On 2012-01-11, David J Taylor <[email protected]> wrote: > "unruh" <[email protected]> wrote in message > news:[email protected]... > [] >> For timing it is only the rise time that is important. The length of the >> pulse is irrelevant, unless your Hardware/software requires a given >> length pulse in order to recognize and interrupt ( and if it did , it >> would be broken since it had better interrupt on the rising edge if it >> is to be useful to timing. ) >> Certainly if you are using TTl-RS232 converters their times are >> important. However most rs232 cards will handle ttl signals fine anyway. > > Distinguish, though, between internal-level signals where a 100 ns pulse > may be quite enough, and signals sent down cables where the pulse length > needs to be sufficient to allow the signal to be recognised at the other > end. I've seen RS-232 inputs with quite strong filtering, and taking > 115,200 baud as a typical maximum for older units, I would not want a > pulse less than 10 microseconds long. First guess estimate.
If rise time is preserved, the pulse is preserved. A 10us pulse will begin to be wiped out once the rise time gets longer than 10us. If the rs232 really filters so strongly that a 10us pulse gets lost, then the timing will never be better than about 10us either. I suspect that the DCD interrupt signal actually gets treated better than that. > > How well the rise-time needs to be preserved depends on the precision > required - I would take more care where sub-microsecond precision was > needed. > > The 100 ms pulse from the GPS 18/x and the Sure board is nice because you > can put an LED (or scope) on that line and see the pulse, giving visual > confirmation of PPS data. The sure board alreae has a very very bright blue led in sync with the PPS. Yes, it is nice for confirmation > > Cheers, > David > _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
