The problem was in serial port connection settings. When I used these i was finnaly able to get response from board:

var port = new SerialPort("COM3", 115600);

port.Open();

port.Parity = Parity.None;
port.StopBits = StopBits.One;
port.DataBits = 8;
port.Encoding = Encoding.UTF8;
port.RtsEnable = true;
port.DtrEnable = true;
port.BreakState = false;

The problem was in serial port connection settings. When I used these i was finally able to get response from board:

var port = new SerialPort("COM3", 115600);

port.Open();

port.Parity = Parity.None;
port.StopBits = StopBits.One;
port.DataBits = 8;
port.Encoding = Encoding.UTF8;
port.RtsEnable = true;
port.DtrEnable = true;
port.BreakState = false;

The problem was in port settings. When I used these i was finnaly able to get response from board:

var port = new SerialPort("COM3", 115600);

port.Open();

port.Parity = Parity.None;
port.StopBits = StopBits.One;
port.DataBits = 8;
port.Encoding = Encoding.UTF8;
port.RtsEnable = true;
port.DtrEnable = true;
port.BreakState = false;

The problem was in serial port connection settings. When I used these i was finnaly able to get response from board:

var port = new SerialPort("COM3", 115600);

port.Open();

port.Parity = Parity.None;
port.StopBits = StopBits.One;
port.DataBits = 8;
port.Encoding = Encoding.UTF8;
port.RtsEnable = true;
port.DtrEnable = true;
port.BreakState = false;