The trends in the development of computer technology are obvious - this is unification, simplification for manufacturers (lower overhead costs) and the combination of many devices in a single package. It is not surprising that many modern novice users do not even know what a PCI bus is. Those who have found the times of the formation of IBM-compatible computers based on 286, 386 and other processors undoubtedly remember the abundance of various buses and hardware interfaces used in computer systems of that time. This is the famous 8-bit ISA bus, then its revision to VLB (known as VESA), the PCI bus, a specialized Intel connector for soft modems, AGP, etc. Moreover, all of them, with rare exceptions, were needed. But at the present time on the motherboards there is a striking “poverty” - only the PCI Express bus, however, of different revisions and with a different number of signal lines. Alas, literally, the end user has to pay for such unification. After all, even if there is, for example, a high-quality sound card, which is a PCI device, it simply has nowhere to connect (inconvenient adapters do not count) and you have to buy a version for another bus or, generally, switch to, frankly, flawed, embedded solution. The situation is reminiscent of the story with the "forcible" transfer of all from AGP to PCI-E.
What is so famous for the PCI bus? Its history began back in 1991, when with the advent of Pentium-class processors it became clear that the widespread 8-bit ISA can no longer provide an acceptable speed for all components to communicate with each other. Recall that at that time there was neither DMI nor Hyper Transport, and the components were connected on the motherboard via the system bus. Although attempts were made to circumvent ISA restrictions, for example, the VESA bus appeared, but because of the restrictions, they did not take root.
In 1992, Intel introduced the PCI bus version 1.0 based on the open standard. Its clock frequency was 33 MHz huge at that time (which simplified the scaling of the processor frequency), depending on the hardware implementation, 32 or 64 bits were supported (compare with 8 bits of ISA). The voltage of the data lines was 5 and 3.3 V, and the throughput was 133 MB. And, most importantly, Plug & Play support has been implemented (down with the jumper!).
In 1993, an improved version 2.0 appeared.
The PCI bus has gained worldwide fame since 1995 (version 2.1). Depending on the implementation, the frequency was 66 MHz. When using the 64-bit version, 533 MB were transferred through this bus. The time was ripe for Windows 95 to finally fully ensure the correct operation of PCI devices with Plug & Play technology. Signal lines allowed switching from 5V to 3.3V.
Then revisions 2.2 - 3.0 were issued. The connector in version 2.2 had a “key” that blocks the installation of voltage-incompatible devices. In 2.3, the transition to 3.3 V continues, so the use of such devices in versions up to 2.1 (5 V) resulted in damage to both the card and, sometimes, the bus. In version 3.0, 5 V was no longer used.
In 1997, PCI had to make room, since Intel also released the AGP bus for video cards that could not fully reach their potential on PCI.
Now the PCI bus, as mentioned earlier, is practically not used, giving way to its successor - PCI Express. The software addressing mechanism has remained the same, but the physical implementation has undergone significant changes. The number of conductors was changed, the operating frequency was increased (along with a decrease in current). The method of configuring transaction initiators (connected devices) has also changed, allowing more convenient to process their requests to the bus.
By the way, on some motherboards you can still find a PCI bus connector. However, we are not talking about native support from the chipset and processor - in this implementation method, a special bridge chip is used that redirects PCI requests to PCI Express.