Datasheet

Identifying Components of Motherboards 15
on the number of lanes supported, which requires a related number of wires. Therefore,
a x8 slot is longer than a x1 slot but shorter than a x16 slot. Every PCIe slot has a 22-pin
portion in common toward the rear of the motherboard, which you can see in Figure 1.8,
which orients the rear of the motherboard to the left. These 22 pins comprise mostly volt-
age and ground leads.
There are three major versions of PCIe currently specified, 1.x, 2.0, and 3.0. For these
three versions, a single lane, and hence a x1 slot, operates in each direction (or transmit and
receive from either communicating device’s perspective), at a data rate of 250MBps (almost
twice the rate of the most common PCI slot), 500MBps, and 1GBps, respectively. Combin-
ing lanes results in a linear multiplication of these rates. For example, a PCIe 1.1 x16 slot
is capable of 4GBps of throughput in each direction, 16 times the 250MBps x1 rate. As
you can see, this fairly common slot doubles the throughput of an AGP 8x slot. Later PCIe
specifications increase this data rate even more.
Up-plugging is defined in the PCIe specification as the ability to use a
higher-capability slot for a lesser adapter. In other words, you can use a
shorter (fewer-lane) card in a longer slot. For example, you can insert a x8
card into a x16 slot. The x8 card won’t completely fill the slot, but it will
work at x8 speeds if up-plugging is supported by the motherboard. Other-
wise, the specification only requires up-plugged devices to operate at the
x1 rate, something to be aware of and investigate in advance. Down-plug-
ging is possible only on open-ended slots, although not specifically allowed
in the official specification. Even if you find or make (by cutting a groove in
the end) an open-ended slot that accepts a longer card edge, the inserted
adapter cannot operate faster than the slot’s maximum rated capability
because the required physical wiring to the PCIe switch on the motherboard
is not present.
Because of its high data rate, PCIe is the current choice of gaming aficionados. Addition-
ally, technologies similar to NVIDIA’s Scalable Link Interface (SLI) allow such users to
combine preferably identical graphics adapters in neighboring PCIe x16 slots with a hard-
ware bridge to form a single virtual graphics adapter. The job of the bridge is to provide non-
chipset communication among the adapters. The bridge is not a requirement for SLI to work,
but performance suffers without it. SLI-ready motherboards allow two, three, or four PCIe
graphics adapters to pool their graphics processing units (GPUs) and memory to feed graphics
output to a single monitor attached to the adapter acting as SLI master. SLI implementation
results in increased graphics performance over single-PCIe and non-PCIe implementations.
Figure 1.8 is a photo of an SLI-ready motherboard with three PCIe x16 slots (every other
slot, starting with the top one), one PCIe x1 slot (second slot from the top), and two PCI
slots (first and third slots from the bottom). Notice the latch that secures the x16 adapters
in place. Any movement of these high-performance devices can result in temporary failure
or poor performance.
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