Simply put, the answer to the question “is PCIe backward compatible?” is a resounding yes!
The PCIe interface is not just backward compatible, it is also forward compatible.
What this means is that if you have an older PCIe expansion card you can install it on a motherboard with a newer PCIe version.
Conversely, if you have an older version of PCIe slots on your motherboard, you can install newer expansion cards on them.
However, there are plenty of caveats that you must be aware of particularly caveats dealing with performance bottlenecks especially if you use newer devices on older slots i.e using a PCIe 4.0 SSD on a PCIe 3.0 slot.
Also, if you were to connect a PCIe 3.0 expansion card on a PCIe 4.0 slots, you will not see any performance boost. The expansion card will conform to PCIe 3.0
In other words, while the PCIe interface is both backward and forward compatible, it is not ideal to build your PC in this configuration.
Ideally, both the PCIe version of the motherboard and the expansion cards you add must match.
PCIe Interface – A Brief Look
PCIe interface is used to connect High Speed Input Output (HSIO) devices to your PC using PCIe slots.
PCIe slots comprise of PCIe lanes which act as information highways through which the CPU communicates with the PCIe devices attached to the slots.
There are different sizes of the PCIe slot i.e x1, x4, x8 and x16. The number after “x” generally denotes the number of PCIe lanes the slot offers.
Depending upon how much data an expansion card would generate, you would choose the corresponding slot for it.
For instance, since a graphics card generates a lot of data, it is designed to be installed into the x16 slot ideally.
An NVMe SSD requires an x4 slot.
Devices that generate low amounts of data such as a Wireless Network cards, dedicated sound cards etc require only an x1 slot.
PCIe Interface Improves with Each Newer Versions
There are different PCIe versions. With each newer PCIe version, the per lane transfer speed of the interface doubles.
For instance a single PCIe lane conforming to version 3.0 has transfer speeds of 0.985 GB/s. A single PCIe lane conforming to version 4.0 has transfer speeds of 1.969 GB/s!
The following table shows PCIe version and their corresponding lane speeds.
Also Read: PCIe 3.0 vs 2.0: What is the Difference?
The most current version that has picked up steam in the market is the PCIe v4.0.
Also Read: Which Intel and AMD CPUs Support PCIe 4.0?
So Is PCIe Backward Compatible?
Yes, PCIe interface is backward compatible. Meaning that you can install a PCIe 4.0 device in a PCIe 3.0 slot.
However, the fact of the matter is that the maximum bandwidth supported by a PCIe 3.0 slot is half as much as that of a PCIe 4.0 slot with the same amount of lanes.
As a rule of thumb, the PCIe interface will work at the maximum speed of the slowest component.
Thus, if you insert a PCIe 4.0 device in a PCIe 3.0, it device may or may not saturate it depending upon how much data it generates.
There are two scenarios that can occur here.
The Newer Device Will NOT Saturate the Older Slower Slot (No Bottlenecks; This is Good)
If you were to install a low demanding expansion card (i.e a card that generates a low amount of data) and if it does not saturate the total throughput rate of the slot, then you will have no performance issues to worry about.
For instance, a version 3.0 WiFi 6 network expansion card uses an x1 slot. Even the fastest WiFi card does not generate enough data to saturate the throughput rate of a single PCIe 2.0 lane let alone PCIe 3.0 or 4.0 lanes.
Therefore, if you were to install a version 3.0 WiFi 6 card on a PCIe 2.0 slot, you will not see any performance drops.
In this case, the backward compatibility aspect of the PCIe interface will serve you well.
Installing a High Demanding Card on an Older Slower Slot (Bottlenecks; This is Bad)
On the flip side, if you were to install a very high demanding card that generates a lot of data on a PCIe slot conforming to an older version, then you may experience performance drops or bottlenecks.
For instance, if you were to install a high end graphics card (say NVIDIA RTX 3090) that is designed for PCIe v4.0 x16 slot on an older PCIe 3.0 or 2.0 x16 slot, then you may experience drastic performance drops.
PudgetSystems.com has a comprehensive study on this. In this study they test an RTX 3090 (V4.0 device) and a Titan RTX (v3.0 device) on PCIe 1.0, 2.0, 3.0 and 4.0 x16 slots for various tasks.
As expected, there were noticeable differences when the GPU was used on an older version.
Image: PudgetSystems.com. Comparison of how RTX 3090 and Titan RTX perform on different PCIe generations. A significant impact can be seen on performance in DaVinci Resolve (video editing software) when using the cards in older PCIe slots.
The difference may not be as pronounced if you are using a low end graphics card that does not generate enough data to saturate the older version PCIe lanes, but for mid – high end graphics card the bottleneck is very real.
Installing Newer NVMe SSDs in Older Slots
The most visible and direct affect that you can see in performance of a component is when using an older version PCIe M.2 slot for newer NVMe SSDs.
NVMe SSDs use 4 PCIe lanes.
A PCIe V4.0 NVMe SSD has nominal speeds of about 5500 MB/s. (Compare this to the 200 MB/s speeds of an average spinning hard disk drive).
If you were to use a V4.0 NVMe SSD on an older V3.0 M.2 slot, then its speeds will essentially be reduced to half (since each V3.0 lane operates at half the speed of V4.0. lanes).
Also Read: Does My Motherboard Support NVMe SSD?
PCIe is Also Forward Compatible
The PCIe interface is also forward compatible, meaning you can use an older device on newer PCIe version.
So for instance, you can use a V3.0 NVMe SSD on a V4.0 M.2 slot. Similarly, you can use a V3.0 graphics card on a V4.0 x16 slot etc.
However, note that you will NOT see any performance gains when using newer slots for older devices.
In other words, a V3.0 NVMe SSD will not perform any faster on a V4.0 M.2.
Also Read: Which Motherboards Support PCIe 4.0?
PCIe is Also Cross-Compatible: x1, x4, x8 card can work in an x16 Slots
PCIe Interface is also cross-compatible meaning smaller devices can work on larger slots. Therefore:
- x1 cards can work in x1, x4, x8 and x16 slots
- x4 cards can work in x8 ad x16 slots
- x8 cards can work in x8 and x16 slots
- x16 slots only work in x16 slots (due to the connector size being physically large).
However, a very important point to note here is that using a larger slot for a smaller expansion card will not give you any kind of performance boost.
It will be waste of resources if you were to use, say, an x16 slot for a mere x1 WiFi expansion card.
The PCIe interface IS backward compatible. This characteristic of the PCIe interface is what makes PC building so modular.
In ideal cases the version of the PCIe slot and its size should match with that of the expansion card.
However, given the backward, forward and cross compatibility aspect of the interface, you can design your PC to suit your needs and budget.