Subject:
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Re: CM-RCX comm
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 31 Jan 2001 12:59:54 GMT
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Original-From:
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Steve Baker <sjbaker1@airmail.AVOIDSPAMnet>
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Reply-To:
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SJBAKER1@AIRMAIL.stopspammersNET
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Viewed:
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2197 times
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"Marco C." wrote:
>
> At 22:30 30-01-2001 -0600, Steve Baker wrote:
> > "Marco C." wrote:
> > >
> > > 2) My first tests with the two pBricks (CyberMaster and RCX) prove that
> > > it's possible to communicate using a (ten times) slower version of VLL
> > > protocol (I call it SlowVLL for short). *It works*, and it transmits data
> > > very accuratly (I'd thought it would suffer some variations in timmings but
> > > no, the Wait() value on the OUTPUT pBrick is exactly the measured Timer()
> > > value on the INPUT pBrick.
> >
> > Wooaahhh! That can't be true. Both machines are running some kind of
> > (hopefully) crystal oscillator to generate the clock that drives the
> > Timer() and Wait() commands. No two cystal oscillators generate the
> > *exact* same timing - so your two clock could easily differ by one
> > part per million or so. Hence, once in a while, you'll find that
> > you either miss a bit or drop a bit. That may be quite rarely or
> > quite frequently depending on the quality of the oscillators that
> > Lego used for the RCX.
>
> That's exactly what my common sense made me believe.
> But various tests I did, using a dump of a Datalog with the timmings
> measured by the RCX, proved me wrong.
>
> Every Wait(10) was correctly measured as a Timer(3)==1, and every Wait(50)
> as a Timer(3)==5 and so on...
Well, yes - but then crystal oscillators are quite accurate (but crucially,
not 100% accurate). You might have to run as long as one or two MILLION
attempts before you see an error - but you *will* see an error if you do
it for long enough.
If your code does one test per second (say) and the clocks differ by one
part per million (which I believe to be a reasonable figure) - then it
might take as long as 11 days for you to see a problem....or it might happen
on the very first try.
--
Steve Baker HomeEmail: <sjbaker1@airmail.net>
WorkEmail: <sjbaker@link.com>
HomePage : http://web2.airmail.net/sjbaker1
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Message is in Reply To:
 | | Re: CM-RCX comm
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| (...) That's exactly what my common sense made me believe. But various tests I did, using a dump of a Datalog with the timmings measured by the RCX, proved me wrong. Every Wait(10) was correctly measured as a Timer(3)==1, and every Wait(50) as a (...) (26 years ago, 31-Jan-01, to lugnet.robotics)
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