[throw82473751]

Voltages yes.. usually its all binary digital signals, running serial/parallel and following some communication protocol. Maybe you should have a look at something really simple/old like UART communication to get some idea how this works and then study next how this is scaled up over PCIE to understand the chat between CPU/GPU?

Or maybe not, one does not need all the details, so often just scaled concepts :)

https://en.m.wikipedia.org/wiki/Universal_asynchronous_recei...

Edit: Wait it is really already QAM over PCIE? Yeah then UART is a gross simplification, but maybe still a good one to start with depending on knowledge level?

[rayiner]

PCI-E isn't QAM. It's NRZ (a type of pulse amplitude modulation with just two amplitude levels), with 8/10b, 64/66b, or 128/130b encoding, and then scrambled to reduce long runs of 0s or 1s. I think PCI-E 6.0 uses PAM-4 with 4 amplitude levels.

[_3u10]

https://pcisig.com/sites/default/files/files/PCI_Express_Ele... It doesn’t say QAM explicitly but it has all the QAM terminology like 128 codes. Inter symbol interference etc. I’m not an RF guy by any stretch but it sounds like QAM to me.

This is an old spec. I think it’s like equivalent to QAM-512 for PCIe 6

[rayiner]

128/130b encoding is used to help embed a clock signal in the data stream. After every 128 bits of data, the receiver will see a preamble of 01 or 10. That voltage transition at predictable intervals helps the receiver keep the clock in sync.

Intersymbol interference likewise applies to pretty much any high-speed digital transmissions. At high frequencies, you have to worry about things like the signal reflecting off the other end of the circuit trace, which creates inter-symbol interference.

QAM is a modulation technique. It specifies how symbols of one or more bits are represented as analog waves on the wire. PCI-E does not use QAM. It simply represents 0s and 1s high and low voltage swings--what's called Pulse Amplitude Modulation. Other modulation techniques encode symbols by varying the amplitude and phase (or both) of a carrier wave. QAM works by modulating the amplitude of two carrier waves 90 degrees out of phase (e.g. sine and cosine) based on a data signal and summing them together: https://www.techtarget.com/searchnetworking/definition/QAM

[_3u10]

Oh cool. Thanks for that. So basically it uses similar terminology because it’s modulated it’s just that QAM is a different kind of modulation?

[rayiner]

It’s similar terminology because it’s dealing with sending digital signals at high speeds using analog signals. “Modulation” just means the scheme by which sequences of bits are represented in analog signals. There necessarily has to be some sort of modulation. But the terminology is more related to common issues that arise from the receiving end reading the small, rapidly changing voltages and recovering the bit stream. Regardless of the modulation you face similar problems with keeping the clock in sync etc.