# Difference between RTU & PLC and also SCADA VS HMI



## Tsmil (Jul 17, 2011)

http://www.tetragenics.com/pdf/TetraViews/nwsmay97a.pdf

http://www.differencebetween.com/difference-between-scada-and-vs-hmi/

These links should give you the basics..


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## Big John (May 23, 2010)

At best I'm a hardware guy, not a programmer, so I never got into the details on this stuff, but here's my understanding:

Any stand alone electronic system that can take inputs from it's environment and use those to trigger pre-programmed outputs is a a PLC. It doesn't require any sort of communication with the outside world to perform it's job.

Within a facility, when you have a variety of interconnected PLCs, often with multiple overlapping functions that are all part of a common process, you have a distributed control system (DCS).

When you have multiple locations spread out over a region, which can consist of something as simple as a remote sensor connected to a modem, or can be as complicated as a large PLC or even a DCS, each one of these is an arm in a SCADA system, and each arm is called a remote terminal unit (RTU). These all feed back to a central location.

At this central location on a SCADA there is a master unit (MTU) that is usually a control center where computers are set up to interact with the RTUs. These computers are examples of human-machine-interfaces, HMIs. But that is not the only type of HMI by any means: Any mechanism by which a person can interact with a PLC, drive, SCADA, DCS, etc is considered an HMI. It could be as simple as a 10 digit keypad or as complex as a PC.


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## Heartnet (Jul 23, 2012)

A book that I read states that SCADA consist of a HMI, PLC and RTU. 

Can SCADA exist if a system only consist of HMI and PLC? Can PLC act like RTU?


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## JRaef (Mar 23, 2009)

SCADA (Supervisory Control and Data Aquisition) is used to describe a _system _that is monitoring and /or controlling a number of sub systems over a wide area. It will consist of an MTU and multiple RTUs. The RTUs cal be dedicated devices or they can be PLCs. It's not a hard and fast rule. 



Heartnet said:


> A book that I read states that SCADA consist of a HMI, PLC and RTU.


It would be pointless to have a supervisory system without some way for the people involved to interact with the system, so you have a Human Machine Interface (HMI) at the MTU. The RTUs are the remote devices that gather the data and enact control decisions made by the MTU. If you need the RTUs to be capable of independent control, you can use a PLC as the RTU. But it's unlikely that you would have an RTU _plus _a PLC, they would be redundant. Many years ago this was sometimes the case because PLCs did not always have communications capabilities, but that has not been the case now for 20+ years. There is nothing a PLC cannot do that a dedicated RTU can do, but if you do not need independent control, simplistic RTUs can sometimes be less expensive.




> Can SCADA exist if a system only consist of HMI and PLC?


Technically yes, but you would not call it a SCADA _system _until you had multiple RTUs. I would just call it a PLC with an HMI.



> Can PLC act like RTU?


Yes, see above.


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## Heartnet (Jul 23, 2012)

Thanks guys appreciate it =)


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## Heartnet (Jul 23, 2012)

JRaef said:


> SCADA (Supervisory Control and Data Aquisition) is used to describe a _system _that is monitoring and /or controlling a number of sub systems over a wide area. It will consist of an MTU and multiple RTUs. The RTUs cal be dedicated devices or they can be PLCs. It's not a hard and fast rule.
> 
> 
> It would be pointless to have a supervisory system without some way for the people involved to interact with the system, so you have a Human Machine Interface (HMI) at the MTU. The RTUs are the remote devices that gather the data and enact control decisions made by the MTU. If you need the RTUs to be capable of independent control, you can use a PLC as the RTU. But it's unlikely that you would have an RTU _plus _a PLC, they would be redundant. Many years ago this was sometimes the case because PLCs did not always have communications capabilities, but that has not been the case now for 20+ years. There is nothing a PLC cannot do that a dedicated RTU can do, but if you do not need independent control, simplistic RTUs can sometimes be less expensive.
> ...


Hi sorry, I just realised that RTU refer to 2 different thing
Remote Terminal Unit 
Remote Telemetry Unit

I was comparing between Remote Terminal Unit with PLC not Remote Telemetry Unit.

SCADA still exist when it only has HMI, and PLC right? 

From what I gather is that, PLC are used for local SCADA monitoring while RTU are used for offshore / wide range monitoring.


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## Heartnet (Jul 23, 2012)

Ok,

Is it safe to say:

SCADA = HMI + PLC for local plant monitoring / controlling

SCADA = HMI + RTU for wide range of control / monitoring

? 

All these things that I find online are making me confused even more


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## erics37 (May 7, 2009)

RTU = Rooftop Unit :jester:


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## JRaef (Mar 23, 2009)

And if you are over 50, it begins to take on a new meaning:

*R*un *T*o *U*rinal...


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## 04-20ma (Aug 21, 2011)

No not at all. A HMI is a local device and scada is a hmi but in a diffrent location say like there are a bunch of oil wells out in the country. The scada would be in the city and it can read and write data to the rtu. And a plc and a rtu are basically the same thing. Just in the way u would use em


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## Heartnet (Jul 23, 2012)

04-20ma said:


> No not at all. A HMI is a local device and scada is a hmi but in a diffrent location say like there are a bunch of oil wells out in the country. The scada would be in the city and it can read and write data to the rtu. And a plc and a rtu are basically the same thing. Just in the way u would use em


for local control and monitoring, having PLC+ HMI together is no a "SCADA" but a "HMI"

having wide range control and monitoring, having PLC+RTU+HMI connected is a "SCADA"

? Both are SCADAs? 

What does your "no" refer :001_huh:


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## 04-20ma (Aug 21, 2011)

All u need for a scada is a plc


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## Trigger_442A (Sep 15, 2012)

Great thread thanks guys for the useful information. I would like to learn more in these fields, is thier books you would recommend?


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## DesignerMan (Jun 13, 2008)

Just to throw my two cents into the mix...A PLC controls a process and the HMI is the operator interface for that process. For most smaller processes this is all you need. If you have a large control system that requires data acquisition and plant level supervisory control then you add a SCADA system as well. :thumbsup:


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## oliquir (Jan 13, 2011)

i would just say a scada is a software on a PC, an HMI is a dedicated screen connected to plc 

and i use a lot of plc/hmi all in one controller like this, only one software to program, it saves a lot of time!
http://www.heapg.com/products/xl6-ocs/


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## gnuuser (Jan 13, 2013)

an rtu is also called a plcc remote plc controller
these devices were continuously connected to the plc and monitored the operation 
for example in a controller office you have a desktop pc connected to a data highway
this computer would have the programming software in it.
the data highway connects to the plc in the machines main panel.
this is an rtu /plcc setup
early rtu systems had a dedicated dumb terminal that simply communicated with the plc (hated those pieces of crap)


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