Microprocessor
The output data lines of microprocessor and memories are usually tristated because

 More than one device can transmit information over the data bus by enabling only one device at a time
 It increases the speed of data transfer over the data bus
 The data line can be multiplexed for both input and output
 More than one device can transmit over the data bus at the same time

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Microprocessor
Ready pin of microprocessor is used

 to indicate that microprocessor is ready to receive inputs
 to introduce wait state
 to indicate that microprocessor is ready to receive outputs
 to provide direct memory access

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