Re: Could you please help me - branch command

On Mar 17, 2:45 am, "Evenbit" <nbaker2...@xxxxxxxxxxx> wrote:
On Mar 13, 6:34 pm, "robertwess...@xxxxxxxxx"

<robertwess...@xxxxxxxxx> wrote:
On Mar 13, 4:48 pm, "Evenbit" <nbaker2...@xxxxxxxxxxx> wrote:

Of the whole set of logic gates, only two are needed to create every
logic circuit you can think of.

Actually only one. Dual input NAND gates can be used to build
anything. Dual input NOR gates are similarly universal. Other
circuit types have similar properties, for example IIL did everything
with inverters and wired-ors (which, as the name suggest, are
completely passive).

Well the "wired-ors (passive circuit)" should be your clue. To find
that which is elemental (or atomic), we must step away from the "high-
level" "abstraction" world of active logic gates. Most of these
(NAND, NOR, XOR, etc) simply can not exist as atomic units in the "low-
level" passive circuits.

To examine this in detail we will look at the "ladder logic" (AKA
"ladder diagrams") used in industrial controls and adopted as the
primary language for PLCs. In this world, there exist [exactly] two
(2) of everything. On one hand we have an energy source (provides for
the input signals) and on the other we have the energy drain (provides
the output state). On one hand we have "normally open" switches and
on the other we have "normally closed" switches. And more to the
point of our discussion - on the one hand we have our switches placed
in a "series" electrical arrangement and on the other we have our
switches placed in a "parallel" electrical arrangement. The "series
circuits" implement a logical AND operation. The "parallel circuits"
implement a logical OR operation. [ as a side note: The last two
statements are true when the goal is to get the output in an
"energized" or "on" state. When analyzing the circuit with an eye to
getting a "non-energized" or "off" state, then the roles of the
circuits are reversed: series = OR; parallel = AND ]

Okay, I know that you can examine a print-out of a typical complex
ladder diagram.. going through it with your trusty red crayon to draw
circles around sections and labeling them with "this is a NAND gate"
or "here is an XOR" -- but this is further proof that they are "high-
level abstractions" because when you look closely, you see that they
are constructed from "discrete" "elemental" "atomic" units: 1) two
switches in series [a logical AND], 2) two switches in parallel [a
logical OR].

Now, to prove that two (2) logic units are required instead of the one
(1) [for instance, NAND] claimed by a few of you, we need a test. So,
from the set of all possible ladder diagrams (infinite) pick any
example to your liking. For every parallel circuit found in a given
diagram, is it possible to redraw the diagram so that only series
circuits remain and the diagram performs the same function? The
answer is no! Likewise, for every series circuit found in a given
diagram, is it possible to redraw the diagram so that only parallel
circuits remain and the diagram performs the same function? Again,
the answer is no! Okay, I know what you are thinking: Just make use
of mechanical interlocking on the switches to eliminate the
"offending" circuits. That will not work! That simply invalidates
the ladder diagram - therefore making it useless. Why? Because the
"control logic" is contained within the "mechanical interlocking" and
since you have removed representation of this "control logic" from
your diagram, the document loses its purpose. If you put this
required information back in, then you end up with the original given
ladder diagram. "Back to square one!"

So you see, one cannot call himself a "self-respecting, card-carrying
Asmer" while at the same time talking about NAND gates as some sort of
magical, all-powerful, fundamental building block. Clearly, those who
speak of such things have their head in the "high-level abstraction"
arena (or somewhere else ;) but certainly they are not thinking in
"low-level" "machine organization" terms.


Errr... You do realize that not all the world is CMOS, right? And
that much logic has been implemented that did *not* use complementary
type of transistors? The 8086, for example. And that very commonly
the pullup (or pulldown) "resistor" in NMOS (PMOS) was implemented as
a plain-old transistor (of the same type), with a fixed input (or none
at all)?

.

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