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Add a identifier to the Modbus RTU request

Created by: Shapovalov_as at: 3/21/2019 8:40 AM (9 Replies)

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3/21/2019 8:40 AM	Rate (0)
Shapovalov_as Advanced Member Joined: 5/22/2014 Last visit: 6/19/2024 Posts: 18 Rating: (0)	Hello experts! The question is very specific. I hope for your help. I work with Modbus RTU. PLC S7-1200. I need to add a identifier to the Modbus RTU request. It looks like this: DN<CR><LF>[Modbus RTU request] DN - identifier <CR><LF> - carriagereturn, linefeed, code 0x0D, 0x0A How can this be realized?

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3/21/2019 2:55 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (323)	Hello, no, this is not possible as these characters do not meet the Modbus specification. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (323)
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3/21/2019 4:21 PM	Rate (0)
Shapovalov_as Advanced Member Joined: 5/22/2014 Last visit: 6/19/2024 Posts: 18 Rating: (0)	Kaulquappe Hello, no, this is not possible as these characters do not meet the Modbus specification. Yes, this is ASCII. Ok, if without these characters? DN [Modbus RTU request] DN - identifier - Hex I'm interested in how to insert a character before the standard request. for example 1 [50 03 00 02 00 0A 69 8C]

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3/21/2019 5:24 PM	Rate (0)
Kaulquappe Posts: 3093 Rating: (323)	Hello, a protocol specification is used that different manufactorers can exchange data without adapting their communication software for each device. If manufacturers think it would be en vogue to modify standard protocols or define their own protocol you must use the standard send and receive blocks and build/check the frames in your PLC program. Regards, Kaulquappe
Kaulquappe Posts: 3093 Rating: (323)
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3/22/2019 7:55 AM	Rate (0)
Shapovalov_as Advanced Member Joined: 5/22/2014 Last visit: 6/19/2024 Posts: 18 Rating: (0)	I need an identifier for an intermediate device (modem, block diagram in the attachment) I understand that I can use blocks "Send_P2P" and "Receive_P2P". But then I will have to write a program for modbas RTU (request, checksum, etc.). Attachment Request.jpg (51 Downloads)

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3/22/2019 8:08 AM	Rate (0)
#dP Posts: 2826 Rating: (686)	For some reason I have doubt that you are in need of using Modbus RTU. Could you detail which instrument you are connecting to the S7-1200?
#dP Posts: 2826 Rating: (686)
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3/22/2019 8:40 AM	Rate (0)
Shapovalov_as Advanced Member Joined: 5/22/2014 Last visit: 6/19/2024 Posts: 18 Rating: (0)	This is a wireless modem, radio network Zigbee. http://embee.ua/index.php?option=com_content&view=article&id=95&Itemid=229&lang=en Unfortunately, the instruction is only in Russian. The principle of data transmission of these modems is that they transmit any packet data on RS 485. In my case, this is a modbus RTU. But for modem identification, before the data packet it is necessary to send DN (Destination Node).

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3/22/2019 10:44 AM	Rate (0)
Kaulquappe Posts: 3093 Rating: (323)	Shapovalov_as I need an identifier for an intermediate device (modem, block diagram in the attachment) I understand that I can use blocks "Send_P2P" and "Receive_P2P". But then I will have to write a program for modbas RTU (request, checksum, etc.). yes. that's it
Kaulquappe Posts: 3093 Rating: (323)
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3/22/2019 11:16 AM	Rate (0)
Shapovalov_as Advanced Member Joined: 5/22/2014 Last visit: 6/19/2024 Posts: 18 Rating: (0)	Kaulquappe yes. that's it I more less understand how to write my program for request ND + modbas RTU. I have a problem with checksum calculation. Maybe someone has a ready-made algorithm?

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3/22/2019 11:53 AM	Rate (0)
Kaulquappe Posts: 3093 Rating: (323)	Just take a look at this thread
Kaulquappe Posts: 3093 Rating: (323)
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