Here is another option if you want to build your own USB to GPIB adapter.
http://www.dalton.ax/gpib/ It uses a Microchip PIC18F2550. I work with Microchip PIC's and have a programmer for them. If you don't have a PIC programmer, It'll add at least another $30 to the build cost. This is not a good choice unless you have a PIC programmer.
This adapter uses the Prologix command set. It appears as a serial port on your computer.
It does not use proper GPIB transceiver chips. It will probably fail if you have a lot of instruments connected. Iv'e used it with three instruments on the bus and initially had no problems. I later had intermittent data corruption from my HP3478A multimeter. I assumed that the problem was from my cheapo home built GPIB adapter. I coughed up the bucks for a real Prologix adapter. That was a troubleshooting fail
After installing the Prologix adapter, I still had the same problem with the 3478A. With more testing, I found that the problem was in the meter. The SN75160B GPIB transceiver was failing. Also, After I popped open my Prologix adapter, I saw that it also does not use GPIB transceiver chips. For what the Prologix adapter costs, I was expecting it to properly support the GPIB standard:)
Since I now have a Prologix adapter, I could compare the Dalton Prologix clone with the real thing. I don't see any significant difference in transfer speed.
The Dalton clone is not 100% compatible with the Prologix adapter. It does not assert the RTS serial control line. This causes problems with the KE5FX ezgpib software.
Also, if you have several instruments, the big cost of GPIB is for the cables and connectors. I was lucky and had a cable to get me started. Anyone remember the Commodore PET computer? It used GPIB to connect to It's peripherals. I had a cable that was used to connect to a PET disk drive. That cable was in poor shape. Insulation was cracking and brittle. It did allow me to get a GPIB interface up and running in one weekend:) I really needed some new cables. I've now had the painful experience of buying more of these expensive cables.
Finally, GPIB (IEEE-488, HPIB) is amazing. It was created in the 1960's. It's only recently been replaced with USB and Ethernet. A long life for a bus protocol!
Attached are two pictures. One of my quick & messy perf board wired Dalton GPIB adapter and another of my Prologix adapter with three GPIB cables connected.