edgecomberts at gmail.com
Sat Jul 12 19:02:59 EDT 2014
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.
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:
> 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
> 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>
> > Hal,
> > As much as I'd like to explain the "big picture" in list, it would make
> > 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).
> > 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
> > the USB bus, but if one was not to put any other USB devices on, nor
> > 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
> > earlier today - it was meant for an ODROID, but it will work with
> > that'll output VT100 codes). Once in an 1RU case, it looks neat, and
> > work just as well as a $500 NTP ethernet time source second hand off
> > 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
> > > controlling them)
> > >
> > > How accurately do the robots have to be synchronized?
> > >
> > >
> > > --
> > > These are my opinions. I hate spam.
> > >
> > >
> > >
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