create a modbus connection for TCP/IPv4
modbus_newTcp(host_IP, host_port)
a string containing target host IPv4 address.
the host port to connect on.
This function will allocate and initialize a Modbus structure to communicate with a Modbust TCP/IPv4 server.
Modbus default port is 502. It's convenient to use a port number greater or equal to 1024 because it's not necessary to have administrator privileges.