[time-nuts] CW12-TIM

Shane Morris edgecomberts at gmail.com
Sat Jul 12 19:04:46 EDT 2014


Said, good measure! Put it into dollars!

This helps! Let me have a think about my budget...


On Sun, Jul 13, 2014 at 9:02 AM, Shane Morris <edgecomberts at gmail.com>
wrote:

> Graham,
>
> Yeah, I suppose I'm not aspiring to the most precise of the measurements,
> I want something that'll give me a reasonable accuracy on a budget.
>
> Inside the RaspberryPi is a free running 1MHz oscillator - if I could
> train it with a 1PPS to a good degree of accuracy (say 10ns to 100ns or so)
> I'd be a happy man. My synchronisation signal across the network is 10kHz,
> so accuracies would need to reflect say twice that for sampling purposes.
> The 10kHz is a "best case" - most equipment will respond slower over
> network links, and thus not generate as much traffic. Additionally, a "no
> change in sensor reading" generates no message over the network, it'll
> intelligently trim that to reduce overhead.
>
> Jason,
>
> I like DIY. I'm sold on this idea of hacking my RPi to get it to provide
> my cluster with NTP signals, and have the 1PPS and 10MHz pop out of a PCI
> slot bracket with BNC connectors in it. I'm even going to buy an 8RU
> desktop rack to put on top of my 18RU "baby" rack to put the RPis in 1RU
> rackmount cases and the 2RU 12VDC power supply shelf in. I'm going to be
> running Plan 9 CPU server on the other RPis, and thats where the NTP data
> will terminate. I hope I learn something good from this!
>
>
> On Sun, Jul 13, 2014 at 1:53 AM, Graham Haddock <gh78731 at gmail.com> wrote:
>
>> Shane:
>>
>> The question I think that is being asked is ...
>> What does "precise" mean to you?
>> To the nearest order of magnitude, what kind of accuracy are you looking
>> for
>> on your three signals.  This defines the kind of system you will need.
>>
>> This group normally aspires to the more accurate end of the scale.
>>
>> If you are doing simple time logging of some process, then  you are
>> probably at the other end of the possible accuracy scale, and can
>> do things much more simply and cheaply.
>>
>> So ...
>>
>> 1 PPS:    +/-   1 ns?  10 ns?  100 ns?  1 us?  10 us ?
>> NTP:       +/-    10 ms? 100 ms? 1 second?
>> 10 MHz:  +/-   10E-6?   10E-9?  10E-12?  10E-14?
>>
>> --- Graham
>>
>> ==
>>
>>
>> On Sat, Jul 12, 2014 at 3:57 AM, Shane Morris <edgecomberts at gmail.com>
>> wrote:
>>
>> > Hal,
>> >
>> > As much as I'd like to explain the "big picture" in list, it would make
>> God
>> > awful noise - if you wish to know any details, I encourage you to
>> respond
>> > to me off list. Given the fact that the robotics is so totally off
>> topic,
>> > I'm not willing to discuss them here. Thats only out of respect to the
>> > topic of the list. The only real stipulations at this design part of the
>> > project is 10MHz out, 1PPS out, and NTP out. Please don't think I'm
>> being
>> > narqy, I'm really not going to pollute the list with off topic chatter.
>> I
>> > am more than happy to discuss off list, as and when.
>> >
>> > David,
>> >
>> > I was planning to use RaspberryPis in some part of the network, and of
>> > course, I must be silly, they have ethernet, and can run Real Time Linux
>> > (the LinuxCNC distros that have been made for control of CNC machines).
>> By
>> > the way, the whole network uses heterogeneous CPU types, I'm pretty
>> > agnostic to CPU type, as long as it does the job I need it to. The
>> actual
>> > ethernet interface won't be as deterministic as we'd like, being
>> chained to
>> > the USB bus, but if one was not to put any other USB devices on, nor
>> attach
>> > anything that draws power, the USB performance would be good enough for
>> > second accuracy NTP frames. This is without any real analysis of any
>> spec
>> > sheets, although I have this link:
>> >
>> > http://www.synclab.org/?tag=raspberry%20pi
>> >
>> > Thats an interesting read in and of itself. An additional link is:
>> >
>> > http://www.geekroo.com/products/795
>> >
>> > Which is a Mini ITX motherboard for RaspberryPi, which can then go
>> nicely
>> > into a 1RU case. Add LCDs and other bits and bobs as needed (I saw a
>> nice
>> > little LCD with an ATMega driver taking TTY strings in the ODROID
>> Magazine
>> > earlier today - it was meant for an ODROID, but it will work with
>> anything
>> > that'll output VT100 codes). Once in an 1RU case, it looks neat, and
>> would
>> > work just as well as a $500 NTP ethernet time source second hand off
>> eBay,
>> > if not much more configurable and hackable.
>> >
>> > Many thanks for the thoughts!
>> >
>> > Shane.
>> >
>> >
>> > On Sat, Jul 12, 2014 at 5:50 PM, Hal Murray <hmurray at megapathdsl.net>
>> > wrote:
>> >
>> > >
>> > > edgecomberts at gmail.com said:
>> > > > I am needing a GPS source of precise time, in three flavours - 10MHz
>> > (or
>> > > > so), 1PPS, and ethernet NTP. In the beginning, the NTP will be most
>> > > > important, and as time goes on, I'll need the 1PPS signal.
>> > > ...
>> > > > If a static CW12-TIM ethernet clock could be made, I would be
>> willing
>> > to
>> > > try
>> > > > my hand at mounting them to mobile robots, again, for synchronised
>> > > timing of
>> > > > events.
>> > >
>> > > I'm missing the big picture.  Are the robots the end target?  What are
>> > you
>> > > going to do before that?
>> > >
>> > > Do the robots have a network connection?  (maybe only WiFi to a local
>> PC
>> > > controlling them)
>> > >
>> > > How accurately do the robots have to be synchronized?
>> > >
>> > >
>> > > --
>> > > These are my opinions.  I hate spam.
>> > >
>> > >
>> > >
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