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Thank for the answers. I will try monitor mode.

Hi! First, sorry because bad english. I want to measure temperatures with arduino and send the values to plc CQM1H-CPU51. I founded the MODBUS RTU Slave Interface for CQM1-CPU4x/CQM1H CPU51/61 PLC in download section. I am uploaded the program with dm memory. For now i want to communicate between pc and plc to try the program, but for now no respons from plc. I try to communicate with modbus softwares (CAS modbus scanner, Symple Modbus Master, qModMaster), but idon't have respons from plc. Just lit one a communication led and thats it. If someone have a functioning example for this type of communication between arduino and CQM1H please answer here. Or someone use this Modbus rtu ladder program tell me how use it. Thank you!

Thank You for the answer. I trying to use CQM1H as Modbus RTU slave. With this ladder program: The problem with the host link communication that i need to change run mode to program mode when i want to write dm memory.   I will check all the links in your post. Thank you!