Why are PCIE's coupling capacitors so large
The larger AC coupling value passes the low-speed 'ping' pulses used for sensing the presence of a link partner. The TX PHY sends the pulses, then watches the signal using the known R-C time constant to detect an open or terminated line.
Here's an excerpt from an article describing this:
&quot;Receiver detection: PCIe uses an ingenious means to recognize both the presence of a physical link and channel width. The specification exploits the fact that an un-terminated, ac-coupled transmission line will have a very different charge time when the line is terminated versus open. Each PCIe transmitter, at the commencement of linkup, produces a low-frequency “ping” on each of the differential TX outputs. The transmitter includes a simple detection circuit to monitor the line response to this ping. With no receiver attached, the edge rate (and amplitude) of the line change is much higher than when a receiver is present. Because the specification has a defined range of coupling capacitance and the receiver termination, a distinct, detectable time constant range defines when a receiver is present or not.&quot;
From here: https://www.electronicdesign.com/technologies/communications/article/21762833/pci-express-and-the-physical-journey-to-gen-3
Once the link partners have found their connections, then the 8b/10b linkup and training process begins.