Question

Hi,

I'm trying to read the GPS latitude from a RUT955. I'm trying to first read a holding register (FC03) and then write a single register (FC06). However after reading the register 143/144 the Router terminates the connection with a FIN/ACK without having received a FIN flag. So I am reeiving data but I have to reconnect after the transmission was terminated by the RUT955. This is kinda annoying as I want a position update every second, plus the transmission is indicated as an error on many Clients.

Here is the wireshark recording.

Master: Modbus Poll; Slave: RUT 955

This is a recording of another Modbus Slave which looks a lot different and works actually fine.

Master: Modbus Poll; Slave Wago 750-8212 (PLC)

Anybody an idea whats going on?

Ian

Answer 1

Hello!

Is it posible for you to send the configuration of both master and slave devices?

BR

Aliaksandr.

Comments

Hi Alex thanks for your response.

I changed my setup and used a RUT955 as Modbus Master and another RUT955 as Slave. I sent you the config files of both, master and slave, as well as the Wireshark capture file captured with TCP dump on the master. With the same results.

Here another screenshot of the capture file just in case anybody else might be able to help.

Regards Ian

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Im talking about the two frames 8 and 18. This is where the Modbus slave sends a FIN, ACK flag out of nothing. In frame 11 the master reconnects and we see the same pattern again.

As mention, I receive the data but it is super anoying, as the transmission is indicated as read/write error under modbus poll (https://www.modbustools.com/modbus_poll.html).

Within a PLC I use a more basic level modbus master compared to modbus poll. So I have to manually reconnect everytime after the socket is closed. This is pretty time consuming and error-prone.

Ian

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Hello, Ian!

Sorry for waiting for a response. Analyzing information about similar problems, I can say that a similar situation occurs when there is any conflict of IP addresses or ports.

So please check if the PLC requires requests and responses from a specific IP address or port?

Best regards, Aliaksandr!

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I have same problem, after reading MB register router terminates the connection and my MODBUS client because of this, does not receive data. 

Is there any firmware fix, which would not terminate the connection after reading? 

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Hello Aliaksandr.

Just now I used a brand new RUT955 with RUT9XX_R_00.06.06.1 frimware and all I changed was to enable Modbus. I left all the IPs and Ports as factory default and received the same results as before.

Have you had the chance to try it out? If so, what was your setup? Was the connection successful?

Best regards Ian