Microsoft Word - Digital Logic Design v_4_6a

• Z indicates high impendence (output is not connected internally)


• X indicates “don’t care”

 One of the most common uses of a tri-state output is for a microprocessor memory bus

where you may have multiple memory banks connected but you only want one to be

interacting with the processor at a time.

Microprocessor

Data bus

(bidirectional)

Control bus

(unidirectional)

Address Bus

(unidirectional)

CE_RAM

ADDR RD WR DATA

CE_ROM

ADDR RD DATA

CE_RAM

ADDR RD WR DATA

Decoder

1 0

0 1 2 3

a bits

c bits

d bits