The formatting for PCIe (or PCI express) connectors can throw some people off. I have seen a few discussions online where people assume that X1, X4, X8, X16, and X32 means that there are that many connectors found in an assembly. I’d thought I’d provide some context on what these format numbers actually mean.
Let’s take a look at an example part number that initially threw me off before I researched.
150-2305200-R-ND
150-2305200-R-ND (Microchip’s part number 2305200-R) is a “Tri-Mode Connectivity Cable For SmartHBA 2200 Host Bus Adapter” that has two PCIe connectors on it. If you look at the datasheet and use CTRL+F to search for 2305200-R, you will find the relevant row of information. The description of the adapter says “x8 SFF-8654” and the end-point says “x4 SFF-8639 U.3.” Without immediately noticing the numbers before these descriptions, I also initially thought it was 8 connectors for the adapter and 4 connectors for the end-point. However, this is actually calling out the number of PCIe lanes instead.
Microsemi Documentation
If you want more technical information on this, check out this detailed product sheet made by Microsemi (now a subsidiary of Microchip). They describe it this way, “PCIe currently supports up to 8 GT/s of throughput per PCIe lane.” Later on they say typical storage implementations utilize, “x2, x4, x8, and x16 lane width interconnect configurations from the host root complex, directly to or through PCIe switches, to endpoint PCIe storage devices.”
Why Does it Matter?
When you split higher value lanes into smaller ones, you need to match the number of lanes evenly. For example, starting with one x8 peripheral, you must have 8 lanes on the output as well. You can do this by splitting this into two x4 connectors or four x2 connectors. You cannot passively split a larger peripheral into options that give you more data lanes than what you started with. In other words, you cannot split an x8 connector into four x4 connectors. Four x4 connectors would result into 16 data lanes, so there would be a mismatch. There is a caveat to this, but it cannot be done passively. There are such things as multiplexers that can “increase” PCIe lanes, but these are expensive and end up being quite large. They basically act as a PCIe switch (like an ethernet switch hub).