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

[Stephan Sandenbergh]

Hi,

NMEA would have been nice - but this is a timing application.

Stephan.

2008/11/19 Stanley Reynolds <stanley_reynolds at yahoo.com>

> Looking at the user guide which is the extent of my experience. Would the
> position version of the M12+ be a better choice for your application ? That
> is trade NMEA-0183 for all the timing features ?
>
> Stanley
>
> Link to user guide :
> http://www.synergy-gps.com/images/stories/guides/m12+userguide.pdf
>
>
>
>
>
>
> ________________________________
> From: Stephan Sandenbergh <stephan at rrsg.ee.uct.ac.za>
> To: Discussion of precise time and frequency measurement <
> time-nuts at febo.com>
> Sent: Wednesday, November 19, 2008 8:29:59 AM
> Subject: [time-nuts] M12+T ASCII interface - I'm confused?
>
> Hi All,
>
> Up until now we've been interfacing my Motorola M12+T's using the Oncore
> software. However, at this point we are trying to have it interfaced
> directly to a FPGA. To my mind this should be simple - the commands are
> discriminated (framed) by looking at the start and terminating bytes
> sequences when they enter the FIFO, check summed, decoded etc.
>
> However, I noted something very peculiar about the motorola ASCII protocol:
> The start bytes @@ and he terminating byte <CR><LF> aren't unique with
> respect to the data bytes. For instance one could receive a time of 13hrs
> and 10mins which would look identical to the terminating characters.
> Initially I thought it made sense since the data is also sent in ascii
> format. It appears not to be the case.
>
> It seems to me that the only way in which a command could be robustly
> identified and check summed is when the interface knows the length of the
> expected return.  Obviously, the data lengths are dependent on both the
> actual command and the specific request. This type of intelligence is
> cumbersome to implement in FPGAs.
>
> It would be of great help if you could point me in the right direction
> here.
> I feel rather stupid in asking such a simple question, but at this point I
> can't seem to see the light. I'm flabbergasted...
>
> Best regards,
>
> Stephan
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