Special Output Gates

It is sometimes desirable to have a logic gate that provides both inverted and non-inverted outputs.

For example, a single-input gate that is both a buffer and an inverter, with a separate output terminal

for each function. Or, a two-input gate that provides both the AND and the NAND functions in a

The general symbology for such a gate is the basic gate figure with a bar and two output lines

protruding from it. An array of complementary gate symbols is shown in the following illustration:

Complementary buffer

Complementary AND gate

Complementary OR gate

Complementary XOR gate

 

 

Complementary gates are especially useful in "crowded" circuits where there may not be enough

physical room to mount the additional integrated circuit chips necessary to provide both inverted

and noninverted outputs using standard gates and additional inverters. They are also useful in

applications where a complementary output is necessary from a gate, but the addition of an inverter

would introduce an unwanted time lag in the inverted output relative to the noninverted output.

The internal circuitry of complemented gates is such that both inverted and noninverted outputs

change state at almost exactly the same time:

Another type of special gate output is called tristate, because it has the ability to provide three

different output modes: current sinking ("low" logic level), current sourcing ("high"), and °oating

("high-Z," or high-impedance). Tristate outputs are usually found as an optional feature on bu®er

gates. Such gates require an extra input terminal to control the "high-Z" mode, and this input is

usually called the enable.

tromechanical relay: a signal-actuated switch contact that may be used to conduct virtually any

type of electric signal. Of course, being solid-state, the bilateral switch has none of the undesir-

able characteristics of electromechanical relays, such as contact "bouncing," arcing, slow speed, or

susceptibility to mechanical vibration. Conversely, though, they are rather limited in their current-

carrying ability. Additionally, the signal conducted by the "contact" must not exceed the power

supply "rail" voltages powering the bilateral switch circuit.

Four bilateral switches are packaged inside the popular model "4066" integrated circuit: