For a headless build I'm currently working on, I did not want to leave the bandwidth of the AGP port unused. But when looking for information online about using the AGP port for anything else than a graphics card you won't find much, except between the lines of this thread maybe. In most other forums when people have been asking about it, they'd be misdirected by some troll with an answer somewhat like "That's not possible because - you know - AGP stands for Accelerated GRAPHICS Port" So I had to search for information elsewhere. And I found some in the AGP Wikipedia article:
An AGP bus is a superset of a 66 MHz conventional PCI bus and, immediately after reset, follows the same protocol.
Now that’s some information I was looking for. Further investigations also lead me to this thread. But as nobody seems to have built the adapter I'm after, I began to tackle it myself. I usually prefer to build prototypes before designing and ordering PCBs so I also chose to try the manual way first in this case. PCI and AGP riser cards were quickly sourced and as soon as I got them the port of the AGP riser was ripped off and wired straight to the PCI riser according to the AGP and PCI specs. After finishing the adapter I noticed that I didn’t have a PCI graphics card at hand and I therefore had to test it blindfolded. So I plugged in an HP NC370T PCI-X Gigabit Ethernet card into the AGP port of my testbed, fired it up and tried to SSH into it. And it worked. Just like that. I still can’t really excite myself about it, as it was just too easy. Usually stuff like that never works on the first try.
After the handbuilt prototype proved that an adapter of this kind is working I started to design a PCB according the AGP spec for impedance and length matching. The PCBs are now finally in and work really well. So far any network card I’ve tried worked flawlessly. Compatibility with SATA RAID cards seems to be more problematic. Of all the RAID controllers I’ve tried, only the Adaptec 2820/2420SA works well enough on the AGP to be usable. If the connected card and the underlying system have the capabilities the full 32bit 66 MHz PCI bandwidth of ~250 MB/s can be achieved.
I naturally got a few of the PCBs made, so if anyone is interested, please get in touch. As I still can't send private messages, please include an e-mail address in your pm.