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.

 

I believe that the Modbus exception Codes 06 is caused by the slave device being busy. See the MODBUS Application Protocol Specification V1.1b, page 49 at the link below:

http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b.pdf

William (Bill) L. Mostia, Jr. PE
Fellow
SIS-TECH Solutions, LP

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