# 4 bit adder 74LS83 to dual 7 segment decoder 74LS47

#### Windoge10

Joined Mar 16, 2018
3
So we are task to create a circuit with the use of the Ic's mentioned that will add from 0-15 and will display the output in a dual 7 segment. Although from what I know the decoder can only display from 0-9 from the 7 segments we will also need to display each inputs making it 3 dual 7 segment displays. But my problem still is the output, how will the decoder of the output display 0-30? What kind of modifications to a special IC to do this?

#### WBahn

Joined Mar 31, 2012
26,398
So we are task to create a circuit with the use of the Ic's mentioned that will add from 0-15 and will display the output in a dual 7 segment. Although from what I know the decoder can only display from 0-9 from the 7 segments we will also need to display each inputs making it 3 dual 7 segment displays. But my problem still is the output, how will the decoder of the output display 0-30? What kind of modifications to a special IC to do this?
You plan to modify a special IC?

If I give you a five-bit binary value and asked you to determine the two digit BCD values it represents, how would you do this by hand?

#### dl324

Joined Mar 30, 2015
13,130
Don't see how it can be done only using those two ICs; assuming you can use more than one of each.

What is this "special IC"?

Please post the complete text of the problem.

#### Ian Rogers

Joined Dec 12, 2012
919
Special IC is the 74ls185... BUT... I think they are made from unobtainium now..

#### Windoge10

Joined Mar 16, 2018
3
can this be done without 74ls185? I ask every electronic stores and none of them sell it and ebay is out of the context as well, and let me rephrase the context of the circuit; by using 74ls83 create a circuit that will add two 4 bit binary digit and display the sum as decimal to a dual 7 segment, i managed to do up to sum of 25 with bcd correction but 26-30 does not match the desired output with bcd, i also looked up on the 74ls185 datasheet but it doesnt have the schematics on it, can this be implemented by basic logic gates? are there any methods i can try?

#### dl324

Joined Mar 30, 2015
13,130
i also looked up on the 74ls185 datasheet but it doesnt have the schematics on it
Every datasheet I've read said it was implemented as a lookup table using ROM.
can this be implemented by basic logic gates?
The binary to BCD conversion problem has been posted a number of times on this and other sites.

There was also a more recent thread on 5 bit converter.
are there any methods i can try?
Make your on lookup table in some memory (EPROM, NOR FLASH, parallel EEPROM, PROM, etc).

#### Windoge10

Joined Mar 16, 2018
3
We cant use memories or microcontrollers yet but I saw exactly something similar to how it suppose to work
. But we still have one more condition to follow so lets leave that for now.

#### Ian Rogers

Joined Dec 12, 2012
919
can this be done without 74ls185? I ask every electronic stores and none of them sell it and ebay is out of the context as well, and let me rephrase the context of the circuit; by using 74ls83 create a circuit that will add two 4 bit binary digit and display the sum as decimal to a dual 7 segment, i managed to do up to sum of 25 with bcd correction but 26-30 does not match the desired output with bcd, i also looked up on the 74ls185 datasheet but it doesnt have the schematics on it, can this be implemented by basic logic gates? are there any methods i can try?
I did it with an Eprom... 2716... Just programmed the first 30 locations to BCD..

#### absf

Joined Dec 29, 2010
1,949
I googled and found a simple schematic to add two 4 bit numbers and display the result in BCD format.

The principle is to add '6' when (S1.S3)+(S2.S3)+C=1

But it can only allow outputs up to decimal 19 max. When the result is 20, it was displayed as "1A"

So I add another 7483 and extend the results to 29. But same problem still happens when "30" is displayed as "2A"

Can you show us how you modified your circuit to display up to 25?

Allen

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