Shane Colton digs into the PCIe PC bus specification and testing in the latest blog post:

Something I found interesting about PCIe in general is that there seems to be a pretty large barrier built up around the black box. Even just finding learning resources is much harder than it should be. The best I found was PCI Express Technology 3.0 and some accompanying material by MindShare, but even that seems like a prose wrapper on top of the specification. There isn’t anything that I would consider a beginner’s guide, like you might find for USB or Ethernet.

This is a used U4301B that I got in what has to be my second-best eBay score of all time, for less than it would have cost me to rent one for a month. There are only ever a handful of them up for auction at any given time, and the market is so small that the price is basically random, so if you’re actually looking for one I can only wish you luck.

… for now I have Gen3 protocol analysis capability, which is good enough for my purposes. I’ve used it a bunch in the past few months to explore the different layers of the PCIe stack and components within.

Read more here.