Forum Home › Forums › Runtime Bugs › Problem with DupAcks of Modbus TCP
- This topic has 9 replies, 3 voices, and was last updated 4 years, 2 months ago by Mikhail.
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January 16, 2020 at 9:00 am #6418vishnuvardhanParticipant
Hi
I am using Rapid Scada as Modbus TCP Client. I am polling 20 registers at a time, a total of 100 registers in a sequence once in a second. Whenever the server sends DupAcks to the client the sequence of polling is getting changed and I am receiving wrong values(which I verified by polling the same registers with some other TCP client software). So, here my doubt is whether the Rapid Scada is capable of handling DupAcks. If it is capable why i’m getting this issue.I’m getting incorrect PDU length in communication log.
https://drive.google.com/open?id=1kgGytct6VpXHIVkTtcOQ5Fuhz2rf30N6
Awaiting your reply..!!Regards,
Vishnu.- This topic was modified 4 years, 2 months ago by Mikhail.
January 16, 2020 at 1:49 pm #6420MikhailModeratorHi Vishnu,
Please post the same log as a text (not image), so I can check it using a tool.
You can find the file in C:\SCADA\ScadaComm\LogJanuary 17, 2020 at 6:11 am #6421vishnuvardhanParticipantHi,
I am posting the log below.
https://drive.google.com/open?id=1rRHtrDnQDPsl3vIkRI4ahcXMauD2LahTRegards
Vishnu.January 17, 2020 at 7:01 am #6422manjey73ParticipantIt feels like your device is sending out packets again
January 17, 2020 at 7:04 am #6423manjey73ParticipantHere you have a request for 5 registers and returned 5
Request element group
Send (12): 00 00 00 00 00 06 01 03 01 38 00 04
Receive (7/7): 00 00 00 00 00 0B 01
Receive (10/10): 03 08 FF ED 00 13 00 00 00 00
OK!And here you have a request for 10 registers but returned 5 from the previous request. And so all the time
Request element group “offet and gain”
Send (12): 00 00 00 00 00 06 01 03 01 63 00 09
Receive (7/7): 00 00 00 00 00 0B 01
Receive (10/10): 03 08 FF ED 00 13 00 00 00 00
Incorrect PDU length!January 17, 2020 at 8:25 am #6424vishnuvardhanParticipantYes, I noticed in wireshark that some TCP Retransmissions have happened and few DupAck’s also. After that the polling pattern has changed, It used to poll at addresses 110,124,138,163,13C before TCP Retransmissions but after that it is polling at same address for twice or thrice continuously and at other addresses it is polling once. Why this would have happened? If I close the connection and re-establish then it’s working fine until some TCP Retransmissions occur.
January 17, 2020 at 8:29 am #6425vishnuvardhanParticipantIf I poll the same addresses using some other Modbus Master Simulator it is working fine..!!
January 17, 2020 at 3:06 pm #6426MikhailModeratorIn the properties of communication channel untick the checkbox “Stay connected”.
Increase the timeout and delay parameters of the device.January 28, 2020 at 12:08 pm #6447vishnuvardhanParticipantYeah, I will do that but I want to know what is the root cause for this problem. Sorry for the late reply..!!
January 28, 2020 at 2:15 pm #6449MikhailModeratorRoot cause will be clear when the problem is solved.
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