In a multiplexed bus system, many devices are connected to 
a common bus. If 2 or more devices attempt to use the bus 
at the same time , then data will be lost. Thus only one 
one device must be allowed to use the bus at a time. O e 
method is to connect the devices through tri-state 
devices , which when disabled will effectively discoonect 
devices from the bus.

A device which has one input,one enable and one output line
 is called tri-state device,if two different devices wants a
common bus then tri-state device are essential to assign bus
to any one of them in bus orientation system.

Tristate also prevents loading effects caused by devices
connected on to the bus...
(adding to what has been written above)