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SCADA design

Created by: Abdel-Qader at: 7/16/2013 1:53 PM (14 Replies)

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7/16/2013 1:53 PM	Rate (0)
Abdel-Qader Silver Member Joined: 1/5/2013 Last visit: 11/22/2024 Posts: 283 Rating: (16)	Hi, my case: eight stations: each station has Schneider SEPAM relays with ACE949-2 which supports Modbus RTU half-duplex with RS485 network. the central site: HMI screens which get stations SEPAMs readings and also when one of these SEPAM relays opened (electricity is off)we will program a button in order to close the SEPAM relay (return electricity). We are planning to purchase 9 Siemens S7-1214c PLCs with GPRS modems and also Communication board RS485 and the required software. Each station will has a SIEMENS PLC S7-1214C and the central site has also S7-1214c. I was thinking that each PLC will act as Modbus master and the communication between the center site (HMI) and each remote station will be profinet or GET/PUT, but this solution is not good since each station PLC will all the time busy the Modbus network receiving the SEPAMs relay reading and for example when one of the SEPAM relays opened and I need to send SEPAM relay reset and close from the central site (HMI) it will not success since the RS485 network at that station will always be busy. So, I think the best design is each station PLC will act as Modbus slave and the PLC at the center site (HMI) will act as Modbus master and the communication between them using GPRS will be as Modbus network. is it right? Thanks

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7/16/2013 6:38 PM	Rate (0)
BBPrudente Experienced Member Joined: 5/28/2013 Last visit: 2/1/2023 Posts: 112 Rating: (3)	Abdel-Qader I was thinking that each PLC will act as Modbus master and the communication between the center site (HMI) and each remote station will be profinet or GET/PUT, but this solution is not good since each station PLC will all the time busy the Modbus network receiving the SEPAMs relay reading and for example when one of the SEPAM relays opened and I need to send SEPAM relay reset and close from the central site (HMI) it will not success since the RS485 network at that station will always be busy. Why do you think that?

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7/16/2013 7:30 PM	Rate (0)
Abdel-Qader Silver Member Joined: 1/5/2013 Last visit: 11/22/2024 Posts: 283 Rating: (16)	imagine we have one station for example and we have a PLC that acts as a modbus master and that PLC will communicate continuslywith the SEPAMs relays slaves to get readings and diagnosis information. and we want for example to write holding register to reset or close SEPAM from the center site (HMI), and for that we trying to do reset, close, or open a SEPAM that message will be sent from the center site (PLC with HMI) to that station. in this case, the RS485 network at that site will be busy since the PLC at that site will continuously get that data. I'm right?

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7/16/2013 7:33 PM	Rate (0)
TIBI68 Gold Member Joined: 12/1/2009 Last visit: 2/9/2025 Posts: 672 Rating: (147)	Hi Abdel-Qader, what about WinCC Telecontrol? WinCC Telecontrol can direct communicate to your relays and no PLC's will be required. Infos here: https://www.automation.siemens.com/mcms/human-machine-interface/en/visualization-software/scada/wincc-options/wincc-telecontrol/pages/default.aspx?HTTPS=REDIR It's maybe a solution for you. Best Regards.
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7/16/2013 7:43 PM	Rate (0)
TIBI68 Gold Member Joined: 12/1/2009 Last visit: 2/9/2025 Posts: 672 Rating: (147)	Not exactly. Usually you program a cycle in reading the data from your relay. In this cycle you Need to put your write commands as well. In example: 1. Read 2. Read 3. Write So it's just a matter of time when your commands will be written to your relay. I am not aware of an EDC "Event driven communication" option on the modbus that will insert automatically a write request when youwant to send a command. Best Regards.
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7/16/2013 8:12 PM	Rate (0)
Abdel-Qader Silver Member Joined: 1/5/2013 Last visit: 11/22/2024 Posts: 283 Rating: (16)	Hi, Do you mean that each PLC will be modbus master in each station and also in the HMI site? If yes what is the recommended communication option between the HMI PLC and each station PLC? And do each station PLC will only responsible for reading and the HMI PLC is responsible for writing in case a sepam relay action is required? Thanks

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7/17/2013 7:25 AM	Rate (0)
TIBI68 Gold Member Joined: 12/1/2009 Last visit: 2/9/2025 Posts: 672 Rating: (147)	Dear Abdel-Qader, In case you go for WinCC with the Telecontrol (Option package) there is no PLC required. If you prefer a PLC based solution: Yes, each PLC in the substation will act as a modbus master to the relay. And at the same time it is connected via TCP/IP to the SCADA. A central PLC is not required. Technically there are more solutions possible, therefore it would be nice to know a bit more about the system specifications/requirements. Some clients request for data storage in the PLC in case ofany communication disturbance.Then you will need to look for something else (SINAUT in example). Best Regards.
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7/17/2013 7:45 AM	Rate (0)
TIBI68 Gold Member Joined: 12/1/2009 Last visit: 2/9/2025 Posts: 672 Rating: (147)	Dear Abdel-Qader, also kindly take a look at this brochure: https://www.automation.siemens.com/mcms/process-control-systems/SiteCollectionDocuments/efiles/pcs7/pdf/76/prdbrief/br_RTU_en.pdf Best Regards.
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7/17/2013 8:41 AM	Rate (0)
Abdel-Qader Silver Member Joined: 1/5/2013 Last visit: 11/22/2024 Posts: 283 Rating: (16)	Dear TIBI68, Thank you, so this is one solution to have each substation PLC as modbus master and i think the center HMI PLC is required in order to establish GPRS connection (GPRS module) with each PLC substation but in this case i think no need to be modbus master (HMI PLC) right? just, its function to establish a GPRS connection to each substation PLC and get the external tags right? in this case, the communication protocol between center HMI PLC and each substation PLC is PROFINET or using GET/PUT in order to get the external tags. can we use another solution: each substation PLC be a modbus slave and the center HMI PLC be modbus master and the communication between them is modbus? is this solution applicable by using GPRS connections between them? Thanks

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7/17/2013 9:35 AM	Rate (0)
TIBI68 Gold Member Joined: 12/1/2009 Last visit: 2/9/2025 Posts: 672 Rating: (147)	Hi Abdel-Qader, we have a small problem when you want to use Modbus RTU (possible) between the SCADA and the PLC's (RTU's = Remote Terminal Units). Your substation PLC ispermanently connected to your relay but you will only receive actualdatawhen the master PLC is connected withthe substationPLC. In between, while it is connecting to the other substation PLC'sthere is data loss. If anything importanthappened in between you will not come to know. Idon't know about your requirements. As per the documents above: With S7-1200 you can have up to 2.000 messages. With S7-300 up to 25.000 and with TIM module up to 60.000 messages. With using modbus RTU no buffering in PLC ! Best Regards.
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