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Unable to read/write Modbus data

Created by: tavistaicu at: 1/17/2020 10:16 AM (8 Replies)

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1/17/2020 10:16 AM	Rate (0)
tavistaicu Posts: 2 Rating: (0)	Hello! I have a problem reading/writing modbus data. A few days ago I established the connection beetween the LOGO and the PC, and everything worked fine, I was able to read/write modbus data to the LOGO using QModMaster. The problem appeared when I tried to write a simple C# program that establishes a connection to PLC and reads/writes modbus data using EasyModbus library. At first, I used EasyModbus Slave simulator to test de C# program and it worked well, but when created the communication with LOGO after I ran the program I got an error message in Visual Studio saying: "System.IO.IOException: "Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host." After that I switched back to QModMaster trying to read/write data and it didn't work anymore. When I try, for example, the function 'Read Coils' and address '0' I get an erro saying: "Read data failed. Error : no error." and also when I use the QModMaster Tools it says that the port is closed.(See the attached images) Attachment modbus3.png (363 Downloads)
tavistaicu Posts: 2 Rating: (0)
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1/17/2020 12:26 PM	Rate (0)
Scorp Platinum Member Joined: 2/22/2008 Last visit: 4/25/2025 Posts: 2698 Rating: (210)	A long description with few details. Maybe you have not closed an earlier connection ? I am not a fit C programmer for this to help you. Offer more details to get help from other people. Regards, Scorp

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1/17/2020 2:45 PM	Rate (0)
sumit_b Experienced Member Joined: 7/29/2019 Last visit: 1/22/2025 Posts: 23 Rating: (3)	Try specifying port 510 instead of 502.

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1/17/2020 4:58 PM	Rate (0)
Scorp Platinum Member Joined: 2/22/2008 Last visit: 4/25/2025 Posts: 2698 Rating: (210)	sumit_b Try specifying port 510 instead of 502. What is the background to use port 510 ? There are 8 static connections possible with Logo8. In case of Modbus server the ports are fixed to 502 up to 509 depend on the connection point, see network project. In case of Modbus client the ports can be defined to any port, but normally the ports are numbered beginning at 502.

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1/17/2020 7:45 PM	Rate (0)
sumit_b Experienced Member Joined: 7/29/2019 Last visit: 1/22/2025 Posts: 23 Rating: (3)	The original posters' screenshot says "Port is closed." He is trying to connect to port 502. When I received my new LOGO, I also had the same problem. It turned out it would only accept connections on port 510. I don't fully understand the network project set up yet, but if OP is able to successfully connect to port 510, that will at least eliminate his C# code as possible cause.

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This contribution was helpful to 1 thankful Users Mutlu_Tuna

1/17/2020 7:53 PM	Rate (0)
ixo65 Grandmaster Joined: 12/18/2014 Last visit: 9/5/2025 Posts: 35928 Rating: (4497)	sumit_b The original posters' screenshot says "Port is closed." He is trying to connect to port 502. When I received my new LOGO, I also had the same problem. It turned out it would only accept connections on port 510. I don't fully understand the network project set up yet, but if OP is able to successfully connect to port 510, that will at least eliminate his C# code as possible cause. In your case: Is the LOGO! the master (client) or the slave (server)?

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1/18/2020 4:50 AM	Rate (0)
sumit_b Experienced Member Joined: 7/29/2019 Last visit: 1/22/2025 Posts: 23 Rating: (3)	ixo65 In your case: Is the LOGO! the master (client) or the slave (server)? The operating mode of the LOGO! is set to "Master". But I am able to read and write from another device (acting as master) as though the LOGO! is the slave.

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1/18/2020 5:12 AM	Rate (0)
ixo65 Grandmaster Joined: 12/18/2014 Last visit: 9/5/2025 Posts: 35928 Rating: (4497)	If the LOGO! If the master (client) is in a Modbus TCP/IP connection, the port number can be freely selected for the Modbus device as slave (server). The LOGO! whowever, the software enters the port number 502 as the default, because Modbus uses the port number 502 for the Server as default. Is the LOGO! the slave specifies port 502 for the first static connection, port number 503 for the second static connection and so on until to the eighth static connection, port number 509.

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1/20/2020 7:07 AM	Rate (0)
tavistaicu Posts: 2 Rating: (0)	Thank you for your answers. I changed the port to 510 and now everything works fine. But I don't really understand what the problem can be because when I used port 502 before, it worked.
tavistaicu Posts: 2 Rating: (0)	Last edited by: tavistaicu at: 01/20/2020 07:15:23
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