>How I could open a drawer of a box-register from my system, using the ports.
>I mean, when the client pays the consumption, in order to place the money inside the bag, how I could open the drawer of my system.

You'll need to check the documentation for the cash register system. There is no single standard for doing this. If it is atached to a serial port on the PC, you can read and write stuff to the COM port using an ActiveX Control like the MSCOMM32 control distribued with VFP or a commercial communications library. The vendor should be able to tell you how to configure the port and what codes need to be sent.