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S7 300 ModbusTCP problem with receive data

Created by: MKuczma at: 3/12/2025 12:46 PM (39 Replies)

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3/12/2025 12:46 PM	Rate (0)
MKuczma Joined: 4/8/2024 Last visit: 3/31/2025 Posts: 20 Rating: (0)	Hi, I'm having an issue with Modbus TCP over Ethernet communication, specifically with receiving data from the server. I'm using an S300 PLC, model 6ES7 318-3FL01-0AB0 with firmware V3.2. Here’s what I’ve figured out so far: The connection to the server is established – on the PC application (acting as the server), I can see that the client is connected. I’m sending a correct request, and the communication block (MODBUSPN) doesn’t report any errors. Using Wireshark, I can see that the server receives the request and sends a response, but the response never reaches the client. The block doesn’t return any output – no error message or confirmation of task completion. Ultimately, the data does not reach its destination. The same server was tested with another S300 PLC, and the data transmission worek without any issues. Regards

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3/17/2025 12:13 PM	Rate (0)
Wüstenräuber Gold Expert Joined: 10/3/2008 Last visit: 7/31/2025 Posts: 996 Rating: (129)	Hi, if you send a request, a response is needed. If no response is received in parameterized monitoring time, then an error message is shown on ERROR and STATUS. The output parameters are shown only 1 plc cycle. Did you save the outputs DONE, ERROR and STATUS to check, what is shown this 1 cycle? Could you attach the online IDB of the modbus block? Best regards, Wüstenräuber

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3/18/2025 10:45 AM	Rate (0)
MKuczma Joined: 4/8/2024 Last visit: 3/31/2025 Posts: 20 Rating: (0)	Thank you for your answer. Yes, I try to reset the bit when an ERROR or DONE occurs. I'm using a MODBUSPN block from Siemens, and yes, I am saving the status. However, the status is empty when I send the request and try to receive data.

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3/18/2025 10:57 AM	Rate (0)
Wüstenräuber Gold Expert Joined: 10/3/2008 Last visit: 7/31/2025 Posts: 996 Rating: (129)	Which bit you try to reset? You start a telegram with a positive edge to ENQ_ENR. After start you have to set the bit to FALSE. It is only a positive edge, not a positive level. When you start a telegram with ENQ_ENR (or ENQ), then there is always a result: either DONE = TRUE, if it is successful or ERROR = TRUE, if it is not successful.

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3/18/2025 11:42 AM	Rate (0)
MKuczma Joined: 4/8/2024 Last visit: 3/31/2025 Posts: 20 Rating: (0)	Initially, I tried to reset ENQ_ENR when DONE or ERROR occurred. ENQ_ENR was set to TRUE only once by me and was not set elsewhere in the program. Then, I also tried to reset another bit, for example M41.0, to check; Ultimately, none of the bits were reset.

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3/18/2025 1:26 PM	Rate (0)
Wüstenräuber Gold Expert Joined: 10/3/2008 Last visit: 7/31/2025 Posts: 996 Rating: (129)	Can you attach the project? It seems that the call/handling of modbus block is not correct.

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3/18/2025 1:44 PM	Rate (0)
MKuczma Joined: 4/8/2024 Last visit: 3/31/2025 Posts: 20 Rating: (0)	I can't attach the project. Maybe only the OB parameters or other things like that. Let me know what you need.

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3/18/2025 1:59 PM	Rate (0)
Wüstenräuber Gold Expert Joined: 10/3/2008 Last visit: 7/31/2025 Posts: 996 Rating: (129)	Without more infos it is not possible to find the reason. You could attach a screenshot of the OB, in which the modbus block is called.

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3/18/2025 2:21 PM	Rate (0)
MKuczma Joined: 4/8/2024 Last visit: 3/31/2025 Posts: 20 Rating: (0)	This is OB1, where ModBus block is calling Attachment CallBlock.PNG (252 Downloads)

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3/18/2025 2:56 PM	Rate (0)
Wüstenräuber Gold Expert Joined: 10/3/2008 Last visit: 7/31/2025 Posts: 996 Rating: (129)	Which values do you parameterized for RECV_TIMEOUT and CONN_TIMEOUT? The parameters are not correct. You have to delete the comments before ENQ_ENR: A “CONTROL_DAT”.ENQ_ENR R “CONTROL_DAT”.ENQ_ENR And you have to set ENQ_ENR, when DONE_NDR or ERROR are set. (not reset) O “CONTROL_DAT”.DONE_NDR O “CONTROL_DAT”.ERROR S “CONTROL_DAT”.ENQ_ENR

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