Except for those that are "Total Bit-Weenies"...
(as in...
"I doan need no stinkin' names!... I like 1s and 0s!")
These examples show just how important it is to develop proper names for the various input conditions.
Certainly you need a string of "true"s to turn on an output... but what does it mean if a particular condition is true?
If the particular element is... ---| |---, and it is true, then the only thing that can be said is that the particular condition is "ON".
If the particular element is... ---|/|---, and it is true, then the only thing that can be said is that the particular condition is "OFF".
Neither case includes a "name". Each case is true, but... what is it that is true? What does being "true" represent?
The Classic Example: This is the PLC version of a hardwired Start/Stop.
START STOP MTR
---| |---+---| |-------( )
|
MTR |
---| |---+
If START is
true and STOP is
true then turn ON the Motor.
So... let's back off a bit and look at common language...
If it is
true that you see a START signal and it is
true that you see a STOP signal... then, Start the Motor.
Hmmm...
Just using a bit of common sense... if someone tells you to do something... and someone else, of equal standing and power, tells you NOT to do that something... what do you do? You obviously have a conflict... it "
does not compute".
The real issue here is the conflict in the names.
Let's look at a real version of a hardwired Start/Stop.
START STOP MTR
---| |---+---|/|-------( )
|
MTR |
---| |---+
There is only one difference between this drawing and the previous drawing.
In the previous drawing, the PLC version, the STOP contact is Normally Open.
In this drawing, the real, hardwired version, the STOP contact is Normally Closed.
In the hardwired world, the STOP signal and its relay indicates that the STOP button is wired through a Normally closed contact. In the hardwired world, you press the STOP button to open the contact and thus drop the circuit. This is how it should be.
It just so happens that you do the same thing, however, with a different contact-type, in the PLC world.
Huuuuuh? (That was a Tool-Man Huuuuh.)
Looking at the hardwired drawing... If START is
true (Start button is pressed) and STOP is
NOT true (Stop button is NOT pressed), then turn ON the Motor.
Again... let's back off a bit and look at common language...
If it is
true that you see a START signal and it is
true that you
DO NOT see a STOP signal... then Start the Motor. Sounds pretty clear to me... how 'bout you?
Looking back at the PLC version...
START STOP MTR
---| |---+---| |-------( )
|
MTR |
---| |---+
.
To be logically consistent... without relying on any "internalized interpretation", the STOP signal should be called... "DON'T STOP".
In that case, if it is
true that you see a
START signal, and it is
true that you see a
DON'T STOP signal... then, Start the Motor.
This is logically correct.
If at any time you
DON'T see the
DON'T STOP signal... then Stop the Motor!
This too is logically correct.
At the very least, the Stop Element should be named "STOP (N/C)"
MGD...
Beyond that example... "names" should indicate the "CONDITION", not the device. For example, if a limit switch is used to indicate that a cylinder is extended... the name of the element should be "Cylinder is Extended"... not LS-2. And, be logically careful about the wiring of that switch... N/O or N/C... and the given name.