How can RS-232 DTR / RTS pins be used for power?

I am currently using the DTR lead for two designs. Like all RS232 leads, the voltage varies from a minus voltage (unasserted) to a positive voltage (asserted). The voltage range is anywhere from ±3v to ±12v. Frankly, I have never seen a voltage amplitude lower than ±5, and that was from a PC COMM port.

The spec for your device says to power the RS-232 interface using +12v on the DTR (asserted), and -12v on the RTS lead (unasserted), for a total of 24 volts. It says it needs 20 ma. This may or may not be possible depending on the RS-232 interface at the other end.

When I did this, there appeared to be a resistor in series with the line at the other end, which acted as a current limiter. I "harvested" between 5 and 10 mA from the DTR line. Even that caused the voltage to drop a bit.

I suggest testing this out first, by measuring the voltage on the DTR and RTS leads with a multimeter. If they're not -12 volts, you can forgot supplying power this way.

If it is -12v, then put a 1210 Ω resistor across the DTR and RTS leads, assert the DTR line and measure the correspond leads to see if the voltage holds up (+12v). You can assert the DTR line either in software (attribute of the COMM function in the Windows API), or for this test, use a program like RealTerm, which has the ability to control the various handshake leads under the Pins tab in its user interface.

I wouldn't worry about the handshaking; if they are telling you to power their interface this way, then it must not be an issue.

RS-232 does not provide for any power standard. Using the RTS and DTS pins as power or GPIO is an abuse of the standard, taking advantage of the nature of electronics in the PC/RS-232 driver. The driver's abililiy to source or sink current determines how much power you can provide to the device. Polarity is determined by the pin state (high or low), while the voltage level is determined by the circuit of the RS-232 driver used.

Any handshaking, if used, can cause reverse voltage or brownout problems, but it will be very short and can maybe be negligible.

Just like using a potato for a battery, just because it works doesn't mean it works well or intended to work that way.