1964 Antique MODEM Live Demo

[Brent Hilpert]

Gordon JC Pearce wrote:
> 
> On Fri, 2009-05-29 at 07:34 -0500, Chris Elmquist wrote:
> > On Thursday (05/28/2009 at 04:03PM -0700), Brent Hilpert wrote:
> > >
> > > (pedantic: His technical description was a little off, it's FSK, not an
> > > 'interrupted' tone.)
> >
> > Agreed.  He started to get on track but then fell off again when he started
> > saying the tones were a function of the voltage level coming in on the
> > RS232 port and that because his laptop had too low of a voltage on that
> > port, he was getting the wrong tones.  This whole discussion in the context
> > of the modem being analog-- implying that he was getting tones that were
> > off frequency due to the incorrect voltage level.  I don't think it went
> > quite like that.
> 
> So, you can't see a possible set of circumstances where having the wrong
> voltage coming in the serial port could cause the frequency shift to be
> wrong?
> 
> How clever do you think the tone generator side is?

It's not inconceivable and the circuit is not overly clever, but it is good
enough to have a trip point over a 0.03 volt range, between +1.26 to +1.29
volts. The tones are pretty solid +/- 4 Hz between there and +/- 25 volts; +/-1
Hz for +/- 3 to 25 volts, so all well within RS-232 requirements. (As measured
on my unit, I suppose the utube guy's unit could have a problem.)

Here's the schematic for amusement (note measurements), pretty simple except
for the specced coils:

  http://www3.telus.net/~bhilpert/tmp/LDSModelAModem.gif

The reason the tone changes when he plugs in the RS-232 connector in the video
is the open-circuit state for the modem modulator is SPACE, while idle-state
for RS-232 is MARK.