[ntp:questions] Garmin GPS 18LVC Setup but questions on best way

Unruh unruh-spam at physics.ubc.ca
Tue Dec 30 21:45:13 UTC 2008


George R. Kasica <georgek at netwrx1.com> writes:

>On Tue, 30 Dec 2008 17:14:52 GMT, Unruh <unruh-spam at physics.ubc.ca>
>wrote:

>>"David J Taylor" <david-taylor at blueyonder.neither-this-part.nor-this-bit.co.uk> writes:
>>
>>>Richard B. Gilbert wrote:
>>>[]
>>>> I think your best help/advice will come from another GPS18LVC user.
>>
>>>I described my own simple setup here:
>>
>>>  http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm
>>
>>>but it's not Linux, and I don't feel competent enough to give "advice". 
>>>It seems likely that the wrong edge is being detected, so why not try 
>>>reversing the polarity?  I only use the:  127.127.20.1  reference clock, 
>>>with GPS configured in the kernel.
>>
>>
>>The other problem is that the the voltage levels might be wrong. A serial
>>port is supposed to be -12V to 12V, not the 0-5V that the PPS line on the garmin
>>delivers.   Now some serial ports are OK with the 0-5V but that is pretty
>>unusual. YOu either need a voltage converter, or you need to use the
>>parallel port instead, which takes 0-5V.
>What you are proposing is contrary to almost everything I've read
>about hooking up this unit.

Yes, well... The RS232 standard says that teh signal levels are -12V and
+12V and that the absolute minimum be -5V and +5V. However, many serial
chip makers have bent those standards and the serial port may or may not
respond to the 0,5 level that the PPS output actually delivers. There is
absolutely no reason why it should. That signal is completely out of spec.
If yours does work, it is because your serial port manufacturer severely
bent the rules. But many (most?) do. But it could well be that the serial
port is flakey on especially the 0V instead of -5 to -12 V end.

The parallel port specs state that the signal levels are 0V and 5 V which
is exactly what the garmin delivers. 




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