Question

2. A BCD code is being transmitted to a remote receiver. The bits are: A3,A2,A1,A0 where A3 is MSB. The receiver includes a BCD error detector circuit that examines the received code to see if it is a legal BCD code. Design the error circuit to produce a high for any error detected using NAND gate only.

Answer 1

according to what i know of digital logic, BCD only goes from digit 0 (0000) to digit 9 (1001), because for 10 you need two digits, so all you've got to do is make a function that produces high for numbers from 10 (1010) to 15 (1111) as follows:
A3 A2 A1 A0 F
0 0 0 0 0
0 0 0 1 0
0 0 1 0 0
.........
1 0 0 0 0
1 0 1 0 1
1 0 1 1 1
1 1 0 0 1
1 1 0 1 1
1 1 1 0 1
1 1 1 1 1

Then simplify the function:
F = A3*A2 + A3*A1
Finally just draw or connect the circuit using NAND

Answer 2

if you don't know already how to implement basic gates using universal gates (NAND and NOR), you can find it here:
http://faculty.kfupm.edu.sa/COE/abouh/Lesson2_6.pdf

Answer 3

@Estebananaya
Thanks a lot.... i appreciate the parting of your knowledge.