Is time stamping and event logging possible with PLCs? I would like to know the MAIN differences between PLCs and RTUs.
I know that RTUs have more communications features, powerful processor compared to PLCs. The only difference i see is time stamping esp upto 1 ms and event logging which is not possible with PLCs.
Am i correct? Can someone shed some light on this??

 

In Plantguard PLC of ABB, 1 msec time stamping is possible.

 

You can do time-stamping with AB ControlLogix PLCs. You can make a PLC do what an RTU does, but you can't make an RTU do what a PLC does. Hope that answers your question.

 

I have written a pseudo sequence of events in an AB SLC5/04 which is adequate for the specific applications purpose. The time stamp is only to 1 second but the sequence is to 1 scan which is running at around 20ms. I could achieve better time stamping but it is not necessary. It's a bit memory hungry for a PLC.

If a fault occurs it enables the maintenance people with a laptop to go straight to a Custom Data Monitor labelled "Event Log" and see the sequence of events that led to the fault. Some faults and diagnostics are not fatal in terms of the application so they can also be checked periodically and appropriate maintenance, calibration or adjustments can be made to the field equipment.

There are also economical ways of transferring the event log to the DCS system. We don't do that at present. We simply send a status file which includes faults and diagnostics.

Having done it on this particular application I will probably do it on all projects in future because I have found it very helpful during commissioning.

Vince Dooley

 

I think i am further confused. I thought its the other way round. RTUs are more powerful. We have to design many systems where the client stresses on RTUs and not PLCs. I have heard that RTUs have certain specs PLC doesn't and hence the system does not comply to client's specifications. I need to know, how can we convince customer the PLC can achieve functionality just the way RTU does if what you are saying is true. Can u tell me the main difference if any?

 

Timestamping can be done with Rockwell and Modicon PLC's. With GE Fanuc you can do a do I/O and log them in a round-robin.

Any how an RTU differentiates form a PLC:
- SOE/SER
- Memory Storage
- Communication
- Low Power Consumption
- Temperature -40 to +70 C

To give competition a chance, RTU's are:
- Kingfisher
- T-box
- Motorola
- etc.

See also the Ian Wiese website... http://members.iinet.net.au/~ianw/

You will see a lot about RTU/SCADA over there.
Hope this helps,

Bart Schaminee

 

If you have not, please take a look at the Matrix offering. http://www.standardautomation.com

 

When you say powerful, it depends on what specific job/field. You see, RTU is only a data concentrator, it doesn't really do logic control or any other calculations which PLC is more powerful in this case.

Normally RTU is installed in the remote area which is far away from the control room, that's why RTU is provided with a large scale of memory storage, the capability to operate in a wide range of temperature till below 0 degree, the capability to do the time stamping of each data coming to the RTU which in this case RTU is more powerful than the PLC.

But PLC nowadays is also starting to be able to do the things that RTU can do and so does RTU I think (I'm a PLC guy).

Hope that wipes out your confusion.

 

You can always find a product that is a PLC but doesn't look like a PLC and then you can call it what ever you want. This is what some of our customers do.

Stephen Luft - President
Entertron Industries
3857 Orangeport Rd.
Gasport, NY 14067
Tel: 716-772-7216
Fax: 716-772-2604
web: www.entertron.com
wwjd - wdjs

 

I agree with Stephen Luft. These days the differences between a DCS and a PLC, a PLC and an RTU, and an RTU and a flow computer are getting blurred. Although they are perhaps 99% the same, each one has minor characteristics
that make them different.

The main differentiator for an RTU is usually a very low power consumption. An RTU often has no line power but must operate from battery and solar panel. This is the one thing they do better than the run of the mill PLC. To
achieve this the RTU use low power processors and employ sleep mode and other techniques.

As far as "powerful" is concerned I would say that a PLC typically is much more powerful: faster, and greater IO capability. This is simply because a PLC is designed to get line power and therefore use powerful processors to
get speed.

Having said that:
- There are RTUs where you can expand IO modules quite a bit
- There are PLCs with low power consumption

If you are a user that need a remote solution, I would not prescribe PLC or RTU. I would write a functional specification saying e.g. that:
- measurements shall be taken every second
- measurements shall be communicated every minute
- the system shall operate round the clock on battery power and solar panel (X hours per day in winter).

Based on this the vendor can use a PLC or RTU. The PLC may need bigger batteries and solar panels, but may do a better job of controlling and updating the data faster.

Jonas Berge
SMAR
===========
[email protected]
www.smar.com
Learn fieldbus and Ethernet at your own pace: www.isa.org/fieldbuses
Learn OPC and automation software at your own pace: www.isa.org/autosoftware