PLC or RTU?

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Thread Starter

Tom DeFreitas

How do you define the difference between an RTU and a PLC? It seems these terms are interchanged regularly. Does anyone sell a true RTU anymore? Many PLC manufacturers have inexpensive MicroPLCs loaded with features. Why would you choose an RTU?
 
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Hakan Ozevin

This can be debated, but I believe that the only difference between an RTU and a PLC is the integrated communication facility (modem or wireless network integration) of the RTU, however this attachment can easily be applied to a PLC nowadays.

PLC's were used to be only *P*rogrammable *L*ogic *C*ontrollers 10-20 years ago, but as time goes by, they are taking the jobs of not only the RTU's, but DCS, CNC systems and even industrial computers as well. Thus I prefer to call them as Programmable Controllers for a long time.

My personal idea is, as the progress in electronics continues, mass production units like PLC's will take over the market share of the specialized units as CNC's, RTU's, IC's and DCS's.

Hakan Ozevin
 
C
>Traditionally, RTUs and PLCs were used in different applications eventhough both of them had similar I/O interfaces. But with fast development cycle in electronics and communications, the distinction between the two is vanishing. RTUs are being built with PLC functionality ( Like IEC 1131 programming ) and PLCs are getting equipped with advanced communication facilities. The only distinction perhaps may be with usage in different applications and the cost effectiveness in a particular application.

>C.S.Nagaraj

>This can be debated, but I believe that the only difference between an RTU and a PLC is the integrated communication facility (modem or wireless network integration) of the RTU, however this attachment can easily be applied to a PLC nowadays.
 
I think each has its place. If you come from an industry that can use "conventional" PLCs in SCADA applications, you can probably also use PCs as master stations, Windows-based HMI, byte-asynchronous comms protocols, one-pass controls, have a database of less than 1000 data points, can afford to loose data occasionally and the cost of inadvertent control activation is low. RTUs are normally uneconomically expensive for these systems. If you come from an industry where you need very high SCADA system availability, multiple operator HMI stations, thousands to millions of data points, need to track event sequences and inadvertent control operation can kill someone or cost thousands of dollars, you need to use RTUs.

PLCs usually do not have the capabilities required by these systems. Modern PLCs and RTUs are (generally) capable of similar processing: PID loops, automated control functions, etc. PLCs are usually cheaper and generally use simpler, less-secure communications protocols. For some particular application an RTU solution or a PLC solution will be more cost effective. It depends entirely on the system functional and performance requirements. IEDs can be considered as a set of separate entities: They usually perform a specific function and perform that function very efficiently or very quickly or with great accuracy. It is usually appropriate to use specific-function devices where these criteria are important. If requirements are less strict then a programmable "general-purpose" RTU or PLC could perform the function in place of an IED. Again the system cost for performing the required functions needs to be considered, including the cost of integrating all the system components and set-up or configuration/database costs.

In summary: Each system will have several possible mixtures of RTUs, PLCs and IEDs that can perform the tasks required by that system. It is the system designer/integrator's task to select an architecture that is cost effective and maintainable. Remember to include cost of data collection and protocol interfacing where necessary.

Crestman
 
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Vivanco Fernando

Normally the RTU's are used in SCADA systems, in this case the process control is given by MCU (Master Central Unit) it has the control algoritms, takes desitions and controls its communications (by modem, radio, microwaves, fiber optic, etc), the RTU's are installed in electrical substations, distribution pols, oil extraction and electric power systems.

In my opinion PLC are used whith autonomous program, like stand alone machines, in this case the algorithms are contained in PLC program and it is independent from external equipment but in sometimes we can have PLC's in a net with a supervisory control.

I wait this information may be usful for you.

 
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