30 May
2009
30 May
'09
11:39 a.m.
Gordon JC Pearce wrote:
On Fri, 2009-05-29 at 07:34 -0500, Chris Elmquist wrote:
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.
On Thursday (05/28/2009 at 04:03PM -0700), Brent
Hilpert wrote:
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?
(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. 