Expansion cards may seem an old concept, but Darien Graham-Smith finds out how PCI Express remains on the cutting edge of interconnect technology

I n a world filled with laptops, smartphones and tablets, the very idea of PCI Express (now commonly shortened to PCIe) might seem outdated. After all, the standard was originally laid down way back in 2003, four years before iPhones – a world where big-box desktop PCs ran Windows XP, and jumbo-sized expansion slots could be used to install anything from a sound card to a modem.

Today, only a minority of us are still using those classic full-sized tower PCs. And even if your motherboard does have PCIe expansion slots, you may never have to use them: most mainstream CPUs include their own built-in graphics processors, so you can get by quite happily without even needing a graphics card, and everything else you need for daily computing is nowadays built into the motherboard.

Yet PCIe as a technology shows no signs of dying. More than 20 years on, it keeps on evolving through subsequent generations, becoming ever faster and more efficient. So why is this technology still so popular, and what’s the secret to its extraordinary longevity?

What is PCI Express?

Simply put, PCI Express is a data bus that connects some sort of outboard device or resource to the motherboard, allowing it to communicate bidirectionally with the CPU and system memory. The technology is developed and maintained by a cross-industry group called PCI-SIG, which includes representatives from AMD, Arm, Dell EMC, IBM, Intel, Nvidia and Qualcomm. PCI-SIG standards specify how signals are exchanged electronically over a PCIe bus, and also define how the connection is physically implemented, in the form of card slots or other connectors – ensuring that any compliant PCIe card will work in any compliant slot, regardless of the manufacturer or card type.

There’s nothing particularly revolutionary about this. The idea of expansion cards dates back at least to the 1980s, when connectors such as ISA and MCA were widely used to add capabilities to basic motherboards. The PCI-SIG itself has been going since 1992, when it created the first PCI card standard.

Longer slots have more pins, so they can carry more “lanes” of data and provide a faster connection