Hi all,
Let me give you a concrete example of why 2 RS-232 ports cannot share
the same port.
Look at the RS-232 Specifications.
The normal voltage for an RS-232 signal line is held at the "Mark level"
of anywhere between -3 and -15 volts in its idle state (as measured at
the receiver).
A "Space" voltage is anywhere between +3 volts and +15 volts (again as
measured at the receiver).
The driver is a voltage source. So imagine 2 drivers trying to put a
signal on a single receiver (or multiple receivers).
The idle driver will be putting a negative voltage on the line, while
the active driver trying to send data will be putting on a positive
voltage. The resulting voltage will be unpredictable and depends on the
impedance of each driver and the resistance of the signal line.
What will happen if you connect two voltage sources (say 2 power
supplies) together, one having a negative voltage and the other having a
positive voltage. They will cancel one another (besides drawing a lot
of current from both supplies).
The net effect is that 2 RS-232 drivers on the same line will "fight"
and proper communications will not be possible.
RS-232 is a point to point operation. A single driver sending data to a
single device. Other receivers may 'listen in' to the communications,
but if 2 drivers are present on the same signal line, the result is chaos.
Virtual ports can take care of that situation, but one cannot simply
parallel live hardware RS-232 signals (with a "Y" cable or other means)
and expect success, so hardware solutions are non-existent. This is a
characteristic of RS-232 communications, and not a problem if you follow
the rules.
73,
Don W3FPR
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