## Circuit help?

### Circuit help?

83837.news.uni-berlin.de:

>> <snip!>

>> 4 full adders wires in parallel.

> Would one half-adder (for the LSB) with three full-adders not be a
> preferable solution as there will be no carry input for the LSB?

>> Now do your own homework.

> OK, sir.

yes that would be fine. I like to keep options open - but yes you don't
need the carry bit on the LSB adder.

### Circuit help?

+---------------
| Here is the easy solution:  go to the library and find a really old Texas
| Instruments TTL data book, with the gold covers from about 1970 or earlier.
| Look up the 7481.  Copy to paper.  Turn in.
+---------------

Awww... Come on, Del, don't be cruel. At least let him use something
from the mid-1980's, like a 74LS381 maybe...

-Rob

-----

627 26th Avenue                 <URL:http://rpw3.org/>
San Mateo, CA 94403             (650)572-2607

### Circuit help?

> > <snip!>

> > 4 full adders wires in parallel.

> Would one half-adder (for the LSB) with three full-adders not be a
> preferable solution as there will be no carry input for the LSB?

I believe the OP wanted to be able to subtract.  This normally
involves setting the input carry (or resetting the input borrow

Also if he is going to use 1's complement, he will need it for the
end around.  So either way, the full adder.

--

Available for consulting/temporary embedded and systems.

### Circuit help?

> +---------------
> | Here is the easy solution:  go to the library and find a really old
Texas
> | Instruments TTL data book, with the gold covers from about 1970 or
earlier.
> | Look up the 7481.  Copy to paper.  Turn in.
> +---------------

> Awww... Come on, Del, don't be cruel. At least let him use something
> from the mid-1980's, like a 74LS381 maybe...

> -Rob

> -----

> 627 26th Avenue <URL:http://rpw3.org/>
> San Mateo, CA 94403 (650)572-2607

I wasn't sure the later books still showed the logic diagrams.  Loved those
schematics/logic diagrams in the TI book.  Saved me many a time.

- Show quoted text -

### Circuit help?

> > +---------------
> > | Here is the easy solution:  go to the library and find a really old
> Texas
> > | Instruments TTL data book, with the gold covers from about 1970 or
> earlier.
> > | Look up the 7481.  Copy to paper.  Turn in.
> > +---------------

> > Awww... Come on, Del, don't be cruel. At least let him use something
> > from the mid-1980's, like a 74LS381 maybe...

> > -Rob

> > -----

> > 627 26th Avenue <URL:http://rpw3.org/>
> > San Mateo, CA 94403 (650)572-2607

> I wasn't sure the later books still showed the logic diagrams.  Loved
those
> schematics/logic diagrams in the TI book.  Saved me many a time.

Ok ok ... walk over to bookshelf on other side of room.
Choose "High-speed CMOS Logic Data Book" from TI, circa 1984.
Open page to "TYPES SN54HC181, ...".
Logic diagrams show ALU plus 4 bit in / out.
ALU functionality is given only by function tables / state tables.
No logic diagram for the ALU.

But wait!
Here is also the very well worn "The TTL Data Book Volume 2"
by TI circa 1985. What is in it I wonder?
Go to page 3-713.
Look! A logic diagram for the ALU.

Ah, yet another one: "ALS/AS Logic Data Book" by TI circa 1986.
It also has the logic diagram.

Put the books back ... oops ... out falls a copy of "A Lightning
Empiricists, literary supplement 1 Preliminary Edition"  from
Philbrick / Nexus Research circa 1969! Have been looking for
this for years. It was used as a bookmark in the TTL databook.

That was fun!

--

... Hank

http://horedson.home.att.net
http://w0rli.home.att.net

### Circuit help?

>+---------------
>| Here is the easy solution:  go to the library and find a really old Texas
>| Instruments TTL data book, with the gold covers from about 1970 or earlier.
>| Look up the 7481.  Copy to paper.  Turn in.
>+---------------

>Awww... Come on, Del, don't be cruel. At least let him use something
>from the mid-1980's, like a 74LS381 maybe...

What is so cruel about the logic diagram of 7481 ?

The older data books even showed the circuit at the transistor and
resistor level :-), well at least for NAND and XOR gates.

IIRC some manufacturer when talking about a package containing a few
XOR gates talked about MSI :-).

Paul