Dear Balucharly!
Thank you for choosing National Instruments Support!
If you search our driver network which can be reached from
here - you will find that the HP 53131A has three drivers, but they all use the GPIB or the IEEE 488.2 interface. You can however, modify these to work with the RS 232 bus.
Another option for you would be to use VISA VIs that can communicate with the serial bus and communicate with your instrument via the RS232 bus.
Hope this helps.
Thanks and Regards
ShekharS