Hi Lucie, OK pin back your ears..
Two ways that I know of :- First thing though is that USB is not really an external serial port as such so you cannot just hook up to it you need some sort of Driver.
The method I have used success fully (so far) involves installation of a Virtual Com port. This will show up in device manager as a serial coms port, you can then Read and write to it using a Com port component like TCommportdriver (there are several of these)
We needed to connect to external devices with USB interface chips made by a company called FTDI. Their website has a lot of stuff on general USB com ports. They supply a driver that creates the virtual comport.
They also supply what they call a 'direct driver', this is a Delphi module that hooks into the USB at low level so you don't need the Virtual comm port, but as with the Virtual comports it only works with their devices.
I have also used a USB to serial converter dongle thing again the user must install the driver supplied.
The reason for this is because all USB things have a pair of ID codes (and a load of other protocol related stuff) that the puter must know before communication can start.
Now you might say when I plug in a Flash dongle it just works (you will see a driver installation message).
This is because all USB things are divided into groups depending on their functionality, some can get away with generic windows USB driver, but the firmware must be able to kick it off so to speak.
So it also depends (a lot) on what it is you want to communicate with). Have a look at FTDI's web site lots of clues and they are Delphi compliant.
Steve: N.M.N.F.
If something is popular, it must be wrong: Mark Twain