[time-nuts] Trimble Thunderbolt 1pps

Attila Kinali attila at kinali.ch
Mon Jan 27 14:08:19 UTC 2014


On Sun, 05 Jan 2014 13:35:43 -0800
Hal Murray <hmurray at megapathdsl.net> wrote:

> 
> attila at kinali.ch said:
> > Also keep in mind that RS-232 relies on the voltage going negative to encode
> > a "1". I.e. getting 0V is not enough and might only work by chance with some
> > RS-232 receivers. 
> 
> I think there are 2 parts to this discussion.  What do the specs say, and 
> what actually happens in the real world?
> 
> I think the specs say that -3 to +3 is no mans land.  A valid signal must be 
> over +3 or under -3.

IIRC that's right. But i haven't had a look at the standard for a very long time.

 
> In practice, the receiver chip only has one power supply.  It would take 
> extra work to make the switching threshold below ground.

That's not correct. Standard transceiver chips (like the MAX232 family)
have an integrated charge pump to get a negative power supply.

I have never done exact measurements, but my experience is that going
a bit (0.5V?) below GND and slightly more above GND is enough to get
a proper 1/0 detection. Of course, if you rely on that you get a very
poor noise performance.



			Attila Kinali

-- 
The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
		-- Tirin, The Dispossessed, U. Le Guin



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