I get this question ALL the time: when is it better to use a PLC over an RTU anbd vice versa? Any words of wisdom?

Depends, dont you love that answer? In earlier times BI (before internet) thier was a clearer distinction now there capabilities overlap quite a bit.

If it is a stand alone application then I would go to with a PLC.

If it will interface with a SCADA network then the next question is what hardware and protocol level will be used and who has that as a native format? Modbus, BSAP, Fisher ROC, CIP, Profibus/Net, etc. Almost everbody has add ones that will get you there but they add cost and lower reliability as add ons.

Then there is aftermarket costs to consider such as support availibility (does a tech need a airplane ticket or passport to get there?).

Also of concern is the number of IO points the Higher counts would go to a PLC.Now where that point is why you hire qualified intregrators.

Another concern is execution times PLC's are typically faster because communication is typically handled by a separate proccessor. So motion control and packaging will probably be a PLC.

But if I was acquiring a few points and sending them back to a scada system I would use something like a bullhorn rtu.

RTU often have communication link embedded (ethernet port, GPRS modem, ...) that are options for PLC. Sometimes they have a webserver on board to control them without an HMI or a SCADA.

I'm currently using PLCs and GSM modems to monitor a large number of remote water sites. We decided so because PLC that we use are signifcantly cheaper than RTU. But I had to program PLC to send SMS and unsolicited alarms. Moreover I had to write some code to interface PLC and the Scada system. Now system is very flexible but if we used RTU work would have been faster.

On the contrary, I struggled a week to program an RTU to send a monostable command.

Dear All,
I can give the best example as i worked on both system for same application development.
My previous organization we used to develop application on Siemens and Allen Bradly PLC for Wind Turbine Control. We done as per requirememnt as it works sucessful.
Allen Bradly PLC done better than Siemens. But in actual appication on field both PLC fail to acquire the target.
And the main reason is response time, scan time. Scan time set by pLC could not control the wind turbine.

And we leave that project.

In RTU, we used c language for programming and we can set scan time in micro sec for certain applications.

I think this is main reason we shift from PLC to RTU.
If any one has doubts , please continue discussion.

Thanks
Kailash
0082 10 9241 1965

Basically, the PLC and RTU hardware is similar. It is the application that's different. PLCs are generally for applications that are processing and control intensive, whereas, RTUs I would say, are for management of data at multiple locations; where you have to gather a large number of data from many geographically diverse places.

Hi All

One interesting piece of information to share with you all!!!

We specialize in system integration of RTUs for Water and Waste Water processes and have been exporting the same.

We use PLCs to build RTUs!!!!


The difference between PLC & RTU is configuration centric

For a process control application, we can divide the whole process into individual process blocks or sub units and we can assign the task of individual control of these blocks to an RTU.

Many RTUs can communicate with centralized control system.

Hope the above helps

@ Sani... Exactly correct. RTU is a "shit-pumping" term.
usually used by overpriced consultants that municipalities
in the US & Canada love to use... Remote Transmitting Unit
was the first acronym I used in the 80's when I did it with AB plc 2's.