Electrical – Pins in IC74138


IC74138(a 3:8 decoder) has 3 enable pins G1, G2A and G2B.

Why are there 3 enable pins and what is the reason for that nomenclature?

Best Answer

The datasheet for the 74138 explicitly says that the three enable inputs were a design decision that is intended to make it easy to use multiple 74138 chips together to for a one out of 24 or one out of 32 decoder.

Extract from the Datasheet:

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With the multiple enables that have different active states (inverted vs. non-inverted,) you can build a one of 24 decoder with no additional inverters in the circuit - saved you a chip full of inverters back in day when they didn't come in SMD form with a single inverter in the package.

This answer from last year shows how the 74138 can be used to build a 1 out of 24 decoder. That example does use additional inverters, but that is because the circuit is doing a bit more than just decoding the inputs - it appears to be using the third enable as a value ot be passed to the outputs. It selects one output, and passes the value at Data-In to that ouput. Were it not for that, there would be no need of the additional inverters, and the 74138s would work together as a 24 output decoder.

Circuit from the linked answer:

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