[ntp:questions] Re: Looking for time sync hardware supported under Linux for under $400?

Hal Murray hmurray at suespammers.org
Wed Jan 18 09:59:25 UTC 2006


[Context is Garmin GPS-18LVC]

>You will need to shift the levels;  RS232 uses both positive and 
>negative voltages for signaling.  I believe that there is a chip that 
>will convert TTL to RS232 levels.  I vaguely recall a number like MC1447 
>but that's just a vague recollection; get the facts before you spend 
>your money!  You will, of course, need both positive and negative supply 
>voltages; I think +/- 12VDC will get the job done and you might even get 
>away with +/- 5VDC.

Beware.  The normal RS-232 level shifting chips include an inverter.
The signal coming out of the Garmin GPS-18LVC is right side up.  If
you add the level shifting chip you will have to add an inverter to
undo the inversion you don't want.

But why bother.  Just plug it in.  It won't have as much noise
rejection as real RS-232 but it will work fine unless you need
a long cable.  (If you need a long cable you have other problems
to worry about.)

Mine is working fine.

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