MAX3232: current limit series resister value

Hi Nam,

To limit the current of an output driver to 60mA while it's shorted to 24V, a 400-ohm resistor would need to be placed in series. Event with this current, the short duration should be limited to prevent exceeding the power dissipation rating of the pin. The receiver pins can withstand +-25V so short-circuit current protection won't be needed for a 24V system. 

Adding large series resistances to RS232 lines impact the resulting signal in two major ways.
1. The received signal voltage will be attenuated as a voltage drop will exist across the signal line. Because the steady-state current of this signal is very small (receiver input impedance 5k-ohm typical), this should not severely impact sufficiently low data rate signals.
2. The RC network created by the line resistance and capacitance will more significantly affect the edge-rate of the signal. Because the output current of the driver will be limited by the resistor, it will take longer for the capacitance of the signal line to charge to its steady-state voltage. Typically, the line capacitance is of more concern here, but significantly increasing the series resistance will have amplify the impact this has. The result will be a slower achievable data rate for the system.

To avoid these negative impacts, I'd recommend using some more advanced components such as TVS diodes to clamp the voltage below the receiver's tolerable threshold and perhaps a series thermistor to limit the current during the fault condition and give some thermal relief to the TVS. 

Let me know if you have any more questions.

Regards,
Eric Schott