what does "not false", "not true" mean.

bear with me, i am new at this. looking at the ladder logic for the system we are looking at, it shows every thing true, except for 1 item that says "not false". does anyone know what this means?

Welcome newbee,
When you say "1 item that says 'not false'", does it say that in a label or description for a contact? If so, and the instruction is not true, then the address needs to change to the opposite state in order to make the rung true.

Sometimes programmers use a description that has a negative word in it that later leads to confusion especially when applied to normally closed contacts. You then have a double negative and have to stop and think about what it actually means.

What kind of PLC are you working on, and can you give more details about the nature of the problem?

With respect to logic, what is the opposite of "TRUE"?

Some would have you think that the opposite of "TRUE" is "FALSE"... not so.

The opposite of "TRUE" is "NOT TRUE". We humans interpret the value of "NOT TRUE" to mean "FALSE". It's a language thing.

The opposite of "FALSE" is not "TRUE"... the opposite of "FALSE" is "NOT FALSE".

To illustrate...

If a normally open contact in a limit switch is used to indicate that a door is "CLOSED"...
What does it mean if the signal is OFF (the contact is open)?

Does it mean the human version of the opposite?
To humans, the opposite of "CLOSED" is "OPENED". Is the door "OPENED"? Maybe... maybe not.

Or does it means the logical version of the opposite?
In logic, the opposite of "CLOSED" is "NOT CLOSED". Is the door "NOT CLOSED"? Definitely so!

Newbee...
Your question was not only taken out of context, it is completely without context. If you haven't already figured it out then please provide a little more context so that we might better understand what it is that you see.

(91)

Here we go again.

91

Click to expand...

What is it this time?
Prime factors of 91=7x13.


91 is a Triangular Number.

91 is a Hexagonal Number.

91 is a Centered Hexagonal Number.

91 is a Centered Nonagonal Number.

91 is a Centered 15-gonal Number.

91 is a Centered 30-gonal Number.

91 is a Square Pyramidal Number.

91 is a Centered Cube Number.

So you want an easy way to determine if the output at the end of the rung should be ON or OFF?

Well, you could read the manual, they always have a great way of explaining it for non-programmers. I mean who’s going to look at the program, non-programmers? It’s not like they made the symbols look like relays so people who are familiar with relay logic would be able to understand it.

Ok, so here’s the simple way to read a rung. Pretend it is relay logic, look at the contacts, if you have a signal that is associated with that contact, the contact will be the opposite that is shown. So if you have a normally-opened contact and that input is on, the contact is closed. If you have a normally-closed contact and that input is on, the contact is opened.

By the way, Not False means True.

"Not false" does not always mean "true", it could mean "not proven to be false"

Yes a program contact will mimic the input device (barring any fault), but is the input device correctly monitoring the real world?

Using Terry's example, if the door is simply "ajar", not completely closed but not completely open, or if the door sensor fails then "not closed" absolutely does not mean "open".

I think this is what Lancie was refering to with his "8 states".
http://www.plctalk.net/qanda/showpost.php?p=107193&postcount=16

I have to agree with Jim, using Terry's example. If the door is NOT CLOSED, it is assumed to be OPEN.

On none critical process applications, we usually only have one valve position limit, typically the closed limit. Then in your logic the valve is closed, or not closed. Unless you have feed back for the open position, I think not closed is preferred to open.

When I think about this one I'm with Terry.


When I program HMI indicators for things such as pneumatic cylinders or thrusters with switches at the end limits I usually include at least the following 4 states.

• Alarm (can be caused by many things)
• Extended (Extended Sw. Actuated + Retracted Sw. NOT actuated)
• Retracted (Retracted Sw. Actuated + Extended Sw. NOT actuated)
• Between (Neither Retracted NOR Extended Sw. Actuated)

The state where both the Extended AND Retracted Switches are on I call "Switch Mismatch" and this will be one of the alarm conditions along with Extend and Retract timeout conditions.

Terry is absolutely correct. I learned all this recently from the Discrete Mathematics course I took. In that class we discussed the foundation of logic, not assumption. Tark, your final comment is an assumption and not, therefore, based in logic. Reasonably and possibly intuitively you would be correct, but from the standpoint of mathematics and logic, you would be incorrect. Terry is right, we need more context to answer your question, but I can assume from past experience, you are probably dealing with XIO and XIC state logic, whether it be your intepretation or psuedocode provided by someone else, that you are trying to follow the reasoning. Please expand for us.


David

Yes, I should have left the last statement out because I was making a large assumption in saying it.

In other logical terms this is the confusion between the "contrary" and the "contrapositive". The "contrary" of an expression states an opposite state (black - white). The "contrapositive" of an expression states all other possibilities (black - not black). If the device sensed by an on/off switch truly only has 2 possible states then "Not False" equals "True" (the contrary equals the contrapositive). If it is measuring only one of many possible states then "Not False" only means "Not False". In this case these also wouldn't be good names. For example, if measuring a cylinder's extreme positions, the "extended" and "retracted" are much more appropriate allowing for the intermediate ("neither extended nor retracted") and the illegal ("extended AND retracted") psossibilities.

Just for the hell of it... What is the meaning of each of the following? That is, what question is being posed?
I'll do the first one for you...

   FALSE

1 --| |-- = This is asking, "Regarding the condition named "False", is it True?





   FALSE

2 --|/|-- =



    NOT

   FALSE

3 --| |-- =



    NOT

   FALSE

4 --|/|-- =





   TRUE

5 --| |-- = 





   TRUE

6 --|/|-- = 



    NOT

   TRUE

7 --| |-- = 



    NOT

   TRUE

8 --|/|-- = 

Just for Ron...
(323)

Just for Ron...
(323)

Click to expand...

lmao...

Motzkin...btw that is some strange stuff, I am not sure I fully grasp it...yet.

"Motzkin"??? Who or What-faaaak-Dat?