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If you are using serial port level shifter (like the max232), you could just tie its output to the external interrupt and set your external interrupt to occur on the proper transition. Whenever the state toggles (such as a character coming down the serial line), your interrupt will fire and you can handle whatever in software.
I am using a pic16F73 now, and I want to put some buttons on the pic I think that portB will do on RB4 t/m RB7.
Can I just put a button on that pin and get RBIE in INTCON set and then I can get a interupt from the buttons.