In digital electronics three-state, tri-state, or 3-state logic tri state buffer pdf an output port to assume a high impedance state, effectively removing the output from the circuit, in addition to the 0 and 1 logic levels. Three-state outputs are implemented in many registers, bus drivers, and flip-flops in the 7400 and 4000 series as well as in other types, but also internally in many integrated circuits. Other typical uses are internal and external buses in microprocessors, computer memory, and peripherals. A tristate buffer can be thought of as a switch.
If B is on, the switch is closed. If B is off, the switch is open.
Three-state buffers can also be used to implement efficient multiplexers, especially those with large numbers of inputs. Three-state buffers are essential to the operation of a shared electronic bus.
The difference lies in the time needed to output the signal. When chip select is deasserted, the chip does not operate internally, and there will be a significant delay between providing an address and receiving the data. An advantage of course, is that the chip consumes minimal power in this case. When chip select is asserted, the chip internally performs the access, and only the final output drivers are disabled by deasserting output enable.
This can be done while the bus is in use for other purposes, and when output enable is finally asserted, the data will appear with minimal delay. A ROM or static RAM chip with an output enable line will typically list two access times: one from chip select asserted and address valid, and a second, shorter time beginning when output enable is asserted.
The PCI local bus provides pull-up resistors, but they would require several clock cycles to pull a signal high given the bus’s large distributed capacitance. To enable high-speed operation, the protocol requires that every device connecting to the bus drive the important control signals high for at least one clock cycle before going to the Hi-Z state.
This way, the pull-up resistors are only responsible for maintaining the bus signals in the face of leakage current. Intel refers to this convention as “sustained tri-state”, and also uses it in the Low Pin Count bus. When devices are inactive, they “release” the communication lines and tri-state their outputs, thus removing their influence on the circuit.