Control System Upgrading


Thread Starter


we have an old fashion control system in our plant (PID single loops) and we would like to update it. please advise us & what is the better system: fieldbus foundatation or DCS?
If you are keeping your existing field devices there is only one answer. That answer is DCS.To install fieldbus would require converting field instruments to Fieldbus units or converters in field. Personally for a retrofit as you describe I would go with the DCS. I would put in remote field I/O.That allows to keep your existing field junction boxes. Assuming everything is electronic already.

If it is grass roots plant then I might consider fieldbus.


Yosef Feigenbaum

You're going to have to provide lots more information.

1st of all, how big is your plant...

Etc. etc. etc.

(8{)} :) .)
What is the matter with a PLC and Device Net? I have found Device Net to be excellent.

DCS relies on a computer. I would not trust my mother in law with Bill Gates. A PLC is much more reliable.
Which is better, apples or oranges? Should I buy a Ferrari or an 18-wheeler?

At the moment, you are asking non-sensical quesitions. You need to find a good vendor to talk to. It will cost you money up front, but will save you a fortune later. They will help you figure out what direction to go in.

My company doesn't do that kind of automation. I would refer you do one of the major process control groups; Honeywell, Invensys, etc.

Good luck.

-James Ingraham
Sage Automation, Inc.
I vote for DCS. We've done this recently here on two plants(built in 1980's). Honeywell and Yokogawa were used due to their specific "smart control", process-specific, software packages that can be implemented on their respective systems. Both "optimize" your plant and both work. Whether the process is batch or continuous will help decide which vendor.

If you want to use BUS, HART, or "smart" instruments......both vendors offer transparent I/O setups that allow the field instruments to "tell the DCS" if they're "healthy or not".

Ian Verhappen

I believe your message really is whether you should convert to analogue or Fieldbus. I am slightly biased, though my feeling is that it will not be any more difficult and in some cases easier for you to convert from your local single loop control to Fieldbus. Since Fieldbus segments need to take into consideration single loop control for their design you are one step ahead of many others. Fieldbus will also likely allow you to integrate multiple single loops on a single segment/network.

By installing Fieldbus, you know you will be using the most current technology widely available that will therefore be supported for a longer time, and also be field upgradeable via software/firmware downloads.

If you have additional questions feel free to contact me directly as well.

Ian Verhappen
[email protected]


I agree with Mark. Stick with a simple DCS or even PLC, depending on how big your plant is or the type of control. If you need faster response, like a bottling or drumming plant, go with a PLC, they are faster.

Since we assume that your instruments are good, don't go with fieldbus. You'll have to buy all new instruments and this is very cost pro-hibitive. Also, be careful of the 'savings' that fieldbus pushers are claiming. They are not all true.
All modern DCS platforms support foundation fieldbus so the two are not mutually exclusive. Pick a good solid DCS (look at Foxboro and Yokogawa) and talk to their local integration partners. Let them help you make the decision on fieldbus technologies, particularly with regard to FFB on valve positioners and DeviceNet on solenoid valve manifolds for actuation of two position valves.

Bart Schaminee

DCS relies on a computer? PLC is more reliable?

I do not know where to start in educating you in the differences between DCS and PLC, but one thing is for sure. DCS is more reliable and stable. It is a PLC made for process, with all functions, features and redundancy. The PLC is just a PLC.

Bart Schaminee
If you keep the 4-20 mA instruments you have a very short-term solution. It is better to jump in now and migrate your instrumentation to Fieldbus early. If not you will find you will have to do it a few years down the road
anyway. Most 4-20 mA instruments from SMAR can be made Fieldbus simply by changing the main circuit board. The pressure sensors etc. remain the same. Other manufacturers make similar claims although you may have to change the
housing in some cases. This makes this migration a bit easier. You can fall back on converters if you really have to. We (SMAR) make both Fieldbus to current, current to Fieldbus, and Fieldbus to pneumatic, as well as for discrete input and output. For certain devices, such as the valve positioners it usually makes sense to change the entire instrument, but the valve actuator and body can remain in place.

The main saving with Fieldbus is not in wiring, rather it is long term savings made possible by advanced diagnostics going beyond a simple "healthy or not" signal.

Unlike popular belief, a large amount of Fieldbus installations are migration rather than green field.

Chapter 5 of the book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" (buy online in hardcopy or download immediately in softcopy):

Chapter 1 makes some comparison of DCS vs Fieldbus, you can download chapter 1 (overview) for free in softcopy form. It's free, but you must register an account. If your email does not support this hyperlink feature correctly,
please copy the entire link and paste it into your Internet browser. Mind the line wrap, make sure to get the complete path all the way to the 4585:

Jonas Berge
[email protected]
Learn fieldbus at your own pace:
A few corrections here.

Firstly, some DCSs like Yokogawa are more relaible than any PLC. I have statistics to prove this.

Secondly, this DCS uses the CPUs in the field control unit for control, so does NOT rely on computers.

Thirdly, the HMIs used for this DCS are masterless, i.e. it does not make use of a server, which means that failure of one p.c. does not affect the interface to the plant.

I suggest you read up on the advantages and cost effectiveness of Yokogawa's CS3000.