The truth table for a NAND gate is as one might expect, exactly opposite as that of an AND gate:Īs with AND gates, NAND gates are made with more than two inputs. To symbolize this output signal inversion, the NAND gate symbol has a bubble on the output line.

The word “NAND” is a verbal contraction of the words NOT and AND.Įssentially, a NAND gate behaves the same as an AND gate with a NOT (inverter) gate connected to the output terminal. It is only with all inputs raised to “high” logic levels that the AND gate’s output goes “high,” thus energizing the LED for only one out of the four input combination states.Ī variation on the idea of the AND gate is called the NAND gate.

An LED (Light-Emitting Diode) provides visual indication of the output logic level: What this truth table means in practical terms is shown in the following sequence of illustrations, with the 2-input AND gate subjected to all possibilities of input logic levels. In case you might have been wondering, AND gates are made with more than three inputs, but this is less common than the simple two-input variety.Ī two-input AND gate’s truth table looks like this: If any input(s) is “low” (0), the output is guaranteed to be in a “low” state as well. One of the easiest multiple-input gates to understand is the AND gate, so-called because the output of this gate will be “high” (1) if and only if all inputs (first input and the second input and. The actual TTL circuitry of these different gates will be explored in subsequent sections. Each basic gate type will be presented in this section, showing its standard symbol, truth table, and practical operation. Since so many combinations are possible with just a few input terminals, there are many different types of multiple-input gates, unlike single-input gates which can only be inverters or buffers. Now, instead of merely inverting or amplifying ( buffering) a single “high” or “low” logic level, the output of the gate will be determined by whatever combination of 1’s and 0’s is present at the input terminals.

This increase in the number of possible input states obviously allows for more complex gate behavior. The number of possible input states is equal to two to the power of the number of inputs: With a single-input gate such as the inverter or buffer, there can only be two possible input states: either the input is “high” (1) or it is “low” (0).Īs was mentioned previously in this chapter, a two input gate has four possibilities (00, 01, 10, and 11). What more can be done with a single logic signal but to buffer it or invert it? To explore more logic gate possibilities, we must add more input terminals to the circuit(s).Īdding more input terminals to a logic gate increases the number of input state possibilities. Inverters and buffers exhaust the possibilities for single-input gate circuits.

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