As a result, no current is drawn from the supply. Note that this third state is NOT equal to a logic level “0” or “1”, but is an high impedance state in which the buffers output is electrically disconnected from the rest of the circuit. Then this type of device has two logic state inputs, “0” or a “1” but can produce three different output states, “0”, “1” or ” Hi-Z ” which is why it is called a “Tri” or “3-state” device. When activated into its third state it disables or turns “OFF” its output producing an open circuit condition that is neither at a logic “HIGH” or “LOW”, but instead gives an output state of very high impedance, High-Z, or more commonly Hi-Z. One being the data input and the other being the enable or control input as shown. Then a tri-state buffer requires two inputs. This control signal can be either a logic “0” or a logic “1” type signal resulting in the Tri-state Buffer being in one state allowing its output to operate normally producing the required output or in another state were its output is blocked or disconnected. This type of Buffer is known as a 3-State Buffer or more commonly a Tri-state Buffer.Ī Tri-state Buffer can be thought of as an input controlled switch with an output that can be electronically turned “ON” or “OFF” by means of an external “Control” or “Enable” ( EN ) signal input. The “Tri-state Buffer”Īs well as the standard Digital Buffer seen above, there is another type of digital buffer circuit whose output can be “electronically” disconnected from its output circuitry when required. Then there is a limit to the number of inputs and outputs than can be connected together and in applications where we need to decouple gates from each other, we can use a Tri-state Buffer or tristate output driver. However, the propagation delay of the gate deteriorates rapidly as a function of fan-in so gates with a fan-in greater than 4 should be avoided. If a digital buffer has a high fan-out rating (current source) it must also have a high “fan-in” rating (current sink) as well. In other words, a digital buffer does nothing as its output state equals its input state. Unlike the single input, single output inverter or NOT gate such as the TTL 7404 which inverts or complements its input signal on the output, the “Buffer” performs no inversion or decision making capabilities (like logic gates with two or more inputs) but instead produces an output which exactly matches that of its input. One type of single input logic gate that allows us to do just that is called the Digital Buffer. When its input signal is “LOW” its output state will NOT be “LOW”, in other words it “inverts” its input signal, hence the name “Inverter”.īut sometimes in digital electronic circuits we need to isolate logic gates from each other or have them drive or switch higher than normal loads, such as relays, solenoids and lamps without the need for inversion. So for example, when the single input to NOT gate is “HIGH”, its output state will NOT be “HIGH”. The digital buffer is the logic gate opposite of an inverter (Not Gate) we look at in the previous tutorial where we saw that the NOT gates output state is the complement, opposite or inverse of its input signal.
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