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I've got a timing card which emits a pulse per second (PPS). The specs for the card state there is a delay of this PPS of

approximately ~7ns for each meter of the cable, or ~200ns per 30 meter cable.

What I don't understand is why this delay is dependent on cable length. Shouldn't the speed of the signal through the cable be constant (assuming constant cable properties throughout) and thus no delay based on length? Where does the root cause of the delay come from?

If this is the incorrect stack exchange for this question, please let me know.

closed as off-topic by Sven Mar 24 '17 at 16:41

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on Server Fault must be about managing information technology systems in a business environment. Home and end-user computing questions may be asked on Super User, and questions about development, testing and development tools may be asked on Stack Overflow." – Sven
If this question can be reworded to fit the rules in the help center, please edit the question.

  • 1
    Because signals in a cable propagate at a finite speed, lower than the speed of light in void (roughly 300,000km/s, which is 3.3ns/m). – user2233709 Mar 24 '17 at 16:36
  • @HåkanLindqvist: Physics Stack Exchange is for active researchers, academics and students of physics and astronomy. I don't think that question is a good fit there. – Sven Mar 24 '17 at 16:42
  • @Sven Ok, then I suppose not so much. – Håkan Lindqvist Mar 24 '17 at 16:45
  • The speed of your signal in it's medium is 142,857km/sec (~ half the speed of light). the root cause is probably topical for physics - after all they have a question about how to weight your own head. – user9517 Mar 24 '17 at 16:48
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The question is rather OT, anyway the answer should be obvious: electric signals take time to travel through a conductor. This is usually not noticeable because we're talking about very high speeds, but it can become relevant for time-sensitive applications and long distances. This is one of the reasons why network latency increases with distance (the other one is how many hops are involved).

Think about the speed of light: it's nearly instantaneous for almost all intents and purposes, but it becomes relevant if you need a ray of light to travel across very long distances, and/or if you care enough about timing that a few (pico/nano/micro)seconds can make a difference. In fact, the same delays exists when using fiber-optic cables.

  • Okay, that makes perfect sense and seems quite obvious now. I was interpreting the meaning of the delay differently. I can remove the question since it's off topic though. – zephyr Mar 24 '17 at 16:39

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