Last time, we looked over diffpairs, their basics, routing rules and the notorious tolerances of PCIe when it comes to diffpairs. Now, let’s take a look at the exact signals that make PCIe tick, as well as give you an overview of which sockets you can get PCIe on.
I separate PCIe sockets into two categories – desktop sockets, which are the usual 1x, 16x, or perhaps x4 PCIe sockets we see on desktop motherboards — and mobile sockets: mPCIe and M.2. There are also sockets like ExpressCard that are still found on some older laptops, but they have definitely fallen out of relevance. On mobile, M.2 is the way forward when it comes to PCIe – if you’re interested, I’ve written a short series talking about everything M.2 before.
On these sockets, most signals will be optional and some signals will be socket-specific, but there are some signals required in any PCIe device. Of course, the first group of required signals is the PCIe diffpairs themselves.
PCIe Is Just Spicy UART
Any PCIe interface has two kinds of diffpairs – TX and RX; for an x1 link, you will have one TX diffpair and one RX diffpair, and a x16 link needs sixteen of each. As you might expect, RX on one end connects to TX on another end, and vice-versa – it’s just like UART, but spicy. Of course, if you’re doing a PCIe extender or socket-to-socket adapter of some kind, you don’t need to do any swapping. If you’re designing a board for one of the PCIe-carrying extension standards, the usual naming is host-side. For example, say you’re putting a PCIe Ethernet co ..
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