Now, the descriptor of a Tag is, is just that. It's a
description, and there's only about one or two
places where you will see this pop up. You will see it in
ProcessBook we we modify a display, or a trend,
here. You have the option to put the description down here in the
corner. You can also do this in DataLink.
Now we also have a,an Engineering Units
that is, for the most part, just the same kind of,
of field. It's an open field. You can put anything you want
there, and about the only purpose for it is that you will
see it show up in ProcessBook and also
you can do this in DataLink, too. See, there's a field here
for Engineering Units. I can turn it on and off.
Let me turn off this description here. Anyway, that's,
that's about the only thing we do with Engineering Units which is kind of
surprising because, you know, you, you would think if we are doing some kind of
a total of a flow we would want to know what the Engineering
Units are. But, we actually end of using
a conversation factor for that. So, that's a completely open
field. Does not really matter what you put in there for Engineering Units.
Now let's look at Zero and Span. You will find Zero
and Span on the Archive Section.
And I will explain why that is in just a second. But let me first
explain how Zero and Span work. Now the Zero
is going to be the smallest amount and
when you add the range, the Span, to the
Zero, you get effectively what the range of this Tag is.
So, for example, as you can see in this illustration here,
if my Zero is 50 and my Span is a hundred,
I end up with a, a range that's
right here, between 50 and a hundred. Now this is more
like what we are use to seeing. With a Zero, Zero and a
Span of a hundred, you get this range. Here we have got a, a Zero
of negative 50, a Span of a hundred, so now
we are, we are saying the range basically is negative 50
to 50. Now, does that mean that we
can only bring in data for
Values within this range? Actually, it does not
mean that. All it really means is that
the Values that we show in
ProcessBook, if you have chosen this option
called DataBase, those Values are going to
be the Values we pull from the Zero on the Span. Now, there are a couple of
other places we use it. When we configure,
for example, multi-state objects. Let me
create a quick multi-state object here.
Yeah, there we go.
I will go in here and I will modify
this as a multi-state.
See, this, this multi-state will actually
go ahead and find the Zero and Fan -- and Span,
from the provided Tag. That's
the only other place where you are going to see this Zero and Span
being made use of within ProcessBook.
So, some of the clients will make use of the Zero
and Span for display purposes, as you saw, but
there is no restriction on what you can actually bring into the
system when you choose a Zero and Span.
You can actually bring in values that are less than the Zero
or greater than the Zero plus the Span. Now, the only exception
to the rule there is if you are using a Float16. That
does limit the values to between the Zero and the Zero plus
the Span. Now there is
one more important purpose of the
Zero and the Span, and that's the reason that you see it in this
Archive Section here. And that is that the Zero and
Span is used when we are figuring out our
compression deviation if that compression deviation is
measured in percent of Span. Now, we will talk a lot
more about that when we talk about data compression.
And we also have a field here called Typical Value, and the Typical
Value's really not used in any of the
applications except in one place, in Performance Equations, for
testing. You can test an equation against Typical Value.
I believe we are doing that in ACE as well. So, we
use it for that so we are, we are suggesting you do is put something that would
represent, you know, what's typical for this
particular Tag. So just put something in there that gives
you as a way of internally documenting what would be typical.
And we do use it for testing in some situations.
Now, we have a, another flag here called Step,
another attribute. And, when Step is turned on,
what it does is it allows you to, to display these,
or this information as a stair-step in
ProcessBook. We, we end up treating it like it's stair-step
data, and it actually does change a little bit of the
behavior when it comes to
exception and compression deviation. Let me, let me explain. First
we will take a look at ProcessBook. Okay, what we are looking at right
here is ProcessBook Trend. It's got six
different Tags on it. They are, they are basically the same sine wave.
But, I have changed some of the attributes to demonstrate
what these attributes look like. So, for example, here's the regular
sine wave. Here's that same sine wave if we have
turned Step on. As you can see, we are displaying
it as a stair-step function. We are just drawing
straight stair-steps in-between the Archive Values.
