In an RS-232 interface, why is the idle or “mark” level a negative voltage?
RS-232 was developed in the days when people were connecting
electromechanical teletypes to telephone lines with modems, and in fact, at
that time the design of any equipment connected to a phone line was tightly
controlled by the phone company.
The reason RS-232 uses a negative voltage for its idle state
is the same reason the phone lines themselves use a negative voltage relative
to ground for power—copper wires in long cables potentially exposed to moisture
are significantly less likely to corrode if they have a negative DC bias on
them.
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