Volt levels

Well, yes I did leave the details out, as I thought they would tend to
obscure the the point that I was trying to make.  Which voltage level is a
"mark" and which is a "space", is somewhat immaterial to the discussion at
hand.  But if you really want to get into it, we can start talking about
current loops and opto-isolated connections and what constitutes a "mark"
and a "space" for those, etc etc.


Yes, RS232D receivers are required to work over the full plus and minus
input voltage range.  This is actually trivial to do.  If you look at the
internal schematic of a typical RS232 line receiver, you'll see why.
Although they typically operate on a single +5V, or +3.3V supply, they can
tolerate excursions well beyond the power supply limits in both the positive
and negative, at the inputs.  A small point of clarification.  Although
RS232D drivers may not be REQUIRED to generate plus and minus voltage
swings, but outside of GPS receivers, I've not seen one that didn't.  Even
amongst GPS receivers, it seems that only the handheld GPS' are the ones
that don't generate true bipolar (+ and -)levels.  Every "mouse" type GPS
receiver that I've examined, does generate true bipolar levels.

When you say "TRUE BIG BLUE", to what equipment are you referring?.
Certainly IBM Aptiva PCs have had RS232D compliant serial ports for a very
long time.  It wouldn't surprise me at all, to hear that IBM mainframes were
still only RS232C compliant.

John Galvin