Wow I did not appreciate that modern interconnect signals are no longer strictly 0/1 digital. Apparently the new 400 Gb ethernet standard also uses PAM4. Is there some underlying shift in process technology that is pushing things in this direction?
>Apparently the new 400 Gb ethernet standard also uses PAM4
I am hoping economy of scale will drive the cost of PAM4 related technology. ( At leats tiny part of these standards ) That is both 200Gb and 400Gb Ethernet as well as 800Gb Ethernet in the future.
I cant wait to see Netflix pushing 800Gbps per box.
How does this stay compatible with PCIe generation 5 and earlier while the encoding changes from interleaved digital (128b/130b from 3.0 to 5.0) in to PAM?
When two PCIe devices are connected to each other, they don't instantly start talking PCIe, they start in a complex autonegotiation mode [0], where they find out the width of the link, the maximum protocol version that each side support, and the maximum speed that the connection between them can support.
[0]: PCIe LTSSM. I sadly cannot link the spec here because it requires registration but have the diagram someone kindly ripped for SO: https://i.stack.imgur.com/QYyCM.png
Presumably by falling back to the lowest version supported by both sides, with negotiation done in 1.0 encoding or so. It's a standard technique. Also note that PCIe is point-to-point so no downgrading for devices just idling on the same bus.
The same way all the other generations stayed compatible; the encoding already changed from 8b10b to 128b/130b in the past. There's a negotiation protocol involved (LTSSM).
