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MODBUS connection to waveshare failing

Created by: AutoGen_9020715 at: 8/7/2024 4:14 PM (6 Replies)

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8/7/2024 4:14 PM	Rate (0)
AutoGen_9020715 Posts: 3 Rating: (0)	I have a problem reading data from a MODBUS device that is connected to LOGO (8.4) via a MODBUS RTU/TCP converter from waveshare (RS485 to Eth) set to the MODBUS RTU to TCP converter. I can access the MODBUS devices data from a PC using various Windows Tools like ModbusPoll etc. This means that the communication between the MODBUS device and the Waveshare converter works fine, and that the configuration on that side should be OK. The Waveshare converter is set to the be a Server (which means that LOGO is configured as a Client). I have tried many things but so far I am not able to get it to work. Normally the Link LED in the converter should be solid blue when the TCP connection is established. It does so when using PC software to read the MODBUS data. But with LOGO reading the data the blue LED just blinks very shortly on each connection attempt then turns off (green). LOGO diagnose says there is no MODBUS connection established. Obviously I am not getting any data. I have tried playing arround with the addresses reading a IR. My starting register on the device is MODBUS 0x0000 which normally should be 1 in LOGO. I have also tried 30001 and 40001 and 2, 30002 and 40002 just to be save... But the result is always the same. The connection breaks. I have tried both Setting a client Ethernet MODBUS connection reading into VM as well as using analog network inputs. Also with the Ethernet MODBUD client connection I have tried relaxing the poll time to several seconds but this also did not change anything. One observation: If I set a wrong MODBUS device ID then the blue light stays ON (and LOGO diagnose says there is a MODBUS connection). This means that LOGO is actually establishing the TCP link and it does not seem to have a problem with this step, but the following request (which I assume is the MODBUS request reading data) seem to break the connection. Unfortunately debugging has become difficult at this point. My attempts to connect wireshark with a very plain switch failed because it seems even they refuse to deliver the data to wireshark... (I guess my switches are not old enough...):-(
AutoGen_9020715 Posts: 3 Rating: (0)
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8/7/2024 4:24 PM	Rate (0)
ixo65 Grandmaster Joined: 12/18/2014 Last visit: 9/9/2025 Posts: 35940 Rating: (4500)	Which LOGO! do you have exactly? Sorry, I have not read the information (8.4). What unit ID (slave address) does the Modbus RS485 device have?
	Last edited by: ixo65 at: 08/07/2024 16:33:49
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8/16/2024 6:52 PM	Rate (0)
AutoGen_9020715 Posts: 3 Rating: (0)	I have tried various. Currently I am using 1. Also I have just tried with a Schneider M221, and it works fine there. It seems that the LOGO device is not happy with the response on MODBUS TCP and cuts the connections. I have tested also with PC software and I get the data fine. It seems that the LOGO device breaks the TCP link for an unknown reason once the Waveshare MODBUS TCP-RTU bridge is responding. Further testing I detected the following: The MODBUS connection is established (to the Waveshare bridge) but when the answer is send it terminates the TCP connection. If I intentially make the RS485 / RTU part of the connection wrong (resulting in no aswer being send back to the LOGO device) then the TCP connection stays alive (I can verify this with the Link LED on the Waveshare bridge).
AutoGen_9020715 Posts: 3 Rating: (0)
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8/16/2024 7:13 PM	Rate (0)
ixo65 Grandmaster Joined: 12/18/2014 Last visit: 9/9/2025 Posts: 35940 Rating: (4500)	What kind of Modbus RTU device is it?

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8/26/2024 1:10 PM	Rate (0)
AutoGen_9020715 Posts: 3 Rating: (0)	ixo65 What kind of Modbus RTU device is it? Eastron SDM630MCT V2, but I have also tried other devices. It does not have anything to do with the RTU side I think. I have done some wireshark traces today, and with this I have some more information... The MODBUS TCP Request and answer seem to be transmitted fine. I can see both in the log. But after that normally there would be only one TCP request to keep the connection life. But with LOGO there are several TCP packets exchanged resulting the the TCP connection to be shut down. This also seems to result in LOGO not processing the MODBUS data returned from the request. I attached the logfile from wireshark. Set the filter ip.addr == 192.168.1.224 (which is the MODBUS gateway). The LOGO is 222 Attachment modbus-logo-waveshare.zip (527 Downloads)
AutoGen_9020715 Posts: 3 Rating: (0)
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3/14/2025 11:29 AM	Rate (0)
@AutoGen_97368 Joined: 10/21/2024 Last visit: 7/17/2025 Posts: 13 Rating: (0)	Hi, could you find a solution?

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3/17/2025 6:22 AM	Rate (0)
AutoGen_380038 Silver Member Joined: 9/11/2014 Last visit: 9/9/2025 Posts: 867 Rating: (37)	Hi, what is the firmware in your LOGO! 8.4? For LOGO! 8.4 there are two firmware updates publisheb by Siemens. At least the update to FW V1.84.03 also improves the communication. Hope this helps

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