These days most computers are built with a USB serial port, and the old RS-232 serial port has become outdated. In spite of that, many devices for data acquisition connect only via the RS 232 DB9 link. Several adaptors are available on the market, in theory permitting you to use a USB computer to control RS-232 devices. In practice it appears that many adaptors however are not suitable for the low date rate RS-232 devices. For example using Datalyse on a recent Dell notebook, only the Prolific Electronic based adaptor seemed to work correctly, giving no communication errors.
Datalyse web site: http://www.datalyse.dk/datauk/
Address: Gilles Carpentier, Faculté des Sciences et Technologies, Université Paris 12 Val-de-Marne, France.
Gilles Carpentier's Web Site: "Computer Data Acquisition for Biochemistry Practice Works: Methods and Examples"