I earlier built the serial port board from the jal tutorial book, which is basically connecting a PIC and a D-9 directly to the MAX232. Now I'm reading AN1310 on the high-speed PIC16/18 bootloader. In figure 7 on page 6 I see this circuit uses 470Ω resistors on the MAX232 outputs. Will the jal-board work with the bootloader? Can it possibly break down the MAX232 or PIC? And most important: why would I use resistors on the MAX232 outputs?
Safety or EMI (Electromagnetic interference) reasons I'd guess. If you have the resistors, you won't be able to kill the PIC if you set the serial port pins to low.
The application note uses 3.3 V supply, so if we configure the UART pins as low, we'll get at most around 8 mA of current, which should be safe for microcontroller to sink.
Also the resistors slow down the signal edges a bit, so there's less EMI.
This is often used for lazy level shifting, such as from 5v to 3.3v.
The internal protection diodes typically present on the lower supply voltage device keep the voltage on the inputs from rising too high, while the resistors limit the current which the diodes have to handle.