RS485 2wires can't read

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I wrote a little program to read (ReadHoldingRegisters) from rs485 (2 wires via 
usb adapter), but reading always timeout

I opened the port using:
ModbusMasterSerial mb = new ModbusMasterSerial(ModbusSerialType.RTU, "COM3", 
38400, 8, Parity.None, StopBits.One, Handshake.None);

Same Query with others client and same hardware software works fine.

something worng in my open?
Thank you
Cesare

Original issue reported on code.google.com by cesare.g...@gmail.com on 29 Apr 2015 at 11:47