Making the 2:4 decoder should be fairly simple. The only thing that continues to confuse me is the truth table. I've explained it here. When using AND gates to make a decoder, the truth table is as follows:   Notice the NOT gates before A, B (select lines) and C (data). <<?? circle before AND? MAINLY, the use of NAND gates instead of AND gates. This causes the truth table to be as given below. (Basically, inverting all the values. 0 when chosen, 1 elsewhere)  STROBE? Here, it's G or G Dash. I understand that it acts as an enable. C is the data. How do I tell what value the enable wants? I understand it from the IC. If G dash is mentioned in the circuit, give it a ZERO. 3:8 DECODER One way is to use two ICs as two separate 2:4 decoders to make a single 3:8 one.  Based on the above interior of the IC 74155, we can use the same IC as a 3:8 decoder if we ensure certain things. (I haven't performed this on my own yet, but assume my theory here is right.) A, B is same. 1G' and 2G' has the same line. 1C and 2C'2 has the same line too.  |
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