3 ms·
If 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 the
by jsolson 4y ago
If 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...
- Arrath 4y agoSo ribbon cables might make a return, huh.
- jsolson 4y agoIn some settings they already have -- the 3M twinax cable the GP mentions is sold in a multi-pair format where, rather than a crimp-style connector, the ends of the cable are soldered onto PCBs that are part of connector assemblies (from, for example, TE's Sliver line: https://www.te.com/usa-en/products/connectors/pcb-connectors/intersection/sliver-connectors.html?tab=pgp-story https://www.te.com/usa-en/products/connectors/pcb-connectors...). Note that on balance I'll take something like an OSFP assembly (https://www.te.com/usa-en/products/connectors/pluggable-connectors-cages/qsfp-dd.html?tab=pgp-story https://www.te.com/usa-en/products/connectors/pluggable-conn...) with a fancy jacket that pulls all of the individual twinax cables together, but it's also substantially more expensive than a simple flat tape-wrapped ribbon. Other places where you'll see coax cables ganged together inside a jacket: high-speed USB cables. For example, here's a type C cable cross-section: https://twitter.com/tubetimeus/status/1125926941469462528 https://twitter.com/tubetimeus/status/1125926941469462528