BCD Adder using 7483

BCD Adder:

1. A BCD adder adds two BCD digits and produces a BCD digit. BCD number cannot be greater than 9.
2. The two given BCD numbers are to be added using the rules of binary addition.
3. If sum is less than or equal to 9 and carry=0carry=0 then correction is necessary. The sum is correct and in the true BCD form.
4. But if sum is invalid BCD or carry=1carry=1, then the result is wrong and needs correction.
5. The wrong result can be corrected by adding six (0110) to it.
6. The 4 bit binary adder IC 7483 can be used to perform addition of BCD numbers.
7. In this, if the four-bit sum output is not a valid digit, or if a carry C3C3 is generated then decimal 6 (0110 binary) is to be added to the sum to get the correct result.
8. Fig1 shows a 1-digit BCD adders can be cascaded to add numbers several digits long by connecting the carry-out of a stage to the carry-in of the next stage.
9. The output of combinational circuit should be 1 if the sum produced by adder 1 is greater than 9 i.e. 1001. The truth table is as follows

Operation: Case1: Sum ≤ 9 and carry = 0

• The output of combinational circuit Y’ = 0. Hence  B3 B2 B1B0  = 0 0 0 0 for adder-2.
• Hence output of adder-2 is same as that of adder-1

Case2: Sum >9 and carry = 0

• If S_3 S_2 S_1 S_0 of adder -1 is greater than 9, then output Y’ of combinational circuits becomes 1.
• Therefore B3 B2 B1B0  = 0 1 1 0 (of adder-2).
• Hence six (0 1 1 0) will be added to the sum output of adder-1.
• We get the corrected BCD result at the output of adder-2.

We can implement BCD addition using 4-Bit Binary Adder i.e. 7483.

Pin Configuration 7483: