Originally Posted by
John P. Myers
Alright. Since i'm back home now and fell guilty about half-assing electrical current flow data, he's the more complex answer:
Yes, an electrical signal travels much slower than the speed of light in a circuit board as Bryan said. For the RAM traces in the top PCB layer, it is generally around 60% (~50% for sub-layers), meaning the signal only travels about 7.08" in 1 ns. And just for completeness, i'll also point out that while the electrical signal travels from positive to negative in a DC circuit, the electrons themselves travel from negative to positive and move only about 0.2 mm/s.
So, in DDR4 2133MHz ECC RAM with a standard CL of 15, you have 1 clock tick every ~0.94ns (0.83ns @ 2400MHz supported by the Xeon v4 CPUs), which is enough time for the electrical signal to travel 6.656" (5.88" @ 2400MHz). Of course, the MHz speed of the DDR bus is half the stated Mhz speed, but since it can spit out data twice per clock, it's effectively the same, because the data is spit out at the start of the clock cycle, then again halfway through, not twice at the same time.