Question:
What is the maximum cable length I can drive with RS-232?
Answer:
The length of cable is not the key limiting factor, but rather the total capacitance of the cable.
RS-232 transceivers are always specified for a guaranteed minimum data rate with a standard load condition. Here is an example: For the MAX3314E that is a 3k Ohm load in parallel with 1000pF and guaranteed 460kbps minimum data rate. Again, the length of cable is not the key limiting factor, but rather the total capacitance of the cable. In general RS-232 is specified for roughly 50 feet or 15 meters, just as a rule of thumb. You need to consider the transmission cable capacitance in your calculation. For example, if the cable has 47pF/meter, then you can use the relation of guaranteed minimum 460kbps driving 1000pF/47pf/m = 21.3m cable length.
RS-232 transceiver datasheets from Maxim often have Typical Operating Characteristic curves with up to 3000pF load. A robust design should always consider the guaranteed minimum and maximum specifications as appropriate, but the typical performance curves provide indication of how the part will operate as conditions are varied.