Now we've got everything hooked up we've got our interface it connects to our data
archive through its buffer subsystem and it also connects to our OPC server.
Our infrastructure is looking a little bit like this. Okay the next thing to do is actually get
data flowing we haven't got any data from our dcs all the way to our data archive yet.
The first thing we need to do to get that happening is create a tag.
Creating a tag effectively creates us a place to store data for this sensor
in the data archive. It stores data in two different ways so it has these two little
places one of them is called the snapshot and the other one is called the archive.
We saw that the data archive can have many archive files it only has really one
snapshot. The snapshot stores the current value of each tag that's been created and the archive
stores historical data for those tags. When you create a tag it allocates a little bit of room
in the snapshot for the most recent value that the tag has received from all the way from the sensor
through the OPC server etc. and it allocates a little record in the most recent archive
to store historical data for the tag. Now this tag doesn't only have data associated with it
also has attributes so this tag has a name, a description, units of measure and a bunch of other
things that we're going to configure. The easiest way to understand what a tag is to create one.
And here we are on PISRV01 and we've got PI system management tools open.
System management tools is the application we use to make tags one by one we'll be making tags in
bulk later in the course using excel but for now navigate through to points and then point builder.
We're going to create a new tag so click new, then we have to fill in some of these details.
We have to give it a name now we're creating a tag for the bearing temperature of pump number one
now this is where we decide on what to call our tag uh this is where it'll be useful if
we had a tag naming convention of some kind usually you'll want to base this on the tag
name convention of your control system but we're going to make one up as we go along.
So this is the bearing temperature of pump one so we're calling it pump one stop bearing temp.
There's a few more things to fill in here there's a descriptor so
bearing temperature of pump one there's a few more things to fill in in this pane. First off
is the point source, the point source must match the point source parameter of the interface we
would like to load this tag. So we want to use our interface that we just created to see this tag,
load it and then gather data from the OPC server.
So this point source needs to match that on the ICU, so we can flick over to our PIINT01.
Now we can see that the point source of our PIINT01 is OPC,
so let's flick back and type in OPC in the point source. Next up engineering units always important
I know that this bearing temperature is in degrees Celsius
and we're going to navigate to the classic tab.
Now the first thing we need to fill in here is the instrument tag, the instrument tag
has to correspond to the item id of this tag on the OPC server. I happen to know that the
item id of this is sample process slash pump one slash bearing temp. So we'll fill that in there.
A few more of these need to match some other thing. Location one must match the interface id of
the interface we want to load it so let's go over to our PIINT01 and see our interface id is one.
Let's go back and fill in one here.
Location three decides if this tag is going to be polled or advised.
We'd like a advice tag, location 4 must correspond to the interface's
location 4 must match the number of the scan class of this interface corresponding to
the scanned rate at which we want to scan this tag. We can go over to our PIINT01
we can see four different scan classes, three of one second and one of two seconds. The one seconds
do seem redundant but different scan classes will be loaded as different groups on the OPC server.
The OPC server will have in its specification how many tags are allowed to be part of a single
group and you can use these scan frequencies to separate them out. So we want our tag to be
a one second advise tag. We're going to talk about advise tags in a little bit.
That corresponds to the first scan class.
The last thing to fill in is the security. Now security has point security and data security.
Anyone with right access to point security can edit the attributes that we're editing right now,
anyone with access to data security can write data to the tag.
So we need to add in our interface identity to the data security. So we're going to go add,
find our interface identity, add it in and ok.
We need to give it write access. We can save our tag now
and we can go take a look at what's happening on the interface side.
So let's go to PIINT01 so the interface will check for new points every two minutes
and then it'll load 25 changes every 30 seconds after that.
We're going to wait a little while and it looks like our point has just been picked up
okay so now we've added point pump1.bearingtemp it's been added to the interface and now we have
at least required percent good tags so we have at least 80 percent of our tags getting good data
our device status is now zero, zero means good that means no errors and we're good.
So we can now go back to PISRV01 and let's see if data is actually coming through for this tag.
To check that we go to data and current values,
let's search for the tag.
Now in this tag search window the easiest way to search for all the tags belonging to
an interface is to type the point source of the interface in here. So as an administrator you'll
get pretty used to the point sources that you use, we used OPC for ours which is very generic
generally you'll name these point sources when you create OPC instances corresponding to where the
data is actually coming from which control system, which location. Okay we found one tag and okay.
So i see that my tag has a value and if i press the play button it will sign up for
updates from this tag. So whenever the value changes we can see the change coming through.
So note that this is a one second advise tag, we controlled that with the location three parameter
so if we go back down to point builder into classic a location three parameter
told us if this was an advise or a poll tag. If it was a poll tag we would get a new value
every second no matter what. If it's an advise tag we get a new value whenever the OPC server decides
that it's changed. So an advise tag is equivalent to the interface asking the OPC server to send it
updates when it feels like but no more often than every one second that's what a one second
advise scan class means. Generally it's best practice to use advise tags in most OPC servers
you'll be able to see this if you read the documentation of the OPC server you're using
generally they perform much better with advise tags. However it's useful to have at least one
poll tag, mainly because if that poll tag breaks then you know data is not flowing.
It's hard to tell if data has stopped flowing if all your tags are advise tags.