Normally, we would connect the dots between Archive Values.
Now, this also has some implication when it comes
to doing your data compression. We will have to talk more
about that when we talk about data compression but, you know,
basically, if you choose Step turning on, we are going to treat
the compression deviation as a straight dead band
instead of compensating for Slope. We will talk more
about that later when we talk about compression. Now,
we also have something here called Display Digits.
You will find that back here. It's called Display Digits
and what we are doing with Display Digits is we are determining
how many digits we are going to display for
purposes of working within PI ProcessBook. Now,
the convention that we use for Display Digits is that
if the number is positive number,
then that's going to stand for decimal places. So,
for example, if the Value coming in is
12.345, and you use this three, it's just going to show
us 12.345. It's three decimal places.
If the Value is this with just the,
the point six at the end there, as you notice, with the Display
Digits, we are going to display that with three decimal places.
Now, if we were to use a negative number,
the negative number indicates significant digits.
So, in the case of this, you know, this number here,
that number, that's going to come back as just
12.3. In the case of this, with three significant digits,
we are just going to round that to 123,000.
So, I will show you the, the difference here.
If we look at our ProcessBook Display,
I will go ahead and show you. This is the regular Sinusoid. Now Sinusoid by
default, all tags by default, use negative
five as its Display Digits. That means five
significant digits. I changed this,
I choose this Display Digits here.
I chose that to be
one decimal place. So, if we look, take a look at this other Tag,
Sinusoid_DisplayDigits.
As you can see,
I put the Display Digits at one, and since that's a positive
number, that indicates it's going to be one decimal place.
And so that's how it shows up here. Now, it also
is going to show up that way if we drag a trend cursor.
So, as I drag my trend cursor,
when I drop it -- you will see right here, this is showing
up with that Display Digits that you have chosen. Now that's
also something that's supported in DataLink. Within DataLink
you can use Display Digits as the,
you know, to set the default of what's displayed. Now,
there are a couple of more flags I would like to go over. One of
them is called Archiving. We will go back and look where that is in a second.
But, just let's look at the effect. When you turn
Archiving off, all it really does is
it turns off archiving. We are no longer are going to archive
these Values. So, as you can see, this particular
Sinusoid here, with Archive turning off, we do not see any
history for this Tag. If I were to make
that first one disappear, as you can see, we are only seeing the
current Value. Now, that will go ahead and continue to update as we
watch it, but there's no history for it. So, that's all we mean by
Archiving turned off. Now, the last thing is we have a,
a setting called Shutdown. And Shutdown --
it's, it's done on the same place as Archiving. Let's go back
in here. This is under Archive.
Yeah, see here's my Archiving Flag, and here's this
one called Shutdown Flag. Now, by default, all the
Tags that you create have Shutdown turned on.
For the Tags that are created on the local PI
Server, I suppose that's fine, as
what Shutdown means is that when you shut down the server,
we will actually write the word Shutdown
into the history for that Tag. So, if I were to
drag a Trend Cursor over here -- and take a look
up in the corner there. Do we see that?
Ah, I am going to have to zoom in a little bit more here.
Yeah, there we go. See the word Shutdown?
We actually wrote that word Shutdown at that
point because that's when the PI Server shut down. If I
were to trend this Shutdown, you will notice
there was no Shutdown written. Let me remove this Trend
Cursor. See, there is no -- we see no Digital
State there, and that's because this particular Tag
had its Shutdown turned off.
So, if I take a look at this -- see, here's the traditional
Sinusoid with Shutdown turned
on. If I look at this, this is Shutdown turned
off, and the result is we -- well, we really
do not know that there's been a shutdown on the PI Server.
And, actually, that may be what you want for
a lot of Tags. For all the Tags that are collected on
Interface Nodes, you really do not want a shutdown of the Server to
introduce these Shutdown -- you know, these, these
Shutdown events into the history
of that Tag, if it's a Tag whose Values are collected elsewhere.