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