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Parasitic Power for RS232

Paul B. Webster VK2BZC says

  1. Derive your +5V from every available RS-232 line through a diode i.e., TXD, RTS, DTR; three diodes directly from these lines to the +5V rail.
  2. If you *must* loopback RTS to CTS and/ or DTR to DSR and DCD, then do so via 1k or 2k resistors to minimize loading by the receivers.
  3. Regulate with a shunt Zener (5.1V) and a large capacitor (220µF) across the +5V supply. RS-232 is current-limited by definition so no problems here. If you *really* need a reference voltage, use a separate reference IC.
  4. The most efficient driver for the TXD line, and protection on the RXD line is a 74HC14 gate. You can use the PIC directly if you want, but the gate is likely cheaper and easier to replace (in the field!).
    Using transistors is counter-productive as you lose current in the pull-up resistors, unless you also wish to use a negative supply derived from another three diodes to provide negative swing to outgoing data.
    Include protective resistors in (incoming) TXD of about 33k ohm and (outgoing) RXD of about 220 ohm.
  5. If you do *not* drive CTS, DSR, DCD and/ or RI, ground them to avoid spurious Line Status interrupts fouling up terminal routines on the PC.

See also:


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