[time-nuts] TEC party file format?
Tom Van Baak
tvb at LeapSecond.com
Tue Jun 28 18:31:18 UTC 2011
> I'm planning on counting 60Hz line cycles with some embedded
> hardware, then dumping the count over RS-232 every minute or so to a
> linux box running ntp. Any thoughts on what data to log?
You have a PC and RS232? Skip the embedded hardware.
An easy trick is to convert the 60 Hz sinewave into a narrow
pulse and send it to -- a serial port. Someone else on the list
can propose the minimum circuit for this (one 555 would do
it, but there is probably a simpler way).
If you make the pulse about 500 microseconds wide and
configure the serial port for 9600 baud, then each pulse will
be magically interpreted as a start bit (and some arbitrary
data bits). Each 60 Hz cycle becomes one 500 us pulse
becomes one RS232 binary byte becomes one read().
Note if the pulse is too short (<100 us at 9600 baud) the
port may miss it. If the pulse is too long (>900 us) the byte
may have a framing error. But anything in between is fine.
Thus simply reading the serial port and counting the bytes,
or hi-res timing the arrival of each byte, will give you the
raw measurement that you need. From that you calculate
things like period, frequency, accumulated time error,
average frequency, etc.
So the PC itself becomes your frequency counter, with NTP
providing the long-term timebase stability you need.
Mains cycles-per-second become RS232 bytes-per-second.
You will get, on average, 60 bytes per second. At the end of
a "perfect" day you would have read 5184000 characters.
In Europe, it's 4320000 (50 Hz * 86400).
The file format I use is mjd,te. mjd = modified julian date, and
te = time error (the cumulative phase error, in seconds).
For detailed study I log data once a cycle or once a second;
for long-term monitoring and pretty plots, once a minute is
more than enough.
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