# BCD adder and subtractor logic diagram

#### roll cast

Joined Apr 3, 2011
11
I have been set this as a sort of extra credit style homework for my electronics class and I just need a bit of help.

I don't need to make it as it is just a theory homework.

It has to add 2 bcd numbers together and have a carry in and out.

So far I have managed to draw out the full binary adders will xors on the B inputs for a 1's complement subtractor.

But I can't figure out how to convert the binary out put into bcd, negatives are really the problem here.

Thanks
AL

ps. I'll post some screnshots of what I have made in circ.wizard in a minute

#### roll cast

Joined Apr 3, 2011
11
Heres my binary adder part done, I am allowed to use a full adder symbol to save me drawing all the gates out but everything else must be made from gates.

The top 4 inputs are A, the second set of 4 are B and the bottom one determines whether it is adding or subtracting.

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#### crutschow

Joined Mar 14, 2008
26,013
Doing BCD addition and subtraction with logic circuits is somewhat complex. You need to detect when each digit value is over 9 and then carry the excess value to the next digit.

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