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Three-state logic is a logic used in electronic circuits wherein a third state, the high-impedance state, is added to the original 1 and 0 logic states that a port can be in. This high-impedance state effectively removes the port from the circuit, as if it were not part of it. So in the third state of high impedance, the output from the port is neither 1 nor 0, but rather the port does not appear to exist.
Three-state logic is also known as tri-state logic.
Three-state logic is used to allow multiple circuits to share the same output or bus lines which may not be capable of listening to more than one device or circuit at a time. In this way, the high-impedance state acts as a selector which blocks out circuits that are not being used. As mentioned, the whole concept of the high-impedance state is to effectively remove the circuit or device's influence from the rest of the circuit as if it were not connected at all. Putting one device on high-impedance is normally used to prevent a short circuit with the other device directly connected in the same way to the same leads, this also prevents both devices being driven at once since this may lead to unintended output or input and cause the whole circuit to malfunction.
Three-state logic is implemented in most bus drivers, registers, flip-flops in the 4000 and 7400 series as well as many others. Three-state logic is commonly used internally in many integrated circuits such as microprocessors, RAM or memory as well as many chips used in peripheral devices. Many of these are controlled by what is called as active-low input which indicates whether the output leads or pins should be placed in a high-impedance state or to drive their loads, that is to either output the standard 1 or 0.