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Is it possible to use DB9 connection of PC to communicate with this RS485 to TTL converter? Sorry if my question is too immature, but I am pretty new to MODBUS and RS485
Most modern PCs do not have a DB9 port, in general they only have USB ports now.
If your PC does have a DB9 port then it is almost certain to be RS232 not RS485.
So the first thing you need to do is is find out the make an model of PC and what the DB9 port is.
If it is RS232 then you can purchase RS232 to RS485 converters.
You have written "To note here is DTU and RTU does not exist in the current system. I have been asked to design them. So I figured out that DTU should have something to process the queries for this I am using arduino".
You can only decide to use an Arduino if you know exactly what the existing system does, at that point you can decide if an Arduino is suitable to replace it.
Perhaps there are language problems here but Ithink you are really getting ouf your depth and do not stand much chance of producing a working system.
Does the existing PLC not work?
If it does work why are they wanting to move to this new DTU / RTU model?
If the only thing they want is to have a wirless link then I wouldreally strongly suggest they look first at simply adding a wireless link to the existing PLC system.
Sorry but I really cannot help further.