[time] tech stuff, was Re: NTP server using GNU/linux and freebsd, was ...

[John Pettitt]

gnu not unix wrote:

>In message <20050204135649.F61239 at teardrop.ca> you write:
>
>  
>
>>Alright I did a little bit of reading of the GPS 18 spec sheet and noticed 
>>how to wire it up to the serial port; I noticed they ignored the PPS line 
>>completely.  Now if I follow the spec sheet exactly and wire the power 
>>source to the 5v lines from the computer's power supply, would this work?
>>    
>>
>
>No; ntpd will not work properly with the NMEA driver (type 20)
>unless there is a PPS signal via DCD provided. Your board should
>work if you also connect a "MAX232" ttl to rs232 driver to the
>PPS signal, and connect the MAX232 to the DCD pin on your serial port.
>
>The MAX232 chips are about $1 us and also need four 1uf capacitors.
>
>  
>
Actually the GPS18 seems to work fine on my system without a MAX232 - 
just hook the pps line to DCD and hook up the rest of the wires per the 
Gamin docs - I have even extended mine 50 feet and it's still fine.   
Older serial boards may have issues with a the low signal level but most 
newer systems will work with anything over about 3v.

If you are getting a GPS18 you should also download the config software 
from Garmin (Windows only) - it will let you turn off junk NMEA data - 
as shipped the GPS18 sends a lot of stuff you don't need - so much that 
it can't manage to it once a second. 

John


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