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Posts: 1,570
Does "255" require eight full adders in a digital circuit? I want to make sure I understand this correctly: A full adder outputs a sum 'plus' a carry bit. The sum could be fed directly to an LED but the carry bit is fed to the next full adder. If I built this circuit is this correct?

• Posts: 1,075
Here is a tutorial on adders.
• Posts: 1,570
edited 2019-01-24 - 20:48:44
@tomcrawford, thanks. I wanted to be sure I understood what I was looking at. The tutorial had a diagram that makes it clear. "255" would require seven full adders plus an XOR gate for the MSB.
(edit: the LSB only needs a half adder)
• Posts: 15,697
If you look at the 74LS181 data sheet you will find the logic equivalent circuit. This implements a carry forward circuit that is much faster. I don’t think any of these 74xx181 are available any more.
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Cluso99 wrote: »
If you look at the 74LS181 data sheet you will find the logic equivalent circuit. This implements a carry forward circuit that is much faster. I don’t think any of these 74xx181 are available any more.

There are a few still available. Jameco is sitting on 175. They do not look like pulls.

https://www.jameco.com/z/74LS181-Major-Brands-4-Bit-Arithmetic-Logic-Unit-DIP-24_46973.html
I still have 16 in a tube.
• Posts: 1,981
74xx181 is EOL'd
74HC283 may be its closest living relative (4 bit full adder)

If you want to emulate a 74xx181 I think a rom lookup table would be the way to go as its stateless as
far as I can tell.
• Posts: 1,075
If you google "74181", you'll find several tutorials on the '181, including its very own wikipedia page.

As to using a rom lookup table, there was once a machine that used a lookup table to do addition. Decimal, I think.

It was a "CADET", which stood for "Can't Add, Doesn't Even Try".
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How do I add new topic?
• Posts: 11,180
Iguanaman wrote: »
How do I add new topic?