Hi, I looked at this a while ago. The "spec" only defines transmission levels, it does NOT specify receive thresholds. All the receiver chips I've looked at, ancient and modern, have only positive thresholds. Most have single supplies and clamp the input at 1 diode drop negative WRT common after an input current limiting resistor, see the MC1489 datasheet. Even the MAX style devices with built-in charge pumps have a Input Logic-Low Voltage range of 0.8 to 1.3V
Going negative does help with noise immunity. If you drive the TX to -9V then you need more than 9V of noise to loose the data. Robert G8RPI. ________________________________ From: paul swed <paulsw...@gmail.com> To: Discussion of precise time and frequency measurement <time-nuts@febo.com> Sent: Monday, 27 January 2014, 14:44 Subject: Re: [time-nuts] Trimble Thunderbolt 1pps OK I can add something here. Yes the spec is correct. But modern receiver chips actually can work with single side signals. You have to look at the specs of the chip to see what they will do. Granted noise immunity is much lower but for most of us in the 10' run distance its good enough. I operate this way on lots of things. Seriously sleazy way to get +/- swings as long as the data flows somewhat all of the time like GPS. Put the unipolar signal through a 1-10UF cap. Smaller is better to a point. Did I really suggest that. Will never admit it. Regards Paul. WB8TSL On Mon, Jan 27, 2014 at 9:08 AM, Attila Kinali <att...@kinali.ch> wrote: > On Sun, 05 Jan 2014 13:35:43 -0800 > Hal Murray <hmur...@megapathdsl.net> wrote: > > > > > att...@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 > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
