英语翻译
Digital devices
The most basic digital devices are called gates.Gates originally got their name from their function of allowing or retarding（“gating”） the flow of digital information.Each gate represents a logic circuit diagram with a representative symbol.Figure 8-1 shows symbols for the common gates.Both sets of symbols illustrated are defined in IEEE/ANSI standard 91-1984.
We use a high voltage level （H）to represent logic 1 and a low voltage（L）to represent logic 0.The output of an AND gate is 1 only if both inputs are 1.The truth table is shown in figure 8-2.A gate is called a combination circuit because its output depends only on the current input combination.
AND gate,OR gate and NOT gate（or inverter）are the three most important kinds of gates.Any digital function can be realized using just these kinds of gates.
A flip-flop is a device that stores either 0 or 1.The state of flip-flop is the value that it currently stores.The stored value can be changed only at certain time determined by a “clock” input,and the new value may further depend on the flip-flop’s current state and its “control” inputs.
A digital circuit that contains flip-flop is called a sequential circuit,because its output at any time depends not only on its current input but also on the past sequence of inputs that have been applied to it.In other words,a sequential circuit has memory of past events.
ANSI：The American National Standards Institute 美国国家标准化组织