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Decoding 8k ram with only looking at 2k at each range.

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darksiders4l

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Hi. I am a student right now and i was given a homework assignment i know i should know. The deal is i have a 8X8k ram. I know all the ranges and domains but i was told to use the enables on the 74LS138 to make it so that i can only read the first 2k of each of the 8 outputs. If this is not a clear enough explaination please let me know.

Andrew
 

You use the outputs to select the chips, obviously.
But then you are required to read only the first 2K of each. Basically, this means: the addresses of the 74x138 must be stable while reading 2K, i.e. "ignore the lower 2048=2^11 addresses".
That means that the inputs of the 74x138 must be connected to adresses 11, 12, 13. Just like if you had 2K RAMs and wanted the contents in a contiguous block.
 

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