1 Jun
2009
1 Jun
'09
4:16 a.m.
Gordon JC Pearce 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?
If you put enough voltage through a serial port signal, you'll break the
modem. Then you'll probably get a fixed frequency regardless of the
input, or no frequency at all. Other than that, no, having the wrong
voltage won't cause the frequency shift to be wrong.
How clever do you think the tone generator side is?
Clever enough that its frequency isn't a linear function of the serial
port signal voltages. Modems with EIA-232 (formerly RS-232) interfaces
detect whether the transmit data signal has a voltage above or below a
threshold, and generate one of two tones based on that. They don't feed
the transmit data signal directly into a VCO, if that's what you were
thinking. Doing it that way wouldn't result in a reliable modem, since
an EIA-232 mark signal can be anywhere from -3V to -25V, and a space
from +3V to +25V.
For current loop, it senses current rather than voltage, but the
principle is the same. It doesn't base the tone on the precise amount
of current flowing, because even in a loop that has a nominal current
(i.e., 20mA), the actually current may be significantly different.