DCE modbus communication problem

[DCIM_Support]

 Dear sir,

    Recently I met some problem with DCE System version 7.4.3, please help me.
    
    1. How the retry time in moniting modbus device works? I tried to modify the retry time,
       but looked at the journal, it seemed no effort, retry times ran out quickly.

    2. I saw many "Received error: Unknown error" in the journal, but I didn't know what happened.
       Is there some details for me? Or somewhere to find the option for details?

    3. I also saw many "Unexpected Response" in the journal, it means the length of received message
       is not equal to that expected. But after checking the content, I found it is just a malposition.
       The system actually had got the answer of last request.

    4. Is there any way to capture the modbus TCP package?

    Attachment is recently journal.

IP:122.192.1.32

250250

(CID:130092500)

[DCIM_Support · Accepted solution]

Dear guanyc,

Thank you for your logs, I looked at them and I can identify a number of common errors:

SLAVE_DEVICE_BUSY

GATEWAY_TARGET_DEVICE_FAILED_TO_RESPOND

SLAVE_DEVICE_FAILURE

Which for obvious reasons lead to the appearance of a DCE daemon scanning error:

modbus_process_timeout_cbk: read timed out for host 122.192.1.32

Therefore, it is possible to say with a high probability: considering, that you have at least 6 slaves connected on the same Modbus serial line, you have an hardware problem of a Modbus serial line.

For this reason, there are a lot of simple recommendations, for example, my answer to the post :

1. reduce number of Modbus slave devices on one IP address (to 5-8);
2. reduce the frequency poll of Modbus slave devices (one poll in 2-3 minutes);
3. increase the number of attempts of poll for each of problem Modbus slave devices (not 3, and 5-6 attempts – for DCE-7.3.1 and newer);
4. increase timeout between unsuccessful attempts poll of Modbus slave devices (for example, up to 15 seconds – for DCE-7.3.1 and newer).

As option it is possible to reduce number of sensors, which are read out on Modbus slave devices.

And at last, to try to understand, and probably to reveal and eliminate defects of the Modbus serial communication line between Modbus devices. Practically, the last recommendation can be the most effective and useful.

I hope this helps you.

If you have any questions, please ask.

With respect.

(CID:130092563)

[DCIM_Support]

Dear sir,

    Thank you for reply!

    1. I have 6 devices divided into 2 groups, one has 2 and the other has 4. The new modbus TCP gateway

    supports isolation(by different ports), it works properly now.

    2. I set the timeout to 60 seconds, but in journal it appears not as long as 60 seconds.

    3. Can you give some test documents or softwares for serial lines?

(CID:130093906)

[DCIM_Support]

Dear guanyc,

2. I set the timeout to 60 seconds, but in journal it appears not as long as 60 sec

If I understand you correctly, and this is really a connection timeout, then this is too much.

Remember: the more you have connected slaves on one same serial line, the less connection timeout should be. Otherwise, the DCE-server will wait too long for communication with only one slave device (especially when the connection is bad) and deny service to other slave devices. Practically, the communication timeout should be selected no more than 10...15 seconds with several slave devices.

To be exact, please show a screenshot from your DCE-7.4.3 where it would be nice to see the scan parameters of any your problem TCP Modbus slave device.

3. Can you give some test documents or softwares for serial lines?

I've been using the simple yet powerful utility Modpoll Modbus Master Simulator for a long time. This utility exists for different platforms, has an excellent help and is easy to use. With the help of this utility it is possible to perform a good diagnosis of your serial communication lines.

Also, do not forget, that the very first point for diagnosing your serial communication line should be directly a TCP/IP Modbus gateway. Any good TCP/IP Modbus gateway should have a statistics menu, where in an accessible form you can find out if there are any errors or problems on the serial communication line? This is a very important point, about which I wrote in detail in a similar topic , for example:

I hope this is what you need.

With respect.

(CID:130094034)

[DCIM_Support]

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