HMI/SCADA Tags in PLC

Addressing HMI/Scada tags

  • Direct reference PLC tags

    Votes: 9 69.2%
  • Remapped dedicated HMI/Scada tags

    Votes: 4 30.8%

  • Total voters
    13
  • Poll closed .

chelton

Member
Join Date
Jan 2012
Location
melbourne
Posts
316
Am I old school?
Now networks are getting vey fast are you directly addressing tags in the PLC or do you create a dedicated HMI/Scada memory area in your controllers?

I am seeing more and more systems that directly address tags in the PLC, a recent project having numerous PLC's on the network and upward of 50k tags, a mixture of most data types, real,long,int,bool,time etc. To me it seems sluggish, but I have not spent enough time to analyse the cause of the problem.

Having done this for many years, I am 1 who has a dedicated memory area for HMI/Scada tags, it takes more planning but I find having all the tags in a consecutive memory location drivers are generally more efficient.

Just interested to see what others are doing.
 
Direct for me, for the most part. In my AOI/UDT definitions (AB guy here) I might have dedicated tags more so for clarity. "ScadaCmd" vs "ProgCmd" to differentiate were the command originated. I may roll up a number of bits into a single DINT for SCADA to read.

Anyhoo...

The majority of tags I use are 1:1 to an AOI/UDT. I'm mostly using Ignition now so I can align Ignition UDTs directly to my AOI/UDTs which makes development quick an easy as it relates to tags.

From a performance standpoint, all my AOI/UDTs are defined as a base array, and SCADA tags are mapped to the array to keep things nicely packaged for transmission. I'm sure I could break a few things up to gain a few ms in response time but I'm not sure the effort is worth it. I just wrapped up a very large system, 70,000+ tags. 150+ programs in 1 PLC and while I do have a slight lag in response I'm confident it's related more to the network firewall/routing rules that are in place than my tag structure.

Plus Rockwell's removal of direct memory access in firmware v21+ is known to have impacted plc to scada comms, at least for Ignition so the extra effort to have dedicated SCADA/HMI tags when I already know of two bottlenecks I can't control...meh.

Someday ....maybe, I'll have time to actually create a few test scenarios to see what, if any real improvements I can find should I change my structures.
 
What I do agree with is locating blocks of tags in sequential addresses, the one thing that slows coms down is accessing tags that are spread all over the address space, it is well known that the header information & checksums etc. often will be larger than a single tag, so that means tags that are spread all over the address space will generate a lot of small send/receives. If data is optimised then one block accessing multiple tags in one transaction is far more efficient as it only has one set of communication header/footer (if you know what I mean).
Tag based rather than physical addressing is a bit of an unknown for example generating a tag in CLX where is it's physical address ? are they located by type and are they located in order of creation.
At least in some platforms like Mitsubishi or Siemens these are physical address spaces so many coms are still done by physical address i.e. "M", "D" etc.
I's like all tech, coms gets faster, platforms get faster, hardware gets faster, but software and ways of processing gets more complicated due to the extra functionality.
Just think about a I7 PC running dos, it would be blisteringly fast compared to windows but lacks all the bells & whistles.
 
I like direct reference tags, but they come at a cost. Example, FT scan rates are per window, and those direct tags only work when the window is open. If you want alarms, historian, etc., now you have to reference those in other areas. That's where a tag database shines.

For Ignition users, using a tag database is a breeze since they allow UDT definitions. Create an instance pointing to a base tag, like you would in FT, and you get all members, alarming, historian that just works. Scan rates can be per tag and defined in the definition. Development time is actually faster in this case than direct reference tags. But, this is comparing Ignition to FT. Wonderware, at least InTouch, not so much better.
 
This obviously scales with the project. If it’s small scale and a proven product I’ll usually just direct reference. If it’s large scale with the potential for multiple revs the. I’ll use mapped alias tags.
 
With the progressive implementation of IEC 61131 on which the variables (tags) of the program no longer have to be linked to a physical address and are referenced by name, I clearly believe that in the future the tags will be read directly from the PLC mainly.

Although this requires protocols that address tags by name such as Ethernet/IP and OPC UA
 
With the progressive implementation of IEC 61131 on which the variables (tags) of the program no longer have to be linked to a physical address and are referenced by name, I clearly believe that in the future the tags will be read directly from the PLC mainly.

Although this requires protocols that address tags by name such as Ethernet/IP and OPC UA

Does that mean that communication drivers are no longer able to optimise read/write. What I am seeing is large complex UDT's/custom data types where only a few boolean/dint tags are read from structures that contain many bytes of elements. This is repeated hundreds of times. In the past drivers some drivers have grouped common memory locations and read out a block of data rather than individual elements.
 

Similar Topics

Hi everybody, i read this forum since a couple of years but i never post on it, cause i found every time the answer i need. But today i made the...
Replies
1
Views
8,253
Hello, I'm working on a machine retrofit project, and am looking for recommendations for HMI/SCADA software. I've used C-More and Horner HMIs...
Replies
6
Views
1,858
We currently use GMail accounts for sending email alarms from quite a few HMI projects. Specifically C-More HMIs and AVEVA Edge runtimes. To...
Replies
8
Views
7,487
Hi guys, I want program a aggregate Scada/HMI, i read the manual but i didn't understand well. Does any one know how to get a value from the...
Replies
7
Views
1,931
Hi Everyone: Has anyone used "FAS-inMOVE Operation & Monitoring – Visualization" from INRAY INDUSTRIE SOFTWARE ?. Any experience, any comment...
Replies
2
Views
1,462
Back
Top Bottom