What’s a three-state bus?

Understanding Tri-State bus is very important for all Electronics students. So let us start from simple electrical concepts.

          In the below diagram a switch is connected between the Input (A) and Output (Y)

Now let us make the switch ON and apply 5V in the Input.

          Since the Switch is ON, Input goes to Output. Since Input is 5V, the output is also 5V

Now let us apply 0V in the Input.

          Since the Switch is ON and the Input is 0V,  the output is also 0V

   Now let us OFF the switch and give Input as 0V or 5V and see what is the output.

     What is the output in both the cases when switch is OFF? In both the cases the general answer from people  is "0". But that is not true. There is big difference between "No Voltage" and "Zero Voltage".

     If the Potential difference between any point and the Common Point (Ground) is Zero then that point is having "Zero Voltage". If that point is not connected to the Common point directly or through any components the that point is having "No Voltage".

    If you connect a 5V supply to 0V point then there will be a Short circuit. But the 5V supply can be connected to "No Voltage" point. Then the "No Voltage point becomes 5V.

     It is like our Railway Network. Train can run on a track, Train may stop on a track or No train on a track. When a Train is stopped on a Track that track can not be used by any other train. But when no train is on a track, that track can be used by other trains.

              In the above diagram based on which switch is ON  the output Y gets A1 or A2 or A3. The only condition is at any given time more than one switch should never be ON.

     No let us replace the Manual switch with Electronic Switch ( Transistor)

    In the above diagram A is a Signal Input, Y is a Signal output and En is Control Input.

 

    Now Let us create a table to show various possibilities of A, Y and En signals.

    In the above table "En" Input is having two states 0 & 1. The  "A" input is having 2 states 0 and 1. But the Y output is having 3 states 0, 1 and Open. This switch is called "Tri-state" switch and the output is called Tri-state output.

    Since we use transistor a switch, due to drifting electrons, a very small current will flow through the Tri-state switch. Also the transistors are modeled as Resistor/Capacitor circuit, the Open state is called as High Impedance State.

   Here is symbol for tri-state switch.

 
   Now let us start discussing about BUS
 
    A group of signals/wires are called BUS. Now in the below diagram we have 8 wires named as D0, D1, D2, D3, D4,D5, D6 and D7.

    These 8 wires can be represented as follows.

 
This bus is called "D" bus and the width of the bus is "8".

This bus is called "A" Bus and the width of the bus is "8"

Now let us  connect both the A and D bus on a single bus called AD Bus

    The AD bus is called Tri-state bus.

When AEN and DEN are Zero the AD bus is a Tri-state bus.
When AEN is 1 and DEN is 0 the AD bus is connected to A Bus
When AEN is 0 and DEN is 1 the AD bus is connected to D Bus
AEN and DEN should never be 1. This is illegal

What's a three-state bus?

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