The 7480 full adder

The 7480 was a one Bit binary full adder.
A similar device with identical pinout and functionality
was the Signetics 8268.

It sure looks a bit odd, especially the low_active
carry output.
Also note the inverted and non_inverted sum outputs
Q and /Q.

Depending on how we connect those funny NAND
gates at the A,B inputs, it is possible to configurate
the adder inputs to be high_ or low_active.
(inverted or non_inverted, that is).

There is a reason for all this, but we are discussing
that later. Now for the two different input configurations:

(Same thing for the A input.)

The first NAND (fed from B1, B2) is an open collector
type (like 7401), and the Signetics datasheet mentions
that the pullup resistor at the NAND output is 4.3 kOhm.

Now to take a look at the circuitry inside the adder box:

And when taking a closer look to the truth table
of the adder, it turns out that our adder also works
well if we change polarity of all the input/output
signals.

Two Bit adder example with inverted and non_inverted
carry signal. Note the /C1 signal:

Side note:
The Bit_serial EDUC8 computer used the 7480 and some logic
gates as ALU, but for best performance I would recommend
taking two 74151 multiplexers as ALU for building Bit_serial
machines... like described in "ALU design Part 3, CM-1".

The first implementations of the 7483 four Bit adder contained
four 7480_like full adders.
Later versions of the 7483 started to use a fast lookahead carry...
until the 7483 eventually went obsolete by the invention of the
74283.

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