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two devices on modbus

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Hellow I'm doing data aquisition from a SCADA on a PC . I have two devices working on Modbus communication protocol but to one of them it has to be on RS-485 (two wire) and for the second device it has to be on RS-422 (four wire). My question is : can i use the same com-port on the PC (let say COM1) with only one coverter RS232 to RS-422/RS485 ? Thanks David

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Hi david, I think you can. Some time back, I had a Nudam make DAQ module which worked whether you needed it as RS-422 or 485. There were TX+ TX- RX+ & RX- terminals. We shorted the TX+ & RX+ together to obtain the 2 wire D+(A) signal. Similarly so were the TX- & RX - shorted to constitute the D- (or B) signal. It worked well but then it was not among other RS-485 slaves. The SCADA was InTouch & the MODBUS DDE server was used. I was guided by diagrams in the book by Jan Axelson "Serial Port Complete". I am convinced that this idea may work & would really like to know what happened after your attempt. Wishing you all the best. R.S.Iyer

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