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.
| Keep up-to-date with our FREE Weekly Newsletter! | Don't miss out on upcoming issues of Circuit Cellar. Note: We’ve made the May 2020 issue of Circuit Cellar available as a free sample issue. In it, you’ll find a rich variety of the kinds of articles and information that exemplify a typical issue of the current magazine. — ADVERTISMENT— |
| Would you like to write for Circuit Cellar? We are always accepting articles/posts from the technical community. Get in touch with us and let's discuss your ideas. |
