Modbus data giving error code 06


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We are interfacing Packagged Air Conditioning unit through modbus. The data is coming to our SCADA but frequently slaves are getting failed and we are getting error code 06 in our log. The frequency of failure is not definite. The 4 slave devices are connected through RS485 to terminal server which is giving data over IP. Is this can be due to terminal server or from device? We also did modbus test utility in our SCADA and found the message as "QueueUserAPC error".

If any help, highly appreciated.