Nuts & Volts - April 2007

FIGURE 2. Useful variations of the MIL/ANSI inverter symbol.

gates in words.

• Buffers — A

buffer is a non-inverting amplifier that has an output drive capacity that is far greater than its input drive requirement, i.e., it has a high fan-out and gives a logic 1 output for a logic 1 input, etc.

(EX-NOR) gate is an EX-OR gate with a negated output, which goes to logic 1 if both inputs are in the same logic state, and goes to logic 0 only if a single input is at logic 1.

• Inverters — An inverter (also known as a NOT gate) is a high fan-out amplifier that gives a logic 1 output for a logic 0 input, and vice versa.

• AND Gates — An AND gate has an output that is normally at logic 0 and only goes to logic 1 when all inputs are at logic 1, i.e., when inputs A and B and C, etc., are high.

• NAND Gates — A NAND gate is an AND gate with a negated (inverted) output; the output is normally at logic 1 and only goes to logic 0 when all inputs are at logic 1.

• OR Gates — An OR gate has an output that goes to logic 1 if any input is at logic 1, i.e., if inputs A or B or C, etc., are high. The output goes to logic 0 only if all inputs are at logic 0.

FIGURE 3. Symbols, truth tables, and Boolean expressions for the eight basic types of logic gates.

• NOR Gates — A NOR gate is an OR gate with a negated output; it has an output that goes to logic 0 if any input is at logic 1, and goes to logic 1 only when all inputs are at logic 0.

• XOR Gates — An exclusive-OR (EX-OR) gate has two inputs, and its output goes to logic 1 only if a single input (A or B) is at logic 1; the output goes to logic 0 if both inputs are in the same logic state.

Figure 3 shows how the functions of the eight basic types of gates can also be presented in tabular form via truth tables (which show the logic state of the output at all possible input logic state combinations) or symbolically via Boolean algebraic terms. Note that, by convention, all logic gate inputs are notated alphabetically as A, B, C, etc., and the output terminal is notated as Y (in counters and flip-flops, etc., the main output is usually notated as Q). The actual logic states may be represented by 0 and 1, as shown, or by L (= Low logic level) and H (= High logic level). Also note in the Boolean expressions that a negated output is indicated by a negation bar drawn above the basic