Sends an TCP command.

Returns {:ok, response}.

Example:

#write 1 to coil at slave 2 address 3200
:ok = TCP.cmd(pid, {:wdo, 2, 3200, 1}, 400)
#write 0 to coil at slave 2 address 3200
:ok = TCP.cmd(pid, {:wdo, 2, 3200, 0}, 400)
#read 1 from coil at slave 2 address 3200
{:ok, [1]} = TCP.cmd(pid, {:rdo, 2, 3200, 1}, 400)

Starts the GenServer.

state is a keyword list to be merged with the following defaults:

%{
  ip: {0,0,0,0},
  port: 0,
  timeout: 400,
}

ip is the internet address to connect to.

port is the tcp port number to connect to.

Returns {:ok, pid}.

Example

Modbus.TCP.start_link([ip: {10,77,0,211}, port: 8899], [name: Modbus.TCP])

Stops the GenServer.

Returns :ok.