4 ms·
Is the PCI-E bus really so noisy?
by tomc1985 4y ago
Is the PCI-E bus really so noisy?
- speed_spread 4y agoPCI-E speed is starting to push against the noise of the universe.
- crote 4y agoAt these kinds of bandwidths, it is extremely noisy. Remember socket AM4, which added support for PCIe 4.0 with chipset X570? Originally, the idea was that non-X570 boards would also have PCIe 4.0, as it doesn't actually depend on the chipset - just the CPU. Turns out most boards simply weren't capable of handling the additional bandwidth, so it was eventually restricted to boards which were designed for it. And that's at one-eighth the bandwidth of PCIe 7.0!
- alias_neo 4y agoInteresting, I could have sworn when I bought my AsRock Rack X470 server board it was advertised as PCI-E 4.0 but their site quite clearly shows 3.0 checking now.
- WithinReason 4y agoI remember for some MBs there were BIOS updates that added 4.0 support, then later BIOS updates removed it.
- Zenst 4y agoIt gets down to the differential tracks connecting the PCIe lanes. Tollerance for PCIe3 compared to 4.0 may see tracks on a motherboard tollerant for 3.0 but fall short for 4.0. Higher the speed, the smallest difference in the length of the track routing pairs can see what works become less stable. Which would explain why some can do 4.0 and others not as well.
- jsolson 4y agoIf it were truly a bus, these signaling rates would be totally impossible. Too many reflections. In practice it's a bunch of point-to-point links, but even then the path across a PCB is pretty brutal in terms of loss. Cables are (surprisingly to me, initially!) better. Overall though, sending some number of additional bits for error correction is often cheaper than driving the line harder to reduce the bit error rate. Put differently: we could manage lower noise, but it would either result in a lower bitrate or (much) higher power. Instead, pick a target goodput and design for power and error correction from both sides.
- chx 4y ago> Cables are (surprisingly to me, initially!) better. That's because the cables in these application are not single wires but complex micro-coaxial assemblies. (The industry best is 3M Twin Axial.) And that will have much more shielding than an inherently unshielded PCB trace. With an appropriate price, of course.
- jl6 4y agoCould you shield PCB traces?
- Lramseyer 4y agoYou can with enough money. Usually you see ground traces and ground planes separate high speed data buses. The problem is that the more layers you have, the more expensive it is.
- _ugfj 4y agoThat's still nowhere near https://www.i-pex.com/sites/default/files/inline-images/2_E.png https://www.i-pex.com/sites/default/files/inline-images/2_E.... this. And they come in less than 1/100 of an inch(!) thickness, look at the OD table https://www.i-pex.com/products/micro-coax-discrete-wire https://www.i-pex.com/products/micro-coax-discrete-wire. It's total bonkers. An 50cm PCIe 4.0 x16 riser from 3M sells for 160 USD retail though...
- Dylan16807 4y agoIt says "GHz" so yes it's extremely noisy. At this point the bits are millimeters long.