# Multi-digit SSD, Encoder Truth Table

Discussion in 'Homework Help' started by jegues, Oct 24, 2010.

1. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
735
43
Hello everyone.

I'm having some trouble with an encoder truth table for a circuit we've been asked to implement.

See figure for circuit as well as the scenario given.

I'm not really sure how I'm supposed to go about determining the values for the truth table. It seems as though on the left hand side of the truth table the encoder bits are being shifted to the left each row.

I'm still not sure what the values of S1 and S0 should be...

My guess for the values of S0, and S1 (based on what I know about priority encoders) would be as follows,

For S0, from top to bottom:
0,1,0,1

For S1, from top to bottom:
0,0,1,1

Essentially counting upwards as we go down each row.

Can someone help me out?

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2. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
What I understand from this exercise is that it wants you to build a 4-to-2 encoder. The output will be the binary representation of the position of 0 in the input. As zero goes from position 0 to position 3, your guess about the values of S0 and S1 find me positive.

You got two logic functions in your hands, one for S0 and one for S1 which must be treated separately. Also the table is an incomplete truth table. Make a 4-input Karnaugh map for each S, and fill it with the known information. The rest of the minterms (rows) that is unspecified should be filled with Don't Cares (X) in the Karnaugh Map.

3. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
735
43
I don't have my scanner with me atm, but here's what I ended up with as logic functions.

S0 = NOT(A&C)

S1 = NOT(A&B)

Hopefully these are correct!?

4. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
Yes, that's what I come up with too.

5. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
735
43
So my encoder running to my multiplexer is simply two nand gates, correct?

6. ### Georacer Moderator

Nov 25, 2009
5,142
1,266
Seems so. I would ask for a second opinion too, however.