# 9's Complement

Joined Oct 22, 2007
2
hello! this is my first post in these forums

and I came here for help with a Digital Logic class

experiment. The problem is:

Design a combinational circuit with four input lines that

represent a decimal digit in BCD and four output lines that

generate the 9's complement of the input digit. Provide a

fifth output that detects an error in the input BCD number.

This output should be equal to logic 1 when the four inputs

have one of the unused combinations of the BCD code.

I need help, my professor has a terrible accent, I can't

ever understand him, and he is an asshole when you ask

him questions. The logic gates you can use are: NAND AND

OR NOR INVERTER and XOR

Thank you, I need an answer asap

#### Dave

Joined Nov 17, 2003
6,969
Have you done anything?

My experience of bad tutors suggests that, if what you are saying is correct, you should make a complaint. You, or someone who has an interest, has paid good money for an education and negligent tuition is not acceptable, particularly if it inhibits your ability to a) learn and b) ask questions.

Dave

Joined Oct 22, 2007
2
it's easy to make suggestions, but the State still wants its percentage of minority Professors. I have paid good money, and its pisses my off....but enough complaining, I have come here for help

I don't have any idea what a 9's complement is and I don't know how to create an error detector. If I knew those things I could at least start

thank you

#### Papabravo

Joined Feb 24, 2006
20,386
The 9's complement of a digit is the number you need to add to it to make that sum equal to 9. Let's use the tilde symbol '~' to mean the 9's complement of.

Rich (BB code):
~0 = 9
~1 = 8
~2 = 7
~3 = 6
~4 = 5
~5 = 4
~6 = 3
~7 = 2
~8 = 1
~9 = 0
An error detector in this context is a logic curcuit that outputs a '1' when the four inputs have any of the following six binary patterns:
Rich (BB code):
1 0 1 0 = 0xA = 10 (decimal)
1 0 1 1 = 0xB = 11 (decimal)
1 1 0 0 = 0xC = 12 (decimal)
1 1 0 1 = 0xD = 13 (decimal)
1 1 1 0 = 0xE = 14 (decimal)
1 1 1 1 = 0xF = 15 (decimal)
It's too bad universities don't have a money back guarantee. I'd certainly make some noise.

#### thingmaker3

Joined May 16, 2005
5,083
and he is an asshole when you ask

him questions.
I suggest having a word with the dean. The professor is being paid to help students with questions, and has an obligation to do so. Part of the dean's responsibility is to make sure the professors live up to theirs.

#### techroomt

Joined May 19, 2004
198
The 9's complement of a digit is the number you need to add to it to make that sum equal to 9
really? i love learning from the wealth of info on this site. i assumed he meant ones complement, or just the complement. thanks papabravo.

#### arthur92710

Joined Jun 25, 2007
307
The 9's complement of a digit is the number you need to add to it to make that sum equal to 9. Let's use the tilde symbol '~' to mean the 9's complement of.

Rich (BB code):
~0 = 9
~1 = 8
~2 = 7
~3 = 6
~4 = 5
~5 = 4
~6 = 3
~7 = 2
~8 = 1
~9 = 0
Well... after your explanation It sounds easy.
I never herd of it.
But binary doesn't go to 9 only 2 (0 and 1)
Would it be like...
0000 = 1001
0001= 1000
0010= 0111
...ect...

#### beenthere

Joined Apr 20, 2004
15,819
Think of Binary Coded Decimal.

0000 = 0
0001 = 1
0010 = 2
0011 = 3
0100 = 4
0101 = 5
0110 = 6
0111 = 7
1000 = 8
1001 = 9

#### Papabravo

Joined Feb 24, 2006
20,386