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I was wondering, what is the absolutely fastest way (lowest latency) to produce external signal (for example CMOS state change from 0 to 1 on electrical wire connected to other device etc.) from PC, counting from the moment, where CPU assembler program knows that signal must be produced.

I know that network device, usb, VGA monitor output have some large latency comapred to other interfaces (SATA, PCI-E). Wich of interfaces or what hardware modification can provide a near-0 latency in output from let's suppose assembler program?

Piotr Müller
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I don't know if it is really the fastest interface you can provide, because that also depends on your definition of "external", but http://en.wikipedia.org/wiki/InfiniBand certainly comes close to what your question aims at. Latency is 200 nanoseconds and below in certain scenarios ...

noamik
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    Under realistic conditions, it's more like 1 microsecond from CPU to external wire. That's still pretty darn good though. – David Schwartz May 28 '13 at 08:16
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    That's why I provided the link so he can read up for himself :) And yes, it all depends on the scenario. Depending on his definition of "external" one could also have named HyperTransport, which would be even faster, but to me that wouldn't really fit the question. – noamik May 28 '13 at 08:23
  • I mean just "external" in any way, even a electrical wire soldered to some CPU register output pins :) I suppose that this question will show some realy interesting answers, but nevermind, it's already buried. – Piotr Müller May 28 '13 at 09:22
  • Well, as I said in my last comment: if you are open to some special definition of external, HyperTransport would certainly be a good candidate. And HyperTransport is found in many modern PCs since it's used by AMD as replacement for the FSB ... – noamik May 28 '13 at 09:24