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Topic Title: Difference between RTU & PLC and also difference between HMI & SCADA
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Created On: 11 March 2013 01:40 AM
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 11 March 2013 01:40 AM
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Heartnet89

Posts: 11
Joined: 23 July 2012

Hi,

Can someone please clarify whats the difference between RTU (remote terminal unit) and PLC (programmable logic controller)?

As well as difference between HMI & SCADA? I am so confused.

Isn't RTU basically PLC? Thanks.
 11 March 2013 03:27 AM
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mustards

Posts: 122
Joined: 01 November 2002

An RTU in its simplest form collects data from sensors and sends it back to a central system via some form of communications and can send commands to actuators. A PLC in its simplest form collects data from sensors and sends commands to actuators and may send data back to a central system via some form of communications. In that sense they are the same. The main differences are:

A PLC will generally have a program running which processes the the data locally whereas an RTU will usually be fairly dumb
A PLC usually communicates with central systems over local area connections (eg LAN) whereas an RTU (hence it's name) operates remotely and communicates over wide area connections (eg telephone, GSM, radio)
A PLC usually communicates with local area type protocols such as DF1, Ethernet/IP, Profibus and an RTU will usually communicate using wide area protocols such as DNP3, IEC60870.

But these days you can buy an RTU that has PLC-like programming available and you can buy PLCs that provide wide area communications so the differences are now much less obvious.

It's a similar story for HMI and SCADA. Both communicate with RTUs and PLCs and display data. Typically the HMI is simpler than the SCADA system and is used for local display and annunciation on one process whereas a SCADA system will typically monitor multiple processes over a wider area. Again these days it is hard to tell the difference as software packages are used more interchangeably.
 05 April 2013 07:41 AM
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sandip

Posts: 21
Joined: 31 March 2013

In PLC a program for a particular process can be developed using ladder diagram ;please let me know whether the same is possible in case of RTU.
 05 April 2013 12:05 PM
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oneye

Posts: 177
Joined: 25 February 2008

No ...
As stated above an RTU in its simplest form collects data from sensors and transmits to a central system, usually a PLC.
 05 April 2013 12:17 PM
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mustards

Posts: 122
Joined: 01 November 2002

I agree, in most cases an RTU can't do execute any form of PLC-like program but as I mentioned you can buy certain types of RTU which do have IEC61131-3 support, meaning you can program in one of five standard languages, such as Ladder Diagram (the others being Structured Text, Function Block Diagram, Instruction List and Sequential Flow Chart). Some older RTUs had proprietary, simple programming environments, usually like Structured Text but generally I would assume an RTU can't do this unless the vendor specifically indicates otherwise.
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