Use of .27Z.27 IEEE 1164

some digital devices support form of three-state logic on outputs only. 3 states 0 , 1 , , z .

commonly referred tristate logic (a trademark of national semiconductor), comprises usual true , false states, third transparent high impedance state (or off-state ) disconnects logic output. provides effective way connect several logic outputs single input, 1 put high impedance state, allowing remaining output operate in normal binary sense. commonly used connect banks of computer memory , other similar devices common data bus; large number of devices can communicate on same channel ensuring 1 enabled @ time.

while outputs can have 1 of 3 states, inputs can recognise two. hence kind of relations shown in table above not occur. although argued high-impedance state unknown , there no provision in electronics interpret high-impedance state state in itself. inputs can detect 0 , 1 .

when digital input left disconnected, digital value interpreted input depends on type of technology used. ttl technology reliably default 1 state. on other hand, cmos technology temporarily hold previous state seen on input (due capacitance of gate input). on time, leakage current causes cmos input drift in random direction, possibly causing input state flip. disconnected inputs on cmos devices can pick noise, can cause oscillation, supply current may dramatically increase (crowbar power) or device may destroy itself.