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

[Brooke Clarke]

Hi Stephan:

Check the manual for "stuffing", it's not a Thanksgiving term here but has to 
do with what happens when a byte in a binary data stream (the data in the 
Motorola protocol is all binary, not ASCII) is the same as the ASCII "@" 
character.  This means that the length of a packet is variable and depends on 
the actual data inside the packet.  But you know the minimum length if you know 
the packet ID.

To start look for the string <CR><LF>@@ and the next characters (maybe 2 or 4 
hex characters) are the packet ID.  Sometimes this happens inside a packet 
because that's what the data looks like.  But go with it and if it's wrong the 
program will cycle around to where you are on packet boundaries.


Have Fun,

Brooke Clarke
http://www.prc68.com

Stephan Sandenbergh wrote:
> 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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