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What are the "Enable Input" pins on a decoder? Answered

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I'm trying to multiplex with an arduino and a 74HC238N decoder. on the datasheet I was looking at I saw that besides the 8 outputs, 3 inputs, GND and Vcc there were also 3 "Inable Input" pins, that I couldn't figure out what they do. Can somebody please explain?

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steveastroukBest Answer (author)2013-07-07

The enable pins are used to switch ON the output pins. Logic chips have THREE states for their outputs, "0", "1" and "Tri-state". WHen they are outputting a "0", the output is switched towards the ground line. When they output a "1", the output is switched to the supply line. When the output is tri-state the output pin "FLOATS", and is what we call "high-impedance".

Its an important feature: it means that pins can share lines with other chips, and their outputs don't fight - in some ancient computer designs, it was possible to connect outputs set to 0 with outputs set to 1 and start a fire !!!

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