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

[Ulrich Bangert]

Stephan,

can you give us a clue what it is all about? What happens with the data
after being decoded by the FPGA? 

Best regards
Ulrich Bangert

> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Stephan Sandenbergh
> Gesendet: Mittwoch, 19. November 2008 15:30
> An: Discussion of precise time and frequency measurement
> Betreff: [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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