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I'm trying out SCAD-LTS with my trusty Modbus simulator ModRSim2.exe as a Docker image on a Windows 10 laptop. I'm having difficulty maintaining a stable connection to the Modbus TCP server. Update rate is at 1s. Data is beeing read, but alarms keep popping up (connection lost, checksum) and the connection keeps beeing reset.

Bigman74066
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  • I found that ckecking "encapsulated" checkbox improves things considerably. I'm still getting "no response from slave 1" messages every now and then though. What is the function of this checkbox? Anyone got a 100% stable connection to Modbus TCP running? – Bigman74066 Jul 01 '21 at 13:57
  • Modbus TCP doesn't use a checksum, Modbus RTU does. They are different protocols. Looks like one side expects a Modbus RTU connection, not TCP – from56 Jul 04 '21 at 16:56
  • Also with Modbus TCP the station number is not used (except if communicating with a RTU network through a TCP to RTU gateway),. Which identifies a slave is its IP address – from56 Jul 04 '21 at 17:07

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The checkbox is for encapsulating the Modbus RTU protocol in TCP data packets. Checking the checkbox was a bad idea (although it seemed to reduce the number of errors it actually made things a lot worse). I ended up increasing the cycle time from 1 to 2 seconds. That made the error messages go away. Weird, because I would not expect protocol errors to arise from short cycle times. I think it might be a bug(?)

Bigman74066
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