THE PCI EXPRESS SLOT

pci and pcie express slotsThe PCI Express slot was developed by Intel as they tried to consolidate desktop, mobile and server requirements into a single architecture. The result was the new PCI Express slot. PCI Express is serial, which is broken up into a series of lanes.

The Intel D915PGN was one of the first motherboards to feature the new PCI Express x16 slot. We were immediately able to acquire a video card for the slot and over time new cards became more widely available.

The first generation PCI Express x16 slot offers double the bandwidth of the old AGP 8x slot.

Some of the improvements include lower pin counts and a smaller physical layout. The performance scaling was also redesigned to allow for forward and backwards compatibility. PCI Express even features hot plug for servers.

PCI Express lanes are bidirectional, which is needed for high speed communication. This make it possible to interconnected all manner of devices to the CPU or northbridge processor. Unlike PCI, PCI Express has no common shared bus so no devices can limit system performance.

Low speed devices such as a PCI Express Wi-Fi card can use a single lane while video cards can use 16 lanes. PCI Express is designed for wide range of bandwidth needs.

PCI Express uses a crossbar type switch similar to the ones used with the old telephone system. This is why the northbridge needs a heatsink to cope with the heat from switching channels in rapid succession. PCI slots can be handled with a separate channel which is then attached to the crossbar. Other logic on the motherboard is handled the same way.

 

PCIe Version Line Code Transfer Rate x1 Bandwidth x4 x8 x16
1.0 8b/10b 2.5 GT/s 250 MB/s 1 GB/s 2 GB/s 4 GB/s
2.0 8b/10b 5 GT/s 500 MB/s 2 GB/s 4 GB/s 8 GB/s
3.0 128b/130b 8 GT/s 984.6 MB/s 3.938 GB/s 7.877 GB/s 15.754 GB/s
4.0 128b/130b 16 GT/s 1.969 GB/s 7.877 GB/s 15.754 GB/s 31.508 GB/s

PCI EXPRESS 4.0

PCI Express has been scaled up to revision 4.0 which is 4x faster lanes. Realistically PCI Express 2.0 slots are fast enough for gaming into 2017. DDR3 bandwidth is the main limiting factor. AMD and recent Intel processors use integrated memory controllers to improve bandwidth. Integrated graphics is increasingly common to support onboard video for lower cost machines.

PCI Express 4.0 is also being designed to increase the power at the slot to 300 watts or more. The goal is to eliminate auxiliary PCI Express cables. Motherboards with dual EPS12V connectors are already known from some extreme Z170 systems,  Even with the minimum 300 watts, there is more than enough power provided by the edge connector to run a GeForce GTX 1080 (reference design 180 watts) and Titan X (reference design 250 watts) without an external power source.

RADEON FURY X

The AMD Radeon Fury X uses high bandwidth memory to improve the overall performance. This chart we found on a forum thread shows the relative performance of the Radeon Fury X which is a very high-end video card. Even when inserted into a PCI Express 1.1 x4 slot, the Fury X does not suffer much at all.

Radeon Fury X PCI Express performance comparison

The higher bandwidth of PCI Express 3.0 will benefit SSD devices which are placing higher bandwidth demands on systems. M.2 slots can exceed 2GB per second.