Tuesday, October 16, 2012

A downloadable nanosecond

I came across a wonderful video of Grace Hopper (if you don't know who she is go read that Wikipedia article first and the come back here) explaining what a nanosecond is using a visual aid. The aid is a length of wire equal to the distance light travels in one nanosecond. That's 299.8mm (or as she puts it 11.8").

That's a handy length because it fits neatly on A4 and US Letter paper. So, here are downloadable nanoseconds that can be used to make the same point as Hopper. I've prepared both A4 and US Letter versions as PDFs.

Seeing the distance light travels in a nanosecond is interesting because it becomes clear that at the very high frequencies that computers operate at the speed of light and length of cabling become significant. This propagation delay is something that designers of very high speed circuits have to take into account. For example, a machine working 1 GHz has a clock that's ticking once every nanosecond.

Here are the two versions.

If you enjoyed this blog post, you might enjoy my travel book for people interested in science and technology: The Geek Atlas. Signed copies of The Geek Atlas are available.


Blogger Dan said...

Another way to illustrate it would be to hold up a 30cm (12inch) ruler.
That's close enough to a light-nanosecond. Maybe we just have 30cm in the UK though.....

3:47 PM  
Blogger John Graham-Cumming said...


5:53 PM  
Blogger Traf Rellik said...

Back in the early days of PC's, the maximum clock speed was about 33 Mhz because of the time it took data to go between the CPU and memory (plus latching). Special layouts allowed some chips to work at slightly higher speeds, but that speed limit wasn't really broken until they started using asynchronous memory and specialized memory controllers to "burst" the signals between them, and then CPU speeds quickly skyrocketed after that. Special considerations also required those early boards to use near-equal lengths for data signals, and slightly longer clock lines so the data signals arrived and stabilized before the data clock arrived.

11:49 PM  

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