Chapter 4
The power comes from otherwise unused outputs. To comply with the stan-
dard, an RS-232 driver’s output must be at least 5V with a 3k load. Each output
can thus source at least 1.6 mA at 5V. In practice, most RS-232 outputs can
source more than this minimum current, but staying within the standard
ensures that a circuit will work on any port..
!
Figure 4-5 shows ways of using RS-232 outputs as a power source. When
asserted, RTS and DTR are between +5 and +15V. Figure 4-5A shows an
unregulated output. To double the output current, tie two lines together as
shown, with a 1N5819 Schottky diode in each line. The diodes prevent current
from feeding back into the interface if the voltages differ. You can use any recti-
fier diodes, but Schottkys have a lower forward voltage than other silicon
diodes. You can even use the TX line as a power source by setting the Break sig-
nal, but of course this means you can’t use the line for data. However, using TXFigure 4-4: This half-duplex interface uses the RS-232 input’s negative voltage to
pull the RS-232 output below ground.