[time-nuts] Fwd: M12+T ASCII interface - I'm confused?

Stephan Sandenbergh stephan at rrsg.ee.uct.ac.za
Thu Nov 20 10:16:40 EST 2008


Hi,

Now I'm slightly confused:

>
> > My gut tells me that  <Checksum><cr><lf><@><@> would be believable more than
> > say 95% (if not 99%) of the time. I've got the following observations:

In the above I assumed no data length checking is employed.

> >
> > - 95% is a bad number in accurate timing applications.
>
> You misunderstand. You can get as close to 100% as you
> want. Some of us have logged data from M12+ receivers without
> error for weeks or months -- gigabytes, error-free.

Sure, I assume you refer to the case when you check the data length as
well? I meant that the <Checksum><cr><lf><@><@> byte string could also
potentially exist in the data itself, but only in very rare cases
(from there the 95% thumb suck).

>
> > - the checksum is one of the few things that are easy to do in VHDL.
> >
> > - one would always miss the first most data stream at startup (not a
> > problem).
>
> Correct, not a problem. Note NMEA, TSIP, and any other
> GPS protocol has the same feature, right?
>
> > - the last message in the data stream will always only be decoded on the
> > next second when the new streams are coming through (a potential problem)
>
> Not true. You can process the entire message as soon as
> the checksum matches. Do you see why?
>
> If you're brave you can process the message, byte by byte,
> field by field, as it arrives in real time and use the checksum
> as a commit.

Again, the checksum could be part of the data string - so without
checking the data length you'll be waiting for the @@ (less probable
to exist in data).

>
> > Thus, I agree: The only real reliable way is to have a lookup of header
> > bytes and data length. Disappointing protocol I must add.
>

Stephan




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