GPIB-USB conversion allows you to connect your GPIB devices to virtually any machine through an available USB port.
Why would you want to do that?
Most newer computers don't come with GPIB ports standard - sure, you can add them with one with a GPIB PCI Card, but why bother if you can do it through a USB port (which is less expensive and doesn't require you open the computer).
Also, if you want to control GPIB devices with a laptop your options are really limited - in short, a GPIB-USB converter is the simplest, fastest, cheapest way to accomplish this.
Prologix GPIB-USB Converter
Pros: (1) The Prologix interface is working great on my HP8562A analyzer, connected to my Windows Vista PC. (2) It was quick to set up and pretty much no-maintenance after that. (3) The Prologix interface costs a fraction of the price of other GPIB interfaces (even those on the used market).
Minor drawbacks: (1) The driver installation instructions were overly complex and overly cautious. Just download the right drivers from the Prologix web site, put them in a directory, install the interface, and when the device installation window pops up, point it to the directory where the driver is. This is just like any other manual driver install in Windows. (2) When the Prologix interface is connected directly to the back of the spectrum analyzer, it sticks out farther than the back legs of the analyzer, so I can't stand the analyzer straight up.
Other Comments: The cons are minor compared to the fact that the interface works like a charm from the get-go.