Connecting a 4-bit adder to a 1-bit adder

Posted on 2004-03-21
Last Modified: 2008-02-01
How do you connect a 1-bit full adder (designed with 9 NAND gates) to a 7483 (4-bit adder)?

I'm having problems with it working 100 percent.

1. Built the 1-bit adder (A,B,Cin); (S,Cout) , tested with the truth table; worked flawlessly.

2. Connected the 7483, making the 1-bit adder the higher-order bit.  I grounded the Cin of the 7483, connected the Cout to the Cin of the 1-bit adder, connected the all Sums to leds, and A&B's to switches.

Some sums are incorrect:

(10011)+(10111)  does not = 1  01010
(11111)+(11111) does not = 1 11110

What could I do to make this 5-bit adder work?
Question by:940775
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Expert Comment

ID: 10644394
First of all, the cirquit would be simpler and less wasteful if you would build a 1-bit *half* adder from nand ports, and make it the lower order bit. The problem might even vanish...
On the other hand, I can't spot any error in your explanation. Are you 100% sure there's no error in the truth table of the full adder?
The bottom line: I think there's no way of telling what the problem is, if you provide the correct sums and not the incorrect output ;-)

Author Comment

ID: 10644469
The instructions  stated a 1-bit adder as the higher.

Truth table
a  b  cin     s   cout
0  0   0       0     0
0  0   1       1     0
0  1   0       1     0
0  1   1       0     1
1  0   0       1     0
1  0   1       0     1
1  1   0       0     1
1  1   1       1     1

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Expert Comment

ID: 10648060
Could you post the values which are incorrect?
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Author Comment

ID: 10648389
The two that would not work for me is listed above.  Since then I was able to get one of them to work ( 11111 + 11111 = 1  11110).

I may have had the 4-bit SUMs LEDs reversed. I "HAD" them in this order (from L to R):

Cout, S(1-bit), S1, S2, S3, S4(4-bit)


Cout, S(1-bit), S4, S3, S2, S1(4-bit)

The only one not working for me is (10011 + 10111).    The ans should equal 1  01010, but I'm getting  1  11010

LVL 27

Expert Comment

ID: 10649930
Either you have a permanent "1" or some wire is wrong.

I'd suggest you do the zero test. Set input A to all zeros and see if the LEDs display input B.
Then reverse the process, B zero test A.
Then add 1 to input A (making a 00001) and test each value of B.
Then set A to 2 and retest
Then set A to 4 and retest
Then set A to 8 and retest.


Author Comment

ID: 10651039
I got it.

What I did wrong was in the switch set up.  I" had" the switches set:

a1-bit, a1, a2, a3, a4        which is wrong

The correct way:

a1-bit, a4, a3, a2, a1        in which a4 is the most significant bit of the 4-bit adder.

Expert Comment

ID: 10651760
"The instructions stated a 1-bit adder as the higher"
Smells like homework to me, but ok, you seem to have found the solution yourself ;)

Author Comment

ID: 10652227
yes, the 1-bit is the highest of all the bits, but when they (1-bit & 4-bit) are cascaded the higher order of the 4-bit is a4 (3,2,1). I had a1(2,3,4).

My graded switch setting:

a1(1-bit) , a4, a3, a2, a1(4-bit)        b1(1-bit), b4, b3, b2, b1(4-bit)


Cout, S1 (1-bit) , S4, S3, S2, S1(4-bit)
LVL 27

Expert Comment

ID: 10656421
I wonder how you did all of this which resulted in such problems.

A very long time ago I did my Computer Science practical in hardware.
First we made a schematic diagram, showing the logic which was to be constructed.
Second we made a hardware schematic showing the physical construction.
Third we made a construction schema, showing in which order (like Swedish furniture) the parts were to be assembled.
And lastly we made a "string list". This was a list which said, connect a wire from a to b and so on.

Only after we had done all this paper work and checked it were we allowed to start construction. In fact one of us did the construction, the other checked what was done. (My lab partner is now a senior IBMer in Kingston NY).

When I worked for a computer manufacturer I was not allowed to do the construction - only the paperwork. That's why I changed to Software. You can spend many happy hours debugging the crap you wrote if you don't start off in a disciplined manner.
LVL 27

Expert Comment

ID: 10947289
Since I put some work in, no refund of points.

Accepted Solution

ee_ai_construct earned 0 total points
ID: 10967556
PAQed, with points refunded (250)

ee_ai_construct - (re-order part number #xm34)
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