Synchronized SCADA control system

maymanyassin

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Join Date
Dec 2013
Location
Alexandria
Posts
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Subject: Synchronized SCADA control system

System construction: SCADA control system consists of 1 master station (controllogix 1756-L63) and 14 remote stations (Micrologix 1400).

System overview: The main target of this system is to compare instantaneous pressure transmitter readings at each remote station by transferring them synchronized to the master station.

System communication Network: cellular network.

System transmission protocol: DNP3 protocol using Prosoft Master/Slave interface module at master station (to act as DNP3 Master) and the Micrologix 1400 controller at each remote site (to act as DNP3 Slave).

Target: We want to ensure synchronization between pressure readings at Master Station to be able to compare instantaneous readings.

Question:
Does the DNP3 Protocol guarantee this synchronization over cellular network? If yes, How come? If no, please advise suitable solution!
 
For a start cellular networks are dicky things - loss of signal - loss of power - interference to signal. I would not use them for that.
If you lose power somewhere it will not happen anyway.
safest way is to wire it but if the software protocol snags up it will not happen anyway.
If it is critical then I would suggest a fibre optic ring topology with UPSs in all locations to hold the PLC up for however long you require it to be held up.
The other option would be bouncing off satellites but even they sometimes play up - particularly during sun spot activity and the like.
 
Thanks for your response.
I'm forced to use Cellular network as the transmission media because the distance between master and some slaves reach to 300KM, so i need to ensure sychronisation through this media.
 
It may work for you. Cellular networks usually have a small generator attached to each station so if power fails the station is only off for about 30 seconds but not sure where you live.
You would still need a UPS at each remote for the cellular to work if there is a power failure.
You would also need to check the signal strength at each remote - if the signal is weak it may not work.
I do not use DNP3 so I cannot comment on it's reliability.
 
I think that there is misunderstanding.
I'm not worried about power failurw as we use solar panels and charging batteries.
I'm worry about time delay due to cellular network that may affect synchronisation between RTUs. how can i overcome it!
 
Hello,

Think about what you are asking.

"Synchronized", "instantaneous", to what order of magnitude. 1 millisecond, 1 second, 10 seconds? etc.?

What is the deadband?
 
well, i think you got what i need.
i need the pressure values to be synchronised at master station to magnitude of 1 millisecond.... is this possible over cellular network?
 
synchronised at master station to magnitude of 1 millisecond.... is this possible over cellular network?

The speed of light is 300,000 km per second
You have a station 300 km from the master
You want 1 millisecond timing (1,000th of a ssecond )

The communications on that site will take 1 millisecond to get there, and 1 millisecond to come back,
a total of 2 milliseconds.
Assuming the electronics or wires don't slow that signal at all.

Now the micrologix probably has a scan time of 20 milliseconds. so can only update its pressure readings at that rate,
Assuming the analog value is ever updated at that rate in the PLC

My thought would be aim for a once per second then you may be getting what your system can really achieve and deliver and any operator could absorb.
 
Last edited:
Hello,

>is this possible over cellular network?

No. Cellular networks are asynchronous. You can sometimes make it appear synchronous if the timing is flexible enough.

I will alter the above. Assuming the master station had 14 cellular channels, one for each remote station, you would have a better chance of 1 millisecond. That assumes many types of errors are allowed.

Still DNP3 was not created/designed for this type of operation. It is an exception based protocol. It has polling but it is not efficient.

>Does the DNP3 Protocol guarantee

DNP3 does not guarantee anything. :)

My2c.
 
well, i think you got what i need.
i need the pressure values to be synchronised at master station to magnitude of 1 millisecond.... is this possible over cellular network?


that isnt going to happen with pretty much any communications protocol.
 

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