I would like to interface an Agilent 53131A Universal Counter and an Agilent E3648A Power Supply to a PC via an Agilent 82357B GPIB-USB interface. I am using an evaluation version of Signal Express (tektronics ed.) but will have a full version installed by the end of the week. I have not been able to find any drivers (IVI or plug and play) to allow for USB connectivity in this fashion. Will I need to drop the USB connection and use serial (RS-232) or buy a GPIB card to be able to controll these instruments with Signal Express?
There are IVI drivers for both of those instruments. Both of them support a GPIB connection which is what you are using. The fact that the GPIB controller connects to the pc with a USB cable does not make a difference. Actually, even if you had instruments that connected to the pc with a direct USB connection, it probably wouldn't make a difference. IVI and most of the other types of drivers listed on NI's site are based on VISA and VISA makes the physical interface a non-issue in most cases. It's just a matter of selecting the correct resource name.
Once you get your GPIB-USB device recognized in MAX and download the IVI drivers for your instruments, you should be able to control the instruments from Signal Express. I see you have a Universal Counter and Power supply; Signal Express only provides a step to directly control the Power supply, and you'll need a user-defined step for the Counter. The links here and here have information on creating user-defined steps, and you can search here for existing user-defined steps. Hope this helps. Thanks!