PCI-Express Versus PCI
As I discuss in depth in my book Upgrading and Repairing PCs, 15th Anniversary Edition, PCI-Express differs in many ways from PCI. The biggest difference is in how signals are transferred. PCI, like other first-generation and second-generation PC expansion buses such as ISA, EISA, VL-Bus, parallel port, SCSI, ATA/IDE, PC Card, CardBus, and AGP, uses parallel signaling. During the first fifteen years of PC development (1981-1997), parallel signaling, which transfers signals eight bits at a time, was considerably faster than serial signaling, which transfers data one bit at a time. However, as signal speeds increase, it's harder and harder to keep parallel signals in sync with each other. To compensate for this, newer parallel signaling standards such as ATA/IDE have adopted shorter cable lengths as speeds have increased.
PCI-Express is the latest technology to switch from parallel data transfers to serial data transfers. However, unlike the painfully slow first-generation RS-232 serial port, PCI-Express runs at very high speeds. To enable system and software vendors to easily support PCI-Express, PCI-Express is backwards-compatible with PCI software drivers and controls. In some ways, PCI-Express's relationship is similar to the relationship between Serial ATA and ATA/IDE (parallel ATA). In both cases, high-speed serial signaling replaces parallel signaling, but both new standards are implemented in such a way that older software is compatible with new hardware.