Johnny's home-built CPU corner (and other stuff, maybe)
7480 - Full Adder
There isn't much to say about this one. It is what is it, a fast 1-bit full adder. As I can see it there are no more than 3 transistors delay here (you have to scroll down a bit), beating every other full adders. The "traditional" way to build a full adder is like this..:
Now, to make an XOR gate the DTL style, the simplest one I have found is this..:
...which by transistors translates into this..:
You can get rid of D1..D4. They are only there for speed improvement.
The A, B and Cin has to travel through 2 XOR gates, equal to 6 transistors. Add, let say 100 nS for each (slow) transistor and we ends up with a total propagation delay of 600 nS.
In the "normal" world that might not matter, but what if there is a need for speed? Then propagation delay do matters. Fast transistors is needed but there is still 6 of them.
The 7480 solves that by making a full adder differently! And there is only a 3 transistor delay!
Note: Even if 2 bit's can be effective added, the CARRY is still a RIPPLE CARRY.
To solve this the 74LS283 4-bit adder was constructed. It's way more complex but by using CARRY LOOK-AHEAD circuitry it's a better option to do 8 bit or 16 bit additions.
The early 7483 was nothing more than a string of four 7480. The 7483 was later upgraded and given the name 74LS283.
The mighty 7480 !
A page from 1976 TI DataBook, showing all the goodies. Next is a cleaned up version and to the far right you can se my DTL version.
