Bus system construction using tri-state buffer

Bus system can be constructed by multiplexers and three-state gates (tri-state buffer). A three state

gate is a digital circuit that exhibit three states.Three states are 0,1, and a high impedance state.The

high impedance state is like open circuit, which means the output is disconnected and does not have a

logic significance.Three-state gates are used to construct AND, NAND and a buffer gate. Buffer gate is

used to construct bus system.

Three-state buffer gate

It has normal and control inputs. The control input determines the output state.When control input is

1, the output is enabled and the gate behaves like any conventional buffer.When control input is 0,the

output is disabled and the gate goes to a high-impedance state, regardless of the value in the normal

input.High impedance is a special feature of three-state gate and not available in normal gates.

Because of high impedance feature, a large number of three-state gate output can be connected with

wires to form a common bus line without endangering loading effects.

The outputs of the four buffers are connected together to form a single bus line.The control inputs to

the buffers determine which of the four normal inputs will communicate with the bus lines.No more

than one buffer may be in the active state at any given time.The connected buffers must be controlled

so that only one three-state buffer has access to the bus line while others are in high

impedance.Decoder can be used to control the active input at any given time. When enable input of

decoder is 0 all the outputs are zero and the lines are in high impedance. When enable is 1 than one

of the four output of decoder is active depending upon binary values of the selection lines.

Que- Construct a bus system using tri state buffer for 2 registers having 4 bits each. Draw a neat and

clean diagram to explain the process.

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