[time-nuts] Trimble Thunderbolt 1pps

[Hal Murray]

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.

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

There is an additional quirk in here.  The original Motorola MC1489 had a 
switching threshold of a diode drop (and some hysteresis).  That chip was 
very popular and turned into a defacto standard.  If you built a RS-232 
receiver chip that required a negative input voltage, all sorts of obscure 
things would break and anybody who used it would have support nightmares. [1]

The typical RS-232 receiver chips actually have good specs.  In particular 
they spec the transition voltages in each direction.

TI Data sheet for MC1489(A) and SN75189(A)
  http://www.ti.com/lit/ds/symlink/mc1489a.pdf

TI Data sheet for MAX232
  http://www.ti.com/lit/ds/symlink/max232.pdf

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Many years ago (early 1980s?), there was a popular brand of modems that sent out a TTL level rather than real RS-232 levels.  Yes, we found that the hard way when we cut a corner.


-- 
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