PCI vs PCIe: Slot Shape
What do PCI and PCIe look like? In general, both PCI and PCIe slots have many physical shapes, depending on how many pins (wires) they use. In general, the more the pins, the faster the speed.
As for PCI, it has two types: 32-bit slot with 124 pins and 64-bit slot with 188 pins. The 32-bit slot is usually used in desktop computers while the 64-bit slot is generally used on servers. The two types of PCI slots are shown as the following picture:
As for PCIe, it has up to 7 physical shapes: x1, x2, x4, x8, x12, x16, and x32, corresponding to 1/2/4/8/12/16/32 lanes, respectively. But only PCI-E x1 (36 pins, 25mm long), PCI-E x4 (64 pins, 39 mm long), PCI-E x8 (98 pins, 56mm long), and PCI-E x16 (164 pins, 89mm long) are commonly used on personal motherboards.
PCI-E x2 is mainly used for internal interfaces rather than expansion slots. Even if it appears on motherboards, it comes in the form of M.2 interface instead of PCI-E slot.
As for PCI-E x12 and PCI-E x32, the former is mainly used in servers and will not appear on consumer platforms, while the latter is only used in some special occasions due to large size and there is almost no the corresponding mass-production product.
The PCI-E x1, x4, x8, and x16 PCIe slots are shown as below:
PCI vs PCIe: Speed
Sometimes, the speed of the interface can decide the usage of the interface. So, before comparing their usage, we need to know the speed of PCI and PCIe.
In general, the 32-bit PCI with the clock frequency @ 33MHz has the max data transfer rate up to 132MB/s. The 64-bit PCI @ 66MHz has the max data transfer rate up to 264MB/s.
Tip: PCI-X, the improved PCI system, can reach a maximum of clock frequency 133MHz (64bit), so that the data transfer rate can be more than 1GB/s.
As for PCIe, its speed varies depending on the number of lanes and the version number. At present, the PCIe version has been updated to 5.0 and the lanes can be up to 32. In Wikipedia, almost all PCIe speeds are listed, except for x12 and x32.
Nowadays, most computers use the PCIe 3.0 version. As you can see, PCIe is much faster than PCI.
PCI vs PCIe: Usage
In general, PCI is mainly responsible for connecting devices like network card, audio card, and SCSI card. It also can be used to connect graphics card, but the speed is very slow. So, it was replaced by AGP (Accelerated Graphics Port).
As for PCIe, it has more usages. It can be used to connect GPU, PCIe SSD, and other devices like SATA hard drives, USB devices, network card, audio card, etc.
Why PCIe can be used to connect so many devices? If you want to know more details about how PCI and PCIe work in a computer, please read PCI vs PCIe: What’s the Difference and How to Distinguish Them?
In general, the PCI-E x8 and x16 slots are used to connect graphics cards; the PCI-E x4 slot is used for installing PCI-E SSDs or M.2 SSDs; the PCI-E x1 slot is used to connect devices including independent network cards, independent sound cards, USB 3.0 / 3.1 expansion cards, etc.
PCI vs PCIe: Compatibility
As we all know, PCIe replaces PCI. Then, is PCIe backward compatible PCI? Actually, the PCI-E interface and the PCI interface are not compatible with each other, because they have different physical shapes.
Then, how about the compatibility between 32-bit and 64-bit PCI interfaces? Actually, the 32-bit PCI slot is not compatible with 64-bit PCI plug, while the 64-bit PCI slot is compatible with 32-bit PCI plug.
Then, how about the compatibility among different PCIe interfaces? This will be much complicated. In general, if the slot and the plug have the same number of lanes, they are compatible with each other, even if they have different PCIe versions.
Why? Because when they have the same number lanes, it means that they have the same length and then the plug can be inserted into the slot.
Then, what if the slot and the plug have different lanes? In general, if the slot is longer than the plug, the plug can be also inserted into the slot (compatible). But if the slot is shorter than the plug, the plug can’t be inserted into the slot (incompatible).