PCI Express uses point-to-point serial connection. Compared with the shared parallel architecture of PCI and earlier computer buses, each device has its own dedicated connection, does not need to request bandwidth from the entire bus, and can increase the data transmission rate to a very high frequency, reaching a high bandwidth that PCI cannot provide.
Video capture cards and graphics cards use this interface. The following introduces the characteristics of the PCI Express interface.
1. Point-to-point connection mode
Compared with the traditional PCI bus, PCI Express has a fundamental innovation in its working method - using a point-to-point bus connection method. We know that the traditional PCI bus works in an exclusive bandwidth mode. Only one device can communicate on the PCI bus at any time. Once the number of devices on the PCI bus increases, the problem of bus control contention will seriously restrict the performance of PCI devices.
The PCI Express bus uses a point-to-point connection method. When each device requires data transmission, it establishes its own transmission channel. This channel is closed to other devices. The channels do not interfere with each other, so the efficiency of data transmission is greatly improved.
2. Serial transmission mode
PCI Express uses serial data transmission, using "voltage differential signal transmission", that is, two lines, with the voltage difference between them as the representation of logic "0" and "1". Every two lines form a lane, and the theoretical transmission rate of each lane is 2.5Gbit/s. In practice, there can be two transmission lanes, divided into upstream and downstream, so that PCI Express can work in duplex state and provide higher transmission rate and quality.
3. High-speed transmission
PCI Express is divided into x1, x2, x4, x8, x16 and x32. In terms of shape, x1 is the shortest, and the higher it goes, the longer it is. They are backward compatible. PCI Express x16 single channel has a bandwidth of 5GB/s (2.5Gb×16/8=5GB/s), but due to the use of 8b/10b encoding, the actual effective bandwidth is 4GB/s (minus 20% of the implanted clock signal). Similarly, the "PCI Express x1" single channel that replaces the current PCI slot should have a bandwidth of 250MB/s, while the bandwidth of the traditional PCI bus is 133MB/s. The high bandwidth of PCI-Express can achieve higher data throughput.
4. Hot-swap support (must be supported by the motherboard)
The data transmission distance of the PCI Express bus is up to 3m, so that each hardware subsystem can be completely separated from each other in space and connected only by cables. It supports hot-swap function and can monitor all access devices in real time, so that hardware manufacturers can design components that meet the modular requirements in shape and size. When users need to expand and upgrade hardware, they only need to unplug the old one and plug in the new one without shutting down.
5. Good compatibility
In addition, the PCI Express bus is also compatible with the PCI specification at the software level, and can be used without updating the operating system and BIOS. In the future, motherboards using the PCI Express bus can still support PCI slots, and expansion cards of various PCI interfaces can operate normally in low-bandwidth mode. This provides a basis for the rapid popularization of PCI Express without the embarrassment of waiting for software.
Because PCI-E is compatible with PCI technology and devices at the software level, it supports the initialization of PCI devices and memory modules, which means that the previous drivers and operating systems do not need to be rebuilt to support PCI-E devices. PCI-E has become the mainstream interface for video capture cards and graphics cards.
