3.4 Planning for PCI Express
Evaluating Platforms for Performance and Reusability
How many of you own a PC with PCIe slot? What about a PCI slot?
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Advances in PC Bus Technology
Do you remember this slide from the bus technology section earlier? We promised we would come back to it, so here we are to discuss the highlighted section—PCI Express. As you can see, it has incredibly low latency and very high bandwidth. In fact, only PCI has lower latency, and nobody has higher bandwidth. This is an incredibly combination for test and measurement applications.
PCI Express
• Why do we care about PCI Express?
• Serial interconnect at 2.5Gb/s
– PCI transactions are packetized and then serialized – LVDS signaling, point-to-point, 8B/10B encoded
– x1 (by 1) gives real-world performance of 200 MB/s/direction – x16 (by 16) gives real-world performance of 3.2 GB/s/direction
• Evolutionary version of PCI
– Uses same software model
It’s FAST
The main thing to take away about PCI Express is it’s speed—it is fast!!! How does it achieve the speed?
It uses a serial interconnect at 2.5 gigabits per second. It takes the PCI transactions that previously existed and packetizes them and then serializes them using LVDS signaling in a point-to-point fashion with 8B/10B encoding.
Note: LVDS stands for low-voltage differential signaling.
Note: 8B/10B encoding is a correction method to balance digital transmissions. It has the effect of sapping 20% of the bitrate in the process, though.
The by-one configuration (with one serial lane) gives performance of 200 megabytes per second per direction. (note the difference between bits and bytes on this slide). The by-sixteen configuration (with sixteen serial lanes) gives performance of 3.2 GB per second per direction.
This is also a purely evolutionary version of PCI, meaning that all software support is 100%
backward compatible. When NI released the PCI Express GPIB board last year, the driver development team didn’t need to make ANY changes to the GPIB driver to support the new board. It looked exactly the same to the software as PCI.
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PCI and PCIe Slots on a Motherboard
Two x1 PCI Express
Slots
One x16 PCI Express
Slot Four PCI Slots
This picture represents how typical motherboards will look for years and probably decades to come. There will be PCI and PCI Express slots side-by-side in the way the PCI and ISA slots have been side-by-side for the last 10+ years. (You can still find ISA slots on motherboards today.) This also shows the size difference between the x1 slots and the x16 slots. The x16 slot in this picture replaces the AGP slot that previously existed for high-performance video applications.
ExpressCard : PCI Express for Laptops
• Laptops shipping January 2005
• 34mm and 54 mm form-factors
ExpressCard is the version of PCI Express that will replace PCMCIA or CardBus in laptop applications. Industry adoption is being driven by a $10–$15 cost reduction for Laptop manufacturers versus Cardbus. There are two main sizes represented here – the ExpressCard 54 and ExpressCard 34. As you can see, the ExpressCard/34 can plug into 54 slot.
As one example of performance, the ExpressCard 54 can have mini rotating hard-drives – which is a lot of performance in a very small package.
Laptops with ExpressCard slots first shipped January 2005
Technical note: both USB 2.0 and PCIe signaling is available on the host.
Coming Modules
• IEEE-1394 Firewire A/B
• Bluetooth
• Flash Memory
• Gigabit Ethernet
• 802.11 Wireless
• and many more to come
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What is the PCI Express Bandwidth?
Bus Bandwidth (MB/s)
PCI (32-bit, 33 MHz) 132 (shared)
x1 PCI Express 250 (per slot)
x4 PCI Express 1000 (per slot)
x16 PCI Express 4000 (per slot)
• Some report both directions versus single directions
• Example: x1 PCI Express
– 500 MB/s (Both Directions) – 250 MB/s (Single Direction)
– 200 MB/s (Single Direction, packet overhead correction)
Intel says x1 PCI Express is 500 Mbytes/s, but that is counting both directions. For Measurement Applications, 250 Mbytes/s is the number NI will be using. Then, with the overhead correction, the real-world number is right at 200 MB/s. But be aware of the different reporting in the industry.
250 MB/s * 8/10 (encoding) = 200 MB/s
Evolution of Industry Standard Buses
1 10 100 1000 10000 100000
1986 1988
1990 1992
1994 1996
1998 2000
2002 2004
2006 USB
Ethernet PC Buses
Speed (Mbits/S)
Year 10 Mbit Ethernet
100 Mbit Ethernet
Gigabit Ethernet
USB 1.1
USB 2.0 ISA
PCI 32/33
PCI Express
40X
This is a picture of how industry-standard buses have evolved in the last 20 years. PC buses have maintained the highest performance over the years, but gigabit ethernet and USB 2.0 were catching up quickly. PCI Express raises the bar immensely, with over 40X the speed of gigabit Ethernet.
Note: 40x is derived the following way: Gigabit Ethernet : 1000 MBit/s (1 Gbit/s) PCI Express x16 : 2.5 GBit/s times 16 lines = 40 Gbit/s
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PCI Express Industry Adoption
• First PCI Express desktops shipped mid 2004
• First ExpressCard laptops shipped January 2005
• PCI and PCI Express are side-by-side in all Intel/Dell roadmaps
• Primary consumer drive is graphics processing (gamers, video editing)
– PCIe x16 slot replacing AGP
As with any new bus, there will be a delayed adoption by the marketplace. PCI Express desktops began shipping mid-2004, and ExpressCard laptops started shipping in early 2005.
All of the roadmaps from Intel and Dell currently have the PCI and PCIe slots side-by-side for many years to come—this is a good thing for technology continuity and will help you preserve and continue your existing investment in PCI. Also, one of the initial drivers for PCI is the graphics processing capabilities from the higher bandwidth.
National Instruments PCI Express Roadmap
2003 2004 2005 2006
PCIe Standard
Complete First Desktops
Shipping
ExpressCard for Laptops
2007 PCIe GPIB
PCIe CameraLink
cPCIe / PXIe Specification Work
PXI 2.2 Compliant Products
PXI and PXIe Products PCI and PCIe Products PXI-8196 w/ ExpressCard
If you look specifically at NI’s roadmap related to PCI Express, here is the timeline.
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PCI Express Bandwidth Demonstration
In applications that need high bandwidth, like high speed video acquisition, PCI Express enabled more data to be streamed to the PC, and thus a more accurate representation of the real-time event.
Previously, applications like this either relied on less data to make a decision, or used very expensive, proprietary systems to meet their bandwidth needs.
Summary
• PCI and PCI Express will be offered side-by-side in Desktop PCs
• Mainstream acceptance of PCI indicates PCI availability for decades
– ISA still available today, remains popular in industrial segments – PCI market size drove software compatibility for PCI Express
• PCI Express will enable new applications for Virtual Instrumentation
• Work to integrate PCI Express and PXI is underway
– Specifications in 2005, products in 2006
– Compatibility will preserve investment, while increasing system performance
