[OT] Splitting PPS?

Hal Murray hmurray at megapathdsl.net
Thu Nov 14 07:55:30 UTC 2019

> It'd be nice if we could also replace the dead GPS master clock with a
> directly connected GPS receiver, so we get PPS into the server (e.g. via a
> serial port). That would make them stratum 1. I recently did this on my own
> with a ublox 6 evaluation kit that I picked up for cheap on eBay. For this
> project, we could buy an equivalent current generation EVK-M8T for $250 from
> e.g. Digi-Key. 

All of Gary's responses are reasonable, but you need to consider the tradeoffs 
between a home brew setup that takes some TLC and purchasing something off the 
shelf that will probably just work.

The signals going in/out of an eBay GPS board are probably logic levels rather 
than RS-232.  The serial signals (Rx, Tx) are probably upside down so they 
come out right when you run them through the typical RS-232 level converter 
chip which has an inverter in both directions.

Many of the eBay RS-232 level converter boards only have 2 sections, one in 
each direction for serial data, nothing for PPS.  Check the fine print and/or 
get a second board for PPS and only use half of it.

The PPS signal is probably not inverted so it will be inverted if you run it 
through a typical level converter chip.  Using the other edge needs a flag on 
a fudge line in ntp.conf.  (gpsd is smart enough to find the right edge, but 
that's another package to depend on)

If you don't like building RS-232 cables, get a commercial cable and cut off 
one end.  If you find a cable with the right connectors, you can use both ends 
after the cut.

Consider running extra cables to the roof rather than using a splitter.  You 
can try/debug a splitter so you have it on hand for when an antenna dies.

Think about lightning.

How far is the cable run to the roof?  Best cable at reasonable price is 
generally RG-6, cable TV stuff, quad shield.  It's 75 ohms rather than 50, but 
loss due to mismatched impedance is small relative to the cable loss if you go 
very far.  (I don't have the cross-over distance handy.)  You can get fire 
grade.  Most low cost RG-58 is not good.

Consider getting different models/brands of GPS receivers so you don't have 
all your eggs in one basket when a software quirk strikes.

If I was doing something like that, I would get a box to hold all the parts 
for one receiver -- just mechanical protection.  Cardboard would be OK.

I would also write down a description of the whole setup, being sure to 
mention all the details and considerations, maybe a pointer to this thread.  I 
would also give a tour for all the people who might be working on it after I'm 
gone or when I'm out of town and off the net.  The tour would cover both the 
hardware and the documentation.  (then update the documentation to cover all 
their questions)

These are my opinions.  I hate spam.

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