Definition of Soft IO in Fieldbus or Profibus


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We are using Profibus network for our under construction process plant.

The initial DCS system contract has been on some IO assumption of 6000 Tags.

Now extracting the required signals from devices using Profibus, we have 18000 number of soft AI and soft DI considering the process values and status bits.

Our system integrator now claims each piece of information received via Profibus, either the status bit or process value, is considered a separate Tag.

We have the argument that each word can be considered as a bunch of 16 bits, each representing a status flag so we have only 1 Tag instead of 16.

Please let me know either we are correct or our system integrator. In oter word is each bit received via Profibus considered a standalone tag?


I think you are getting screwed!

One of the main differences between DCS tags and PLC tags is the basic structure. In a DCS system tags are not values but structures that contain a number of bits of information. These are often sperated out by the standard dot method you see in most programming languages. For an alanog input named "tag", tag.pv is usually the actual process value, tag.stat is the status bit.

Basically in a dcs system each I/O point has 1 tag not several. How many I/O points does your system have?

You haven't said which system you have. It may be the nodes out on the profibus system behave differantly to native system I/O and you do need a tag for each bit a data. What do you have out on the profibus system?

How many non-IO tags does your system have?
We do think so as well!

It is Yokogawa CS3000.
We send most of the plant's process information via Profibus (PT, TT, FT, LT, AT) as well as all the MOV, all the LV/MV switchgear info and VFDs.

E.g. lets say for one single starter we take 6 AI and 5 DI to DCS, so the number of non-IO tags are huge. The system integrator is considering 11 tags. We disagree with them.