1964 Antique MODEM Live Demo

[Tony Duell]

> 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?

I think it;'s unlikely. One of my old modems has a transmitter that's 
essentially a VCO. The control voltage comes from a couple of pots (one 
for mark, the other for space), these are swtiched by the incoming RS232 
signal. But that signal is first applied to a transistor that's either 
cut off or staturated, so there's no way to get an 'in between' tone.

> 
> How clever do you think the tone generator side is?

Given that the RS232 spec is pretty wide (+/-3V to +/-25V, and the +ve 
and -ve votltages don't have to be the same), no modem should make any 
assuptions about the exact voltage on the TxD line.

-tony