Re:Even a laptop can run RAM externally thanks to a little-k...
by _trampeltier on Thursday October 19, 2023 @01:01PM
(#37945448)
Long long time ago, when Ethernet used coax cables, the max. length of the segment was not because of the signal strength. It was because with the max. length of an Ethernet segment the first half of the ethernet data packet was on the cable, if now the computer on the other side started to send a packet too, a collision would not be detected, and the packet would be lost.
Electrical Engineers since at least DDR2, probably earlier, need to ensure that all lines are delay matched to about 100picoseconds.
That is, in DDR2, if DataBit#1 takes 1.1nanoseconds from start-of-wire to end-of-wire, then all other bits must be somewhere between 1.0ns to 1.2ns in length.
This requires impedance controlled pcbs from the manufacturer, and length tracking software for PCB design.
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The dielectric of FR4 (the glass/resin used to make PCBs) is what determines the speed of light of the copper actually. And it can be 3.6 or 4.2 or whatever, but the PCB manufacturer will tell you in the PCB specsheets.
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I think I heard that DDR4 is now 10picosecomds (10x more accuracy in delay matching), which is doable with modern CAD and PCB software. I dunno the delay requirements of DDR5.