then stays HIGH until the RESET input momentarily goes LOW.
Note that most latch circuits actually have a second output that is simply the first output inverted. In other words, whenever the first output is HIGH, the second output is LOW, and vice versa. These outputs are usually referred to as Q and Q-bar with the latter notated as follows:
The notation is usually pronounced either “bar Q” or “Q bar,” though some people pronounce it “not Q.” The horizontal bar symbol over a label is a common logical shorthand for inversion. That is, Q-bar is the inverse of Q. If Q is HIGH, Q-bar is LOW, and if Q is LOW, Q-bar is HIGH.
You can easily create an active-high latch from a pair of NOR gates. (The output of a NOR gate is HIGH if both inputs are LOW; otherwise, the output is LOW.) In this circuit, the SET input is connected to one of the inputs of the first NOR gate, and the RESET input is connected to one of the inputs of the second NOR gate.
The trick of the latch circuit is that the output of the NOR gates are cross-connected to the remaining NOR gate inputs. In other words, the output from the first NOR gate is connected to one of the inputs of the second NOR gate, and the output from the second NOR gate is connected to one of the inputs of the first NOR gate.
The next schematic is for an active-low latch. The only difference between this schematic and the one shown previously is that the active-low latch uses NAND gates instead of NOR gates. Notice also in this diagram that the inputs are referred to as SET-bar and RESET-bar rather than SET and RESET, which indicates that the inputs are active-low.
