How to get power from RS-232 port.pdf | | How to get power from RS-232 port
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Get power out of PC RS-232 port
Copyright Tomi Engdahl 1997-2000 This text tries to clear out the mystery of how to get power out of PC serial power. There are quite many small circuit which take all their operating power from serial port, where there are no real power output pin. Examples for this type of circuits are PC mouse and software protection dongle. You might wonder how this is possible. There is one way to get some power out of serial port: steal it from signal lines. When you are developing you own circuit which connects only the PC, then the only line which can be used are the output signal lines from PC serial port: DTS, RTS and TD. In normal operation situation DTR and RTS are raised so they give positive voltage output (about +12V when not loaded). TD pin is in logic 1 when no data is sent which means that it is most time at negative voltage (-12V when not loaded) most of the time. The voltage at these outputs drop quite fast w |