I think it's even broader than that Mike, like +/-5 up to +/-25.
From MAXIM's MAX232 datasheet:
"The typical driver output voltage swing is ±8V when loaded with a nominal 5kΩ RS-232 receiver and VCC =+5V.
Output swing is guaranteed to meet the EIA/TIA-232E and V.28 specification, which calls for ±5V minimum
driver output levels under worst-case conditions."
For those not in the know:
Parts like MAX232 use the capacitors and an internal voltage doubler & inversion circuits to take 3.3V power on VCC pins to make +/- 6V output swing, and 5V power to make +/-10V swing:
"The MAX220–MAX249 have two internal charge-pumps that convert +5V to ±10V (unloaded) for RS-232 driver
operation. The first converter uses capacitor C1 to double the +5V input to +10V on C3 at the V+ output. The
second converter uses capacitor C2 to invert +10V to -10V on C4 at the V- output."
"The MAX3222/MAX3232/MAX3237/MAX3241’s internal power supply consists of a regulated dual charge pump
that provides output voltages of +5.5V (doubling charge pump) and -5.5V (inverting charge pump), regardless of
the input voltage (VCC) over the 3.0V to 5.5V range."
Some versions don't even use external caps, everything is internal, altho these are limited in the speed they can support.
Read all about the different versions here.http://datasheets.maxim-ic.com/en/ds/MAX220-MAX249.pdfhttp://datasheets.maxim-ic.com/en/ds/MAX3222-MAX3241.pdf