create a Modbus RTU connection
modbus_newRtu(device, baud, parity, data_bits, stop_bits)
a string containing the name of the serial port handled by the OS.
the baud rate of the communication, eg. 9600, 19200, 57600, etc.
a string for the parity. Should be 'N' for none, 'E' for even, 'O' for odd.
The data_bits argument specifies the number of bits of data, the allowed values are 5, 6, 7 and 8.
The stop_bits argument specifies the bits of stop, the allowed values are 1 and 2.
This function will allocate and initialize a Modbus structure to communicate in RTU mode on a serial line.