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

 

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.

 

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.