[time-nuts] FASTRAX GPS

Hal Murray hmurray at megapathdsl.net
Wed Jul 2 04:04:37 EDT 2014

> To you and Hal who suggested it, is this unit suitable for outputing a1pps
> timing signal?  Wouldn't the long serial option Hal suggestion mess that
> up, vs. using this method to put the Fastax as close as possible to a
> system which which would have the systems gpio and serial ports attached? 

Mumble.  It depends.  How far do you want to go?  What sort of cables are you 
going to use?  How time-nutty are you feeling today?

Take this with a grain of salt (or 2):

RS-232 drivers are current limited.  The cables have a capacitance per foot.  
For reasonable length cables, a lumped circuit rather than a transmission 
line is a reasonable assumption.  That means the signal is roughly a 
trapezoid with a tradeoff between cable length and bit rate.  You can get 
special low-capacitance cables that allow you to go farther and/or faster.

If you google for >RS232 cable length< you will get all sorts of opinions.  
The environment matters.  Are you wrapping the cable around an arc welder?  
Are the grounds the same?  Ballpark is 100 feet at 9600, 10 ft at 115K.

If you want to go farther or faster, it's probably wise to shift to 
differential signaling.  There are various options.  RS-422 is a good place 
to start.  LVDS is also popular.  You can also use ECL/PECL.

Note that some/many low cost GPS units don't have real RS-232 drivers.  YMMV 
and such.

As a first cut, you can just run the PPS signal through the same set of level 
converters.  The slow rise time of the trapezoid means that voltage noise 
turns into time noise/jitter.  It would be interesting to see if a PC running 
NTP could measure that.

The next step is to use a real driver and terminate the far end.  There are 
lots of choices.  RG-6 is widely used by the cable TV industry.  It's easily 
available, low cost, and works well.  Cat-5 with RS-422 or LVDS drivers and 
similar receivers is another choice.

If you have a scope, a bit of experimentation might be interesting.

These are my opinions.  I hate spam.

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