I have a question about this circuit:

enter image description here

Suppose that the input is 0000. Then ROM A is Enabled, and the output will be 0111. But what will be the order of the output? Will 0 be on D0, 1 on D1, 1 on D2, 1 on D3 or 1 be in D0, 1 in D1, 1 in D2, 0 in D3?

I mean where is the MSB in the output going?

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    \$\begingroup\$ This is inadequately specified. There is no way to tell what A3 is. Or are the D0 - D3 lines tied to the A0 - A3 lines? \$\endgroup\$ – WhatRoughBeast May 3 '14 at 21:56

That depends on the rom used, it should be written in the datasheet. I'd say that \$D_0\$ is the LSB and \$D_3\$ is the MSB, but that's just a guess based on the fact that \$D_0\$ represents \$2^0\$, \$D_1\$ represents \$2^1\$ and so on.


As no IC number or datasheet is given, you can guess D0 is the LSB and D3 is MSB. There are two logical foundations for this assumption.

  1. The '0' in D0 may represent the '0' in \$ 2^0\$.

  2. Most of the time, digital electronics books use 0 denotation as lower bit representation, and I assume you found this in some book.


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