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

[Stephan Sandenbergh]

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