Snail's Pace Reviews: PCIe slots

[molluscsandmonsters]

A PCIe is a type of expansion bus. Lots of other things are also buses! A bus is made of single wires (serial) or groups of wires (parallel) that carry data from one location to another. Most modern bus systems use lanes, which are pairs of serial wires with one sending data and the other receiving at the same time.

 

Expansion buses allow you to plug extra hardware into your motherboard as easily, in most cases, as you plug a game cart into a gameboy (or Switch, I’m old). Most additional functionality that comes from hardware is stuff plugged into expansion bus slots. The most common form of expansion bus used to be PCI (Peripheral Component Interconnect), and you still sometimes see PCI slots. They look like this:
 

However, on most modern motherboards you’ll see PCIe (Peripheral Component Interconnect express) slots. They look like these four slots at the top:



(Above: PCIe x4, PCIe x16, PCIe x1, PCIe x16, and a PCI slot)

 

PCIe have different numbers of lanes, indicated by x(#) or “by (number)” notation. For instance, a PCIe x1 slot has only one lane, while a x16 slot has sixteen lanes. Peripheral cards need a certain number of lanes to function. They also won’t physically fit in a slot with too few lanes. They can usually fit in slots with more lanes than they need, but there’s no mechanical benefit to doing so.

 

There are currently four versions of PCIe. Each one doubles the data rate of the last one; PCIe v1 has a data rate of 2 Gb/s (250 MB/s), PCIe v2 has a data rate of 4 Gb/s (500 MB/s), v3 is 8 Gb/s (1 GB/s), and v4 is 16 Gb/s (2 GB/s). This means each wire in the lane can transfer that much data per second. To figure out how much the whole bus can transfer at once, coming and going, you need to take into account how many lanes there are too.

 

Example: a PCIe x4 slot is v3

 

1 GB per wire    2 wires    4 lanes
------------------- * ---------- * ---------- = 8 GB/s for this slot
1 second            1 lane      1 slot

(In case screenreaders made a mess of that or the formatting doesn't hold up on other screens, it says that 1 GB per wire divided by 1 second times 2 wires divided by 1 lane times 4 lanes divided by 1 slot equals 8GB for this slot, or 1 GB per wire per second times 2 wires per lane times 4 lanes per slot equals 8 GB for this slot.)

The same motherboard will commonly have more than one version of PCIe slot on it, because the slower kinds are cheaper and still work fine for many applications. Your higher rated PCIe slots, which are x16 and v3 or v4, connect directly to the CPU or “Northbridge” in order to move faster, while any other PCIe slots will connect to the chipset or “Southbridge”. Most chipset PCIe slots are x1, which is also the size of most peripheral cards aside from the fancy video cards and such that you’d want in those higher-end slots anyway. Occasionally you’ll also see x4 or x8 slots. It’s easy to tell the differences between them just by comparing their sizes, but you can also tell them apart by the number and configuration of gaps.

 

Important components that may be installed via PCIe slot include:

- GPUs
- WiFi cards
- dedicated audio cards
- cards that add extra ports, such as more USB ports
- some kinds of very fast SSDs (may require additional Stuff)
- adapters for weirder things

 

The most important thing here is to understand how your available number and types of PCIe slots affect what you can do and how you’re going to do it. When building a PC or replacing a part, you need to know whether the peripherals you need will work with the slots on your motherboard. Also, if a peripheral component isn’t communicating with the rest of the machine, you need to know that a bad PCIe slot is a potential culprit.