Comment on iFixit hails replaceable LPCAMM2 laptop memory as a 'big deal'

• itsmect@monero.town ⁨1⁩ ⁨week⁩ ago

  The signal does not care about how it gets from the sender to the receiver. The only thing that matters is that at the receivers end 0s and 1s can be separated. One common measurement is the eye pattern. If the eye is “open” enough (=matches the spec), communication is possible.

  Impedance mismatch causes reflections (visible as oscillation after rising/falling edge), differential pair line mismatch degrades the slop of the signal transition (rising/falling edge). Geometric features only matter if they are large compared to the signal wavelength. As a rule of thumb features smaller then 1/20th can be safely ignored, often times a ratio as large as 1/5 works just fine. USB3 uses 2.5Ghz (5Gbit/s) or 5Ghz (10Gbit/s), where 1/20th result in 3.4mm and 1.7mm respectively (assuming an effective dialectic of 3.17). This is still grossly simplified, because in many real systems you don’t control the entire transmission line (eg. user buys a random cable and expects it to work), so it makes sense that the USB consortium specifies eye patterns and factors in various system uncertainties.

  RAM on the other hand uses 16/32/64/128 single ended data lines, with a dedicated clock line. Data does not have to arrive perfectly at the same time, but the margin may be as little as 1/10th of a clock cycle. Here accurate length matching is absolutely required. Its also the reason why the same CPU + RAM combination may archive higher stable clock rates on some mainboards then on others.

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  • gregorum@lemm.ee ⁨1⁩ ⁨week⁩ ago

    Ok, wow. Thank you for educating me on a great deal I didn’t know when I asked the question. And while it does a great deal to bridge that gap… the question remains unanswered: how is this breakthrough achieved?

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    • itsmect@monero.town ⁨1⁩ ⁨week⁩ ago

      Which breakthrough do you mean? Can you rephrase your question?

